Flash-Friendly File System (F2FS)

개요·LFS 문제와 F2FS 핵심 기능

SSD, eMMC, SD card 같은 NAND flash storage는 rotating disk와 특성이 다르므로 상위 layer인 filesystem 설계도 이에 맞춰야 합니다. F2FS는 Log-structured File System(LFS)을 기반으로 하되 wandering tree의 연쇄 갱신과 높은 cleaning overhead를 완화하도록 설계됐습니다.

device 내부 geometry와 Flash Translation Layer(FTL) 관리 방식이 서로 다르기 때문에 F2FS와 userland tool은 on-disk layout, allocation algorithm, cleaning algorithm을 조정하는 다양한 parameter를 제공합니다.

`f2fs-tools` git tree는 `mkfs.f2fs`, `fsck.f2fs`, `dump.f2fs`를 제공합니다. patch는 `[email protected]`, bug는 kernel Bugzilla의 f2fs component로 보냅니다.

LFS는 모든 변경을 log 형태로 순차 기록해 file write와 crash recovery를 빠르게 합니다. 빠른 새 write를 위한 큰 free area를 유지하려고 log를 segment로 나누고 cleaner가 심하게 fragmented된 segment의 live data를 압축 이동합니다.

file data를 log 끝으로 옮기면 direct pointer, indirect pointer, inode, inode map, checkpoint가 재귀적으로 갱신되는 현상을 wandering tree problem이라 합니다. F2FS는 inode와 pointer block을 모두 `node`로 다루고 모든 node 위치를 NAT에 기록해 leaf data write의 update propagation을 끊습니다.

out-of-place write는 obsolete block을 넓게 흩뿌립니다. cleaner는 SIT로 victim segment를 고르고 SSA의 summary로 parent index를 읽은 뒤 data-parent cross-reference를 검사해 valid data만 이동합니다.

F2FS는 random write area를 넓히면서 spatial locality를 유지하고 FTL operation unit에 data structure를 정렬합니다. background cleaning, greedy·cost-benefit victim policy, hot/warm/cold multi-head log, adaptive logging을 지원합니다.

.. SPDX-License-Identifier: GPL-2.0

=================================
Flash-Friendly File System (F2FS)
=================================

Overview
========

NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
been equipped on a variety systems ranging from mobile to server systems. Since
they are known to have different characteristics from the conventional rotating
disks, a file system, an upper layer to the storage device, should adapt to the
changes from the sketch in the design level.

F2FS is a file system exploiting NAND flash memory-based storage devices, which
is based on Log-structured File System (LFS). The design has been focused on
addressing the fundamental issues in LFS, which are snowball effect of wandering
tree and high cleaning overhead.

Since a NAND flash memory-based storage device shows different characteristic
according to its internal geometry or flash memory management scheme, namely FTL,
F2FS and its tools support various parameters not only for configuring on-disk
layout, but also for selecting allocation and cleaning algorithms.

The following git tree provides the file system formatting tool (mkfs.f2fs),
a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).

- git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git

For sending patches, please use the following mailing list:

- [email protected]

For reporting bugs, please use the following f2fs bug tracker link:

- https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs

Background and Design issues
============================

Log-structured File System (LFS)
--------------------------------
"A log-structured file system writes all modifications to disk sequentially in
a log-like structure, thereby speeding up  both file writing and crash recovery.
The log is the only structure on disk; it contains indexing information so that
files can be read back from the log efficiently. In order to maintain large free
areas on disk for fast writing, we divide  the log into segments and use a
segment cleaner to compress the live information from heavily fragmented
segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
implementation of a log-structured file system", ACM Trans. Computer Systems
10, 1, 26–52.

Wandering Tree Problem
----------------------
In LFS, when a file data is updated and written to the end of log, its direct
pointer block is updated due to the changed location. Then the indirect pointer
block is also updated due to the direct pointer block update. In this manner,
the upper index structures such as inode, inode map, and checkpoint block are
also updated recursively. This problem is called as wandering tree problem [1],
and in order to enhance the performance, it should eliminate or relax the update
propagation as much as possible.

[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/

Cleaning Overhead
-----------------
Since LFS is based on out-of-place writes, it produces so many obsolete blocks
scattered across the whole storage. In order to serve new empty log space, it
needs to reclaim these obsolete blocks seamlessly to users. This job is called
as a cleaning process.

The process consists of three operations as follows.

1. A victim segment is selected through referencing segment usage table.
2. It loads parent index structures of all the data in the victim identified by
   segment summary blocks.
3. It checks the cross-reference between the data and its parent index structure.
4. It moves valid data selectively.

This cleaning job may cause unexpected long delays, so the most important goal
is to hide the latencies to users. And also definitely, it should reduce the
amount of valid data to be moved, and move them quickly as well.

Key Features
============

Flash Awareness
---------------
- Enlarge the random write area for better performance, but provide the high
  spatial locality
- Align FS data structures to the operational units in FTL as best efforts

Wandering Tree Problem
----------------------
- Use a term, “node”, that represents inodes as well as various pointer blocks
- Introduce Node Address Table (NAT) containing the locations of all the “node”
  blocks; this will cut off the update propagation.

Cleaning Overhead
-----------------
- Support a background cleaning process
- Support greedy and cost-benefit algorithms for victim selection policies
- Support multi-head logs for static/dynamic hot and cold data separation
- Introduce adaptive logging for efficient block allocation

Mount option: GC·inline·allocation·fault

첫 mount option 집합은 background/foreground GC 협력, roll-forward recovery, discard, xattr·ACL, active log, inline storage, flush ordering, extent cache, reserved 공간, fault injection과 allocation mode를 제어합니다.

