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1 .. SPDX-License-Identifier: GPL-2.0
4 WHAT IS Flash-Friendly File System (F2FS)?
7 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
13 F2FS is a file system exploiting NAND flash memory-based storage devices, which
14 is based on Log-structured File System (LFS). The design has been focused on
18 Since a NAND flash memory-based storage device shows different characteristic
20 F2FS and its tools support various parameters not only for configuring on-disk
23 The following git tree provides the file system formatting tool (mkfs.f2fs),
26 - git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
30 - linux-f2fs-devel@lists.sourceforge.net
34 - https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs
39 Log-structured File System (LFS)
40 --------------------------------
41 "A log-structured file system writes all modifications to disk sequentially in
42 a log-like structure, thereby speeding up both file writing and crash recovery.
48 implementation of a log-structured file system", ACM Trans. Computer Systems
52 ----------------------
61 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
64 -----------------
65 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
75 3. It checks the cross-reference between the data and its parent index structure.
86 ---------------
87 - Enlarge the random write area for better performance, but provide the high
89 - Align FS data structures to the operational units in FTL as best efforts
92 ----------------------
93 - Use a term, “node”, that represents inodes as well as various pointer blocks
94 - Introduce Node Address Table (NAT) containing the locations of all the “node”
98 -----------------
99 - Support a background cleaning process
100 - Support greedy and cost-benefit algorithms for victim selection policies
101 - Support multi-head logs for static/dynamic hot and cold data separation
102 - Introduce adaptive logging for efficient block allocation
123 disable_roll_forward Disable the roll-forward recovery routine
124 norecovery Disable the roll-forward recovery routine, mounted read-
125 only (i.e., -o ro,disable_roll_forward)
126 discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
129 no_heap Disable heap-style segment allocation which finds free
166 extent_cache Enable an extent cache based on rb-tree, it can cache
169 increasing the cache hit ratio. Set by default.
170 noextent_cache Disable an extent cache based on rb-tree explicitly, see
215 fragmentation/after-GC situation itself. The developers use these
216 modes to understand filesystem fragmentation/after-GC condition well,
219 position. With this, we can simulate the after-GC condition.
230 recommend to re-format the filesystem after using these options.
255 non-atomic files likewise "nobarrier" mount option.
290 "lz4", "zstd" and "lzo-rle" algorithm.
294 lz4 3 - 16
295 zstd 1 - 22
327 cache compressed block, in order to improve cache hit ratio of
330 files using the blk-crypto framework rather than
331 filesystem-layer encryption. This allows the use of
332 inline encryption hardware. The on-disk format is
334 Documentation/block/inline-encryption.rst.
335 atgc Enable age-threshold garbage collection, it provides high
350 age_extent_cache Enable an age extent cache based on rb-tree. It records
355 "panic", "continue" and "remount-ro", respectively, trigger
357 the partition in read-only mode. By default it uses "continue"
360 mode continue remount-ro panic
363 syscall errors -EIO -EROFS N/A
366 pending non-dir write drop keep N/A
380 - major file system information managed by f2fs currently
381 - average SIT information about whole segments
382 - current memory footprint consumed by f2fs.
390 The files in each per-device directory are shown in table below.
393 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
398 1. Download userland tools and compile them.
411 # mkfs.f2fs -l label /dev/block_device
412 # mount -t f2fs /dev/block_device /mnt/f2fs
415 ---------
417 which builds a basic on-disk layout.
422 ``-l [label]`` Give a volume label, up to 512 unicode name.
423 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
426 ``-o [int]`` Set overprovision ratio in percent over volume size.
429 ``-s [int]`` Set the number of segments per section.
432 ``-z [int]`` Set the number of sections per zone.
435 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
436 ``-t [0 or 1]`` Disable discard command or not.
444 ---------
445 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
446 partition, which examines whether the filesystem metadata and user-made data
447 are cross-referenced correctly or not.
452 -d debug level [default:0]
457 ---------
461 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
462 It shows on-disk inode information recognized by a given inode number, and is
468 -d debug level [default:0]
469 -i inode no (hex)
470 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
471 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
475 # dump.f2fs -i [ino] /dev/sdx
476 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
477 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
482 ----------
489 -----------
490 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
496 -----------
504 -------
506 f2fs-specific ones, which is very useful for QA tests.
513 On-disk Layout
514 --------------
524 align with the zone size <-|
525 |-> align with the segment size
543 - Superblock (SB)
548 - Checkpoint (CP)
552 - Segment Information Table (SIT)
556 - Node Address Table (NAT)
560 - Segment Summary Area (SSA)
564 - Main Area
567 In order to avoid misalignment between file system and flash-based storage, F2FS
576 ------------------------------
587 +--------+----------+---------+
589 +--------+----------+---------+
593 +-------+-------+--------+--------+--------+--------+
595 +-------+-------+--------+--------+--------+--------+
598 `----------------------------------------'
601 ---------------
614 |- data (923)
615 |- direct node (2)
616 | `- data (1018)
617 |- indirect node (2)
618 | `- direct node (1018)
619 | `- data (1018)
620 `- double indirect node (1)
621 `- indirect node (1018)
622 `- direct node (1018)
623 `- data (1018)
631 -------------------
635 - hash hash value of the file name
636 - ino inode number
637 - len the length of file name
638 