Mount Options
=============


======================== ============================================================
background_gc=%s         Turn on/off cleaning operations, namely garbage
                         collection, triggered in background when I/O subsystem is
                         idle. If background_gc=on, it will turn on the garbage
                         collection and if background_gc=off, garbage collection
                         will be turned off. If background_gc=sync, it will turn
                         on synchronous garbage collection running in background.
                         Default value for this option is on. So garbage
                         collection is on by default.
gc_merge                 When background_gc is on, this option can be enabled to
                         let background GC thread to handle foreground GC requests,
                         it can eliminate the sluggish issue caused by slow foreground
                         GC operation when GC is triggered from a process with limited
                         I/O and CPU resources.
nogc_merge                 Disable GC merge feature.
disable_roll_forward         Disable the roll-forward recovery routine
norecovery                 Disable the roll-forward recovery routine, mounted read-
                         only (i.e., -o ro,disable_roll_forward)
discard/nodiscard         Enable/disable real-time discard in f2fs, if discard is
                         enabled, f2fs will issue discard/TRIM commands when a
                         segment is cleaned.
heap/no_heap                 Deprecated.
nouser_xattr                 Disable Extended User Attributes. Note: xattr is enabled
                         by default if CONFIG_F2FS_FS_XATTR is selected.
noacl                         Disable POSIX Access Control List. Note: acl is enabled
                         by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
active_logs=%u                 Support configuring the number of active logs. In the
                         current design, f2fs supports only 2, 4, and 6 logs.
                         Default number is 6.
disable_ext_identify         Disable the extension list configured by mkfs, so f2fs
                         is not aware of cold files such as media files.
inline_xattr                 Enable the inline xattrs feature.
noinline_xattr                 Disable the inline xattrs feature.
inline_xattr_size=%u         Support configuring inline xattr size, it depends on
                         flexible inline xattr feature.
inline_data                 Enable the inline data feature: Newly created small (<~3.4k)
                         files can be written into inode block.
inline_dentry                 Enable the inline dir feature: data in newly created
                         directory entries can be written into inode block. The
                         space of inode block which is used to store inline
                         dentries is limited to ~3.4k.
noinline_dentry                 Disable the inline dentry feature.
flush_merge                 Merge concurrent cache_flush commands as much as possible
                         to eliminate redundant command issues. If the underlying
                         device handles the cache_flush command relatively slowly,
                         recommend to enable this option.
nobarrier                 This option can be used if underlying storage guarantees
                         its cached data should be written to the novolatile area.
                         If this option is set, no cache_flush commands are issued
                         but f2fs still guarantees the write ordering of all the
                         data writes.
barrier                         If this option is set, cache_flush commands are allowed to be
                         issued.
fastboot                 This option is used when a system wants to reduce mount
                         time as much as possible, even though normal performance
                         can be sacrificed.
extent_cache                 Enable an extent cache based on rb-tree, it can cache
                         as many as extent which map between contiguous logical
                         address and physical address per inode, resulting in
                         increasing the cache hit ratio. Set by default.
noextent_cache                 Disable an extent cache based on rb-tree explicitly, see
                         the above extent_cache mount option.
noinline_data                 Disable the inline data feature, inline data feature is
                         enabled by default.
data_flush                 Enable data flushing before checkpoint in order to
                         persist data of regular and symlink.
reserve_root=%d                 Support configuring reserved space which is used for
                         allocation from a privileged user with specified uid or
                         gid, unit: 4KB, the default limit is 12.5% of user blocks.
reserve_node=%d                 Support configuring reserved nodes which are used for
                         allocation from a privileged user with specified uid or
                         gid, the default limit is 12.5% of all nodes.
resuid=%d                 The user ID which may use the reserved blocks and nodes.
resgid=%d                 The group ID which may use the reserved blocks and nodes.
fault_injection=%d         Enable fault injection in all supported types with
                         specified injection rate.
fault_type=%d                 Support configuring fault injection type, should be
                         enabled with fault_injection option, fault type value
                         is shown below, it supports single or combined type.

                         ===========================      ==========
                         Type_Name                        Type_Value
                         ===========================      ==========
                         FAULT_KMALLOC                    0x00000001
                         FAULT_KVMALLOC                   0x00000002
                         FAULT_PAGE_ALLOC                 0x00000004
                         FAULT_PAGE_GET                   0x00000008
                         FAULT_ALLOC_BIO                  0x00000010 (obsolete)
                         FAULT_ALLOC_NID                  0x00000020
                         FAULT_ORPHAN                     0x00000040
                         FAULT_BLOCK                      0x00000080
                         FAULT_DIR_DEPTH                  0x00000100
                         FAULT_EVICT_INODE                0x00000200
                         FAULT_TRUNCATE                   0x00000400
                         FAULT_READ_IO                    0x00000800
                         FAULT_CHECKPOINT                 0x00001000
                         FAULT_DISCARD                    0x00002000
                         FAULT_WRITE_IO                   0x00004000
                         FAULT_SLAB_ALLOC                 0x00008000
                         FAULT_DQUOT_INIT                 0x00010000
                         FAULT_LOCK_OP                    0x00020000
                         FAULT_BLKADDR_VALIDITY           0x00040000
                         FAULT_BLKADDR_CONSISTENCE        0x00080000
                         FAULT_NO_SEGMENT                 0x00100000
                         FAULT_INCONSISTENT_FOOTER        0x00200000
                         FAULT_TIMEOUT                    0x00400000 (1000ms)
                         FAULT_VMALLOC                    0x00800000
                         ===========================      ==========
mode=%s                         Control block allocation mode which supports "adaptive"
                         and "lfs". In "lfs" mode, there should be no random
                         writes towards main area.
                         "fragment:segment" and "fragment:block" are newly added here.
                         These are developer options for experiments to simulate filesystem
                         fragmentation/after-GC situation itself. The developers use these
                         modes to understand filesystem fragmentation/after-GC condition well,
                         and eventually get some insights to handle them better.
                         In "fragment:segment", f2fs allocates a new segment in random
                         position. With this, we can simulate the after-GC condition.
                         In "fragment:block", we can scatter block allocation with
                         "max_fragment_chunk" and "max_fragment_hole" sysfs nodes.
                         We added some randomness to both chunk and hole size to make
                         it close to realistic IO pattern. So, in this mode, f2fs will allocate
                         1..<max_fragment_chunk> blocks in a chunk and make a hole in the
                         length of 1..<max_fragment_hole> by turns. With this, the newly
                         allocated blocks will be scattered throughout the whole partition.
                         Note that "fragment:block" implicitly enables "fragment:segment"
                         option for more randomness.
                         Please, use these options for your experiments and we strongly
                         recommend to re-format the filesystem after using these options.

Mount option: quota·checkpoint·compression·error

후반 mount option은 quota, fsync, dummy encryption, checkpoint disable/merge, compression, inline crypto, zoned discard, memory 절약, critical error policy, NAT와 casefold lookup을 제어합니다.

compression enable 우선순위는 directory flag보다 `compress_extension`·`nocompress_extension`이 높고, 개별 file의 compress/no-compress flag가 가장 높습니다. `nocompress_extension`은 `compress_extension=*`의 예외로도 동작합니다.

usrquota                 Enable plain user disk quota accounting.
grpquota                 Enable plain group disk quota accounting.
prjquota                 Enable plain project quota accounting.
usrjquota=<file>         Appoint specified file and type during mount, so that quota
grpjquota=<file>         information can be properly updated during recovery flow,
prjjquota=<file>         <quota file>: must be in root directory;
jqfmt=<quota type>         <quota type>: [vfsold,vfsv0,vfsv1].
usrjquota=                 Turn off user journalled quota.
grpjquota=                 Turn off group journalled quota.
prjjquota=                 Turn off project journalled quota.
quota                         Enable plain user disk quota accounting.
noquota                         Disable all plain disk quota option.
alloc_mode=%s                 Adjust block allocation policy, which supports "reuse"
                         and "default".
fsync_mode=%s                 Control the policy of fsync. Currently supports "posix",
                         "strict", and "nobarrier". In "posix" mode, which is
                         default, fsync will follow POSIX semantics and does a
                         light operation to improve the filesystem performance.
                         In "strict" mode, fsync will be heavy and behaves in line
                         with xfs, ext4 and btrfs, where xfstest generic/342 will
                         pass, but the performance will regress. "nobarrier" is
                         based on "posix", but doesn't issue flush command for
                         non-atomic files likewise "nobarrier" mount option.
test_dummy_encryption
test_dummy_encryption=%s
                         Enable dummy encryption, which provides a fake fscrypt
                         context. The fake fscrypt context is used by xfstests.
                         The argument may be either "v1" or "v2", in order to
                         select the corresponding fscrypt policy version.
checkpoint=%s[:%u[%]]         Set to "disable" to turn off checkpointing. Set to "enable"
                         to re-enable checkpointing. Is enabled by default. While
                         disabled, any unmounting or unexpected shutdowns will cause
                         the filesystem contents to appear as they did when the
                         filesystem was mounted with that option.
                         While mounting with checkpoint=disable, the filesystem must
                         run garbage collection to ensure that all available space can
                         be used. If this takes too much time, the mount may return
                         EAGAIN. You may optionally add a value to indicate how much
                         of the disk you would be willing to temporarily give up to
                         avoid additional garbage collection. This can be given as a
                         number of blocks, or as a percent. For instance, mounting
                         with checkpoint=disable:100% would always succeed, but it may
                         hide up to all remaining free space. The actual space that
                         would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
                         This space is reclaimed once checkpoint=enable.
checkpoint_merge         When checkpoint is enabled, this can be used to create a kernel
                         daemon and make it to merge concurrent checkpoint requests as
                         much as possible to eliminate redundant checkpoint issues. Plus,
                         we can eliminate the sluggish issue caused by slow checkpoint
                         operation when the checkpoint is done in a process context in
                         a cgroup having low i/o budget and cpu shares. To make this
                         do better, we set the default i/o priority of the kernel daemon
                         to "3", to give one higher priority than other kernel threads.
                         This is the same way to give a I/O priority to the jbd2
                         journaling thread of ext4 filesystem.
nocheckpoint_merge         Disable checkpoint merge feature.
compress_algorithm=%s         Control compress algorithm, currently f2fs supports "lzo",
                         "lz4", "zstd" and "lzo-rle" algorithm.
compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
                         "lz4" and "zstd" support compress level config.

                         =========      ===========
                         algorithm        level range
                         =========      ===========
                         lz4                3 - 16
                         zstd                1 - 22
                         =========      ===========
compress_log_size=%u         Support configuring compress cluster size. The size will
                         be 4KB * (1 << %u). The default and minimum sizes are 16KB.
compress_extension=%s         Support adding specified extension, so that f2fs can enable
                         compression on those corresponding files, e.g. if all files
                         with '.ext' has high compression rate, we can set the '.ext'
                         on compression extension list and enable compression on
                         these file by default rather than to enable it via ioctl.
                         For other files, we can still enable compression via ioctl.
                         Note that, there is one reserved special extension '*', it
                         can be set to enable compression for all files.
nocompress_extension=%s         Support adding specified extension, so that f2fs can disable
                         compression on those corresponding files, just contrary to compression extension.
                         If you know exactly which files cannot be compressed, you can use this.
                         The same extension name can't appear in both compress and nocompress
                         extension at the same time.
                         If the compress extension specifies all files, the types specified by the
                         nocompress extension will be treated as special cases and will not be compressed.
                         Don't allow use '*' to specifie all file in nocompress extension.
                         After add nocompress_extension, the priority should be:
                         dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
                         See more in compression sections.

compress_chksum                 Support verifying chksum of raw data in compressed cluster.
compress_mode=%s         Control file compression mode. This supports "fs" and "user"
                         modes. In "fs" mode (default), f2fs does automatic compression
                         on the compression enabled files. In "user" mode, f2fs disables
                         the automaic compression and gives the user discretion of
                         choosing the target file and the timing. The user can do manual
                         compression/decompression on the compression enabled files using
                         ioctls.
compress_cache                 Support to use address space of a filesystem managed inode to
                         cache compressed block, in order to improve cache hit ratio of
                         random read.
inlinecrypt                 When possible, encrypt/decrypt the contents of encrypted
                         files using the blk-crypto framework rather than
                         filesystem-layer encryption. This allows the use of
                         inline encryption hardware. The on-disk format is
                         unaffected. For more details, see
                         Documentation/block/inline-encryption.rst.
atgc                         Enable age-threshold garbage collection, it provides high
                         effectiveness and efficiency on background GC.
discard_unit=%s                 Control discard unit, the argument can be "block", "segment"
                         and "section", issued discard command's offset/size will be
                         aligned to the unit, by default, "discard_unit=block" is set,
                         so that small discard functionality is enabled.
                         For blkzoned device, "discard_unit=section" will be set by
                         default, it is helpful for large sized SMR or ZNS devices to
                         reduce memory cost by getting rid of fs metadata supports small
                         discard.
memory=%s                 Control memory mode. This supports "normal" and "low" modes.
                         "low" mode is introduced to support low memory devices.
                         Because of the nature of low memory devices, in this mode, f2fs
                         will try to save memory sometimes by sacrificing performance.
                         "normal" mode is the default mode and same as before.
age_extent_cache         Enable an age extent cache based on rb-tree. It records
                         data block update frequency of the extent per inode, in
                         order to provide better temperature hints for data block
                         allocation.
errors=%s                 Specify f2fs behavior on critical errors. This supports modes:
                         "panic", "continue" and "remount-ro", respectively, trigger
                         panic immediately, continue without doing anything, and remount
                         the partition in read-only mode. By default it uses "continue"
                         mode.

                         ====================== =============== =============== ========
                         mode                        continue        remount-ro        panic
                         ====================== =============== =============== ========
                         access ops                normal                normal                N/A
                         syscall errors                -EIO                -EROFS                N/A
                         mount option                rw                ro                N/A
                         pending dir write        keep                keep                N/A
                         pending non-dir write        drop                keep                N/A
                         pending node write        drop                keep                N/A
                         pending meta write        keep                keep                N/A
                         ====================== =============== =============== ========
nat_bits                 Enable nat_bits feature to enhance full/empty nat blocks access,
                         by default it's disabled.
lookup_mode=%s                 Control the directory lookup behavior for casefolded
                         directories. This option has no effect on directories
                         that do not have the casefold feature enabled.

                         ================== ========================================
                         Value                    Description
                         ================== ========================================
                         perf                    (Default) Enforces a hash-only lookup.
                                            The linear search fallback is always
                                            disabled, ignoring the on-disk flag.
                         compat                    Enables the linear search fallback for
                                            compatibility with directory entries
                                            created by older kernel that used a
                                            different case-folding algorithm.
                                            This mode ignores the on-disk flag.
                         auto                    F2FS determines the mode based on the
                                            on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL`
                                            flag.
                         ================== ========================================
======================== ============================================================

debugfs·sysfs·사용법과 userland tool

`/sys/kernel/debug/f2fs/`는 mount된 모든 F2FS partition 정보를 보여 줍니다. `status`에는 현재 주요 filesystem 정보, 전체 segment의 평균 SIT 정보, F2FS memory footprint가 포함됩니다.

mount된 filesystem의 sysfs 정보는 device name별 `/sys/fs/f2fs/<devname>`에 있으며 자세한 file 목록은 `Documentation/ABI/testing/sysfs-fs-f2fs`를 참조합니다.

사용 순서는 userland tool build, 필요 시 `f2fs.ko` module 삽입, mount directory 생성, `mkfs.f2fs` format, `mount -t f2fs`입니다.

`fsck.f2fs` quick option은 `-d debug_level`(기본 0)입니다. `dump.f2fs`는 `-d`, hex inode용 `-i`, SIT segment 범위용 `-s`, SSA segment 범위용 `-a`를 지원하며 `0~-1`은 전체 범위입니다.

Debugfs Entries
===============

/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
f2fs. Each file shows the whole f2fs information.

/sys/kernel/debug/f2fs/status includes:

 - major file system information managed by f2fs currently
 - average SIT information about whole segments
 - current memory footprint consumed by f2fs.

Sysfs Entries
=============

Information about mounted f2fs file systems can be found in
/sys/fs/f2fs.  Each mounted filesystem will have a directory in
/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
The files in each per-device directory are shown in table below.

Files in /sys/fs/f2fs/<devname>
(see also Documentation/ABI/testing/sysfs-fs-f2fs)

Usage
=====

1. Download userland tools and compile them.

2. Skip, if f2fs was compiled statically inside kernel.
   Otherwise, insert the f2fs.ko module::

        # insmod f2fs.ko

3. Create a directory to use when mounting::

        # mkdir /mnt/f2fs

4. Format the block device, and then mount as f2fs::

        # mkfs.f2fs -l label /dev/block_device
        # mount -t f2fs /dev/block_device /mnt/f2fs

mkfs.f2fs
---------
The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
which builds a basic on-disk layout.

The quick options consist of:

===============    ===========================================================
``-l [label]``     Give a volume label, up to 512 unicode name.
``-a [0 or 1]``    Split start location of each area for heap-based allocation.

                   1 is set by default, which performs this.
``-o [int]``       Set overprovision ratio in percent over volume size.

                   5 is set by default.
``-s [int]``       Set the number of segments per section.

                   1 is set by default.
``-z [int]``       Set the number of sections per zone.

                   1 is set by default.
``-e [str]``       Set basic extension list. e.g. "mp3,gif,mov"
``-t [0 or 1]``    Disable discard command or not.

                   1 is set by default, which conducts discard.
===============    ===========================================================

Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.

fsck.f2fs
---------
The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
partition, which examines whether the filesystem metadata and user-made data
are cross-referenced correctly or not.
Note that, initial version of the tool does not fix any inconsistency.

The quick options consist of::

  -d debug level [default:0]

Note: please refer to the manpage of fsck.f2fs(8) to get full option list.

dump.f2fs
---------
The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
file. Each file is dump_ssa and dump_sit.

The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
It shows on-disk inode information recognized by a given inode number, and is
able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
./dump_sit respectively.

The options consist of::

  -d debug level [default:0]
  -i inode no (hex)
  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]

Examples::

    # dump.f2fs -i [ino] /dev/sdx
    # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
    # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)

Note: please refer to the manpage of dump.f2fs(8) to get full option list.

sload.f2fs
----------
The sload.f2fs gives a way to insert files and directories in the existing disk
image. This tool is useful when building f2fs images given compiled files.

Note: please refer to the manpage of sload.f2fs(8) to get full option list.

resize.f2fs
-----------
The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
all the files and directories stored in the image.

Note: please refer to the manpage of resize.f2fs(8) to get full option list.

defrag.f2fs
-----------
The defrag.f2fs can be used to defragment scattered written data as well as
filesystem metadata across the disk. This can improve the write speed by giving
more free consecutive space.

Note: please refer to the manpage of defrag.f2fs(8) to get full option list.

f2fs_io
-------
The f2fs_io is a simple tool to issue various filesystem APIs as well as
f2fs-specific ones, which is very useful for QA tests.

Note: please refer to the manpage of f2fs_io(8) to get full option list.

On-disk layout과 metadata shadow copy

F2FS volume은 고정 2MiB segment로 나뉘고, 연속 segment가 section을, section 집합이 zone을 이룹니다. 기본 section·zone 크기는 각각 segment 하나지만 `mkfs`로 바꿀 수 있습니다.

volume은 SB, CP, SIT, NAT, SSA, Main의 여섯 area로 나뉩니다. SB를 제외한 area는 여러 segment로 구성됩니다.

mount 시 CP area를 scan해 마지막 valid checkpoint를 찾습니다. scan 시간을 줄이려고 CP 사본은 두 개만 두고 하나가 항상 마지막 valid data를 가리키는 shadow copy mechanism을 사용합니다. NAT와 SIT도 각각 두 사본으로 같은 방식을 씁니다.

Design
======

On-disk Layout
--------------

F2FS divides the whole volume into a number of segments, each of which is fixed
to 2MB in size. A section is composed of consecutive segments, and a zone
consists of a set of sections. By default, section and zone sizes are set to one
segment size identically, but users can easily modify the sizes by mkfs.

F2FS splits the entire volume into six areas, and all the areas except superblock
consist of multiple segments as described below::

                                            align with the zone size <-|
                 |-> align with the segment size
     _________________________________________________________________________
    |            |            |   Segment   |    Node     |   Segment  |      |
    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
    |____________|_____2______|______N______|______N______|______N_____|__N___|
                                                                       .      .
                                                             .                .
                                                 .                            .
                                    ._________________________________________.
                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
                                    .           .
                                    ._________._________
                                    |_section_|__...__|_
                                    .            .
                                    .________.
                                    |__zone__|

- Superblock (SB)
   It is located at the beginning of the partition, and there exist two copies
   to avoid file system crash. It contains basic partition information and some
   default parameters of f2fs.

- Checkpoint (CP)
   It contains file system information, bitmaps for valid NAT/SIT sets, orphan
   inode lists, and summary entries of current active segments.

- Segment Information Table (SIT)
   It contains segment information such as valid block count and bitmap for the
   validity of all the blocks.

- Node Address Table (NAT)
   It is composed of a block address table for all the node blocks stored in
   Main area.

- Segment Summary Area (SSA)
   It contains summary entries which contains the owner information of all the
   data and node blocks stored in Main area.

- Main Area
   It contains file and directory data including their indices.

In order to avoid misalignment between file system and flash-based storage, F2FS
aligns the start block address of CP with the segment size. Also, it aligns the
start block address of Main area with the zone size by reserving some segments
in SSA area.

Reference the following survey for additional technical details.
https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey

File System Metadata Structure
------------------------------

F2FS adopts the checkpointing scheme to maintain file system consistency. At
mount time, F2FS first tries to find the last valid checkpoint data by scanning
CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
One of them always indicates the last valid data, which is called as shadow copy
mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.

For file system consistency, each CP points to which NAT and SIT copies are
valid, as shown as below::

  +--------+----------+---------+
  |   CP   |    SIT   |   NAT   |
  +--------+----------+---------+
  .         .          .          .
  .            .              .              .
  .               .                 .                 .
  +-------+-------+--------+--------+--------+--------+
  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
  +-------+-------+--------+--------+--------+--------+
     |             ^                          ^
     |             |                          |
     `----------------------------------------'

Node index와 multi-level directory hash

data 위치를 관리하는 핵심 구조는 `node`입니다. inode, direct node, indirect node의 세 종류가 있습니다. 4KiB inode block은 data index 923개, direct node pointer 2개, indirect node pointer 2개, double-indirect pointer 1개를 담습니다.

file 하나가 다루는 최대 범위는 `4KiB * (923 + 2*1018 + 2*1018*1018 + 1018*1018*1018)`, 약 3.94TiB입니다. 모든 node block 위치는 NAT가 변환하므로 leaf data write가 상위 node 주소 갱신으로 전파되지 않습니다.

directory entry 하나는 11바이트이며 file name hash, inode number, name length, directory·symlink 등의 file type으로 구성됩니다. 4KiB dentry block에는 valid bitmap 27바이트, reserved 3바이트, 11바이트 dentry 214개, 8바이트 name slot 214개가 들어갑니다.

directory는 multi-level hash table을 사용합니다. level `n < MAX_DIR_HASH_DEPTH/2`이면 bucket당 2 block, 이후에는 4 block입니다. bucket 수는 전반부에서 `2^(n + dir_level)`, 후반부에서 `2^((MAX_DIR_HASH_DEPTH/2)-1)`입니다.

file 생성은 name을 담을 연속 empty slot을 level 1부터 N까지 lookup과 같은 방식으로 찾습니다. directory의 logical file size는 hole이 있어도 마지막 배치 위치까지 반영되므로 child 수와 같지 않을 수 있습니다.

Index Structure
---------------

The key data structure to manage the data locations is a "node". Similar to
traditional file structures, F2FS has three types of node: inode, direct node,
indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
indices, two direct node pointers, two indirect node pointers, and one double
indirect node pointer as described below. One direct node block contains 1018
data blocks, and one indirect node block contains also 1018 node blocks. Thus,
one inode block (i.e., a file) covers::

  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.

   Inode block (4KB)
     |- data (923)
     |- direct node (2)
     |          `- data (1018)
     |- indirect node (2)
     |            `- direct node (1018)
     |                       `- data (1018)
     `- double indirect node (1)
                         `- indirect node (1018)
                                      `- direct node (1018)
                                                 `- data (1018)

Note that all the node blocks are mapped by NAT which means the location of
each node is translated by the NAT table. In the consideration of the wandering
tree problem, F2FS is able to cut off the propagation of node updates caused by
leaf data writes.

Directory Structure
-------------------

A directory entry occupies 11 bytes, which consists of the following attributes.

- hash                hash value of the file name
- ino                inode number
- len                the length of file name
- type                file type such as directory, symlink, etc

A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
used to represent whether each dentry is valid or not. A dentry block occupies
4KB with the following composition.

::

  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
                      dentries(11 * 214 bytes) + file name (8 * 214 bytes)

                         [Bucket]
             +--------------------------------+
             |dentry block 1 | dentry block 2 |
             +--------------------------------+
             .               .
       .                             .
  .       [Dentry Block Structure: 4KB]       .
  +--------+----------+----------+------------+
  | bitmap | reserved | dentries | file names |
  +--------+----------+----------+------------+
  [Dentry Block: 4KB] .   .
                 .               .
            .                          .
            +------+------+-----+------+
            | hash | ino  | len | type |
            +------+------+-----+------+
            [Dentry Structure: 11 bytes]

F2FS implements multi-level hash tables for directory structure. Each level has
a hash table with dedicated number of hash buckets as shown below. Note that
"A(2B)" means a bucket includes 2 data blocks.

::

    ----------------------
    A : bucket
    B : block
    N : MAX_DIR_HASH_DEPTH
    ----------------------

    level #0   | A(2B)
            |
    level #1   | A(2B) - A(2B)
            |
    level #2   | A(2B) - A(2B) - A(2B) - A(2B)
        .     |   .       .       .       .
    level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
        .     |   .       .       .       .
    level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

The number of blocks and buckets are determined by::

                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
  # of blocks in level #n = |
                            `- 4, Otherwise

                             ,- 2^(n + dir_level),
                             |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
  # of buckets in level #n = |
                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
                                      Otherwise

When F2FS finds a file name in a directory, at first a hash value of the file
name is calculated. Then, F2FS scans the hash table in level #0 to find the
dentry consisting of the file name and its inode number. If not found, F2FS
scans the next hash table in level #1. In this way, F2FS scans hash tables in
each levels incrementally from 1 to N. In each level F2FS needs to scan only
one bucket determined by the following equation, which shows O(log(# of files))
complexity::

  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)

In the case of file creation, F2FS finds empty consecutive slots that cover the
file name. F2FS searches the empty slots in the hash tables of whole levels from
1 to N in the same way as the lookup operation.

The following figure shows an example of two cases holding children::

       --------------> Dir <--------------
       |                                 |
    child                             child

    child - child                     [hole] - child

    child - child - child             [hole] - [hole] - child

   Case 1:                           Case 2:
   Number of children = 6,           Number of children = 3,
   File size = 7                     File size = 7

Block allocation·cleaning·write hint·fallocate

runtime에는 Main area 안에 Hot/Warm/Cold node와 Hot/Warm/Cold data의 active log 여섯 개가 있습니다.

free-space 관리에서 copy-and-compaction(cleaning)은 sequential write가 빠른 device에 적합하지만 사용률이 높으면 cleaning overhead가 큽니다. threaded log는 random write가 생기지만 cleaning이 필요 없습니다. F2FS는 기본적으로 cleaning을 쓰다가 filesystem 상태에 따라 threaded log로 전환하는 hybrid policy를 사용합니다.

F2FS는 FTL GC unit과 맞추려고 section 단위로 segment를 할당하고, FTL mapping granularity를 고려해 active log section을 가능한 한 서로 다른 zone에서 할당합니다.

on-demand cleaner는 VFS call에 제공할 free segment가 부족할 때 실행되고 greedy algorithm으로 valid block이 가장 적은 victim을 고릅니다. idle 때 kernel thread가 실행하는 background cleaner는 age와 valid block 수를 함께 보는 cost-benefit algorithm으로 log block thrashing을 줄입니다. Main area 전체 block validity는 bitmap으로 추적합니다.

기본 `fallocate()` mode 0은 `offset..offset+len`에 disk space를 할당하고 필요하면 file size를 늘리며 기존 data가 없던 부분을 0으로 초기화합니다. 이는 `posix_fallocate(3)`를 효율적으로 구현하는 정책입니다.

먼저 `ioctl(fd, F2FS_IOC_SET_PIN_FILE)`을 호출하면 이후 `fallocate(fd, 0, 0, size)`가 0 또는 random data를 가진 on-disk block address를 고정 할당합니다. `fibmap()`으로 주소를 얻어 block device를 직접 여는 사용 시나리오에 유용합니다.

Default Block Allocation
------------------------

At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
and Hot/Warm/Cold data.

- Hot node        contains direct node blocks of directories.
- Warm node        contains direct node blocks except hot node blocks.
- Cold node        contains indirect node blocks
- Hot data        contains dentry blocks
- Warm data        contains data blocks except hot and cold data blocks
- Cold data        contains multimedia data or migrated data blocks

LFS has two schemes for free space management: threaded log and copy-and-compac-
tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
for devices showing very good sequential write performance, since free segments
are served all the time for writing new data. However, it suffers from cleaning
overhead under high utilization. Contrarily, the threaded log scheme suffers
from random writes, but no cleaning process is needed. F2FS adopts a hybrid
scheme where the copy-and-compaction scheme is adopted by default, but the
policy is dynamically changed to the threaded log scheme according to the file
system status.

In order to align F2FS with underlying flash-based storage, F2FS allocates a
segment in a unit of section. F2FS expects that the section size would be the
same as the unit size of garbage collection in FTL. Furthermore, with respect
to the mapping granularity in FTL, F2FS allocates each section of the active
logs from different zones as much as possible, since FTL can write the data in
the active logs into one allocation unit according to its mapping granularity.

Cleaning process
----------------

F2FS does cleaning both on demand and in the background. On-demand cleaning is
triggered when there are not enough free segments to serve VFS calls. Background
cleaner is operated by a kernel thread, and triggers the cleaning job when the
system is idle.

F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
In the greedy algorithm, F2FS selects a victim segment having the smallest number
of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
according to the segment age and the number of valid blocks in order to address
log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
algorithm.

In order to identify whether the data in the victim segment are valid or not,
F2FS manages a bitmap. Each bit represents the validity of a block, and the
bitmap is composed of a bit stream covering whole blocks in main area.

Write-hint Policy
-----------------

F2FS sets the whint all the time with the below policy.

===================== ======================== ===================
User                  F2FS                     Block
===================== ======================== ===================
N/A                   META                     WRITE_LIFE_NONE|REQ_META
N/A                   HOT_NODE                 WRITE_LIFE_NONE
N/A                   WARM_NODE                WRITE_LIFE_MEDIUM
N/A                   COLD_NODE                WRITE_LIFE_LONG
ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
extension list        "                        "

-- buffered io
------------------------------------------------------------------
N/A                   COLD_DATA                WRITE_LIFE_EXTREME
N/A                   HOT_DATA                 WRITE_LIFE_SHORT
N/A                   WARM_DATA                WRITE_LIFE_NOT_SET

-- direct io
------------------------------------------------------------------
WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
WRITE_LIFE_NONE       "                        WRITE_LIFE_NONE
WRITE_LIFE_MEDIUM     "                        WRITE_LIFE_MEDIUM
WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG
===================== ======================== ===================

Fallocate(2) Policy
-------------------

The default policy follows the below POSIX rule.

Allocating disk space
    The default operation (i.e., mode is zero) of fallocate() allocates
    the disk space within the range specified by offset and len.  The
    file size (as reported by stat(2)) will be changed if offset+len is
    greater than the file size.  Any subregion within the range specified
    by offset and len that did not contain data before the call will be
    initialized to zero.  This default behavior closely resembles the
    behavior of the posix_fallocate(3) library function, and is intended
    as a method of optimally implementing that function.

However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having
zero or random data, which is useful to the below scenario where:

 1. create(fd)
 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
 3. fallocate(fd, 0, 0, size)
 4. address = fibmap(fd, offset)
 5. open(blkdev)
 6. write(blkdev, address)

Compression 구현과 fs/user mode

compression 기본 단위인 cluster는 `4 << n`(`n >= 0`) logical page로 구성되며 file을 여러 cluster로 논리 분할합니다. cluster마다 독립적으로 압축 여부를 결정합니다.

특수 block address가 compressed cluster인지 normal cluster인지 표시합니다. compressed cluster의 metadata는 cluster를 `1 .. (4 << n)-1` physical block에 mapping하고 compress header와 compressed data를 저장합니다.

overwrite write amplification을 피하려고 write-once file만 압축하며, cluster의 모든 logical block에 valid data가 있고 압축률이 threshold보다 낮을 때만 압축합니다.

regular inode compression은 `chattr +c file`, 상위 directory의 `+c` 상속, `compress_extension=ext`, `compress_extension=*`로 켭니다. `chattr -c file` 또는 `nocompress_extension=ext`로 끕니다. 개별 file flag가 extension과 directory flag보다 우선합니다.

압축 공간은 향후 update 가능성을 보장하려고 기본적으로 user-visible free space로 돌려주지 않습니다. `F2FS_IOC_RELEASE_COMPRESS_BLOCKS`로 회수하면 inode flag가 write를 막으며, `F2FS_IOC_RESERVE_COMPRESS_BLOCKS`로 다시 예약하거나 file size를 0으로 truncate해야 write가 다시 가능합니다.

`compress_mode=fs`는 기본 mode로 writeback 때 자동 압축합니다. `compress_mode=user`는 자동 압축을 끄고 user가 `F2FS_IOC_DECOMPRESS_FILE`과 `F2FS_IOC_COMPRESS_FILE` ioctl로 대상과 시점을 정합니다.

Compression implementation
--------------------------

- New term named cluster is defined as basic unit of compression, file can
  be divided into multiple clusters logically. One cluster includes 4 << n
  (n >= 0) logical pages, compression size is also cluster size, each of
  cluster can be compressed or not.

- In cluster metadata layout, one special block address is used to indicate
  a cluster is a compressed one or normal one; for compressed cluster, following
  metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
  stores data including compress header and compressed data.

- In order to eliminate write amplification during overwrite, F2FS only
  support compression on write-once file, data can be compressed only when
  all logical blocks in cluster contain valid data and compress ratio of
  cluster data is lower than specified threshold.

- To enable compression on regular inode, there are four ways:

  * chattr +c file
  * chattr +c dir; touch dir/file
  * mount w/ -o compress_extension=ext; touch file.ext
  * mount w/ -o compress_extension=*; touch any_file

- To disable compression on regular inode, there are two ways:

  * chattr -c file
  * mount w/ -o nocompress_extension=ext; touch file.ext

- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:

  * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
    should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
    can enable compress on bar.zip.
  * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
    compresse, bar.zip and baz.txt should be non-compressed.
    chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
    and baz.txt.

- At this point, compression feature doesn't expose compressed space to user
  directly in order to guarantee potential data updates later to the space.
  Instead, the main goal is to reduce data writes to flash disk as much as
  possible, resulting in extending disk life time as well as relaxing IO
  congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
  interface to reclaim compressed space and show it to user after setting a
  special flag to the inode. Once the compressed space is released, the flag
  will block writing data to the file until either the compressed space is
  reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is
  truncated to zero.

Compress metadata layout::

                                [Dnode Structure]
                +-----------------------------------------------+
                | cluster 1 | cluster 2 | ......... | cluster N |
                +-----------------------------------------------+
                .           .                       .           .
          .                      .                .                      .
    .         Compressed Cluster       .        .        Normal Cluster            .
    +----------+---------+---------+---------+  +---------+---------+---------+---------+
    |compr flag| block 1 | block 2 | block 3 |  | block 1 | block 2 | block 3 | block 4 |
    +----------+---------+---------+---------+  +---------+---------+---------+---------+
               .                             .
            .                                           .
        .                                                           .
        +-------------+-------------+----------+----------------------------+
        | data length | data chksum | reserved |      compressed data       |
        +-------------+-------------+----------+----------------------------+

Compression mode
--------------------------

f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
With this option, f2fs provides a choice to select the way how to compress the
compression enabled files (refer to "Compression implementation" section for how to
enable compression on a regular inode).

1) compress_mode=fs

   This is the default option. f2fs does automatic compression in the writeback of the
   compression enabled files.

2) compress_mode=user

   This disables the automatic compression and gives the user discretion of choosing the
   target file and the timing. The user can do manual compression/decompression on the
   compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
   ioctls like the below.

To decompress a file::

  fd = open(filename, O_WRONLY, 0);
  ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);

To compress a file::

  fd = open(filename, O_WRONLY, 0);
  ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);

NVMe ZNS와 device aliasing

ZNS의 zone capacity는 zone size와 같거나 작으며 zone에서 사용할 수 있는 block 수를 뜻합니다. mount 시 capacity보다 뒤에서 시작하는 segment는 free bitmap에서 permanently used로 표시해 write allocation과 GC 대상에서 제외합니다.

zone capacity가 기본 2MiB segment size에 정렬되지 않으면 capacity 앞에서 시작해 경계를 가로지르는 segment도 usable합니다. 다만 그 segment 안에서도 capacity 뒤 block은 unusable입니다.

device aliasing file은 일반 F2FS node structure 없이 storage device 전체를 하나의 큰 extent로 mapping합니다. 이 영역은 pinned 상태로 공간을 보유하여 F2FS 영역 일부를 다른 filesystem이나 용도로 임시 예약할 수 있습니다.

핵심은 partition size나 filesystem format을 바꾸지 않고 `/dev/vdc`에서 임의 file operation을 수행하면서 그 공간은 F2FS의 `/data` 사용량으로 계산하고, 사용 후 alias file 삭제로 회수하는 것입니다.

NVMe Zoned Namespace devices
----------------------------

- ZNS defines a per-zone capacity which can be equal or less than the
  zone-size. Zone-capacity is the number of usable blocks in the zone.
  F2FS checks if zone-capacity is less than zone-size, if it is, then any
  segment which starts after the zone-capacity is marked as not-free in
  the free segment bitmap at initial mount time. These segments are marked
  as permanently used so they are not allocated for writes and
  consequently are not needed to be garbage collected. In case the
  zone-capacity is not aligned to default segment size(2MB), then a segment
  can start before the zone-capacity and span across zone-capacity boundary.
  Such spanning segments are also considered as usable segments. All blocks
  past the zone-capacity are considered unusable in these segments.

Device aliasing feature
-----------------------

f2fs can utilize a special file called a "device aliasing file." This file allows
the entire storage device to be mapped with a single, large extent, not using
the usual f2fs node structures. This mapped area is pinned and primarily intended
for holding the space.

Essentially, this mechanism allows a portion of the f2fs area to be temporarily
reserved and used by another filesystem or for different purposes. Once that
external usage is complete, the device aliasing file can be deleted, releasing
the reserved space back to F2FS for its own use.

.. code-block::

   # ls /dev/vd*
   /dev/vdb (32GB) /dev/vdc (32GB)
   # mkfs.ext4 /dev/vdc
   # mkfs.f2fs -c /dev/[email protected] /dev/vdb
   # mount /dev/vdb /mnt/f2fs
   # ls -l /mnt/f2fs
   vdc.file
   # df -h
   /dev/vdb                            64G   33G   32G  52% /mnt/f2fs

   # mount -o loop /dev/vdc /mnt/ext4
   # df -h
   /dev/vdb                            64G   33G   32G  52% /mnt/f2fs
   /dev/loop7                          32G   24K   30G   1% /mnt/ext4
   # umount /mnt/ext4

   # f2fs_io getflags /mnt/f2fs/vdc.file
   get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable
   # f2fs_io setflags noimmutable /mnt/f2fs/vdc.file
   get a flag on noimmutable ret=0, flags=800010
   set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable
   # rm /mnt/f2fs/vdc.file
   # df -h
   /dev/vdb                            64G  753M   64G   2% /mnt/f2fs

So, the key idea is, user can do any file operations on /dev/vdc, and
reclaim the space after the use, while the space is counted as /data.
That doesn't require modifying partition size and filesystem format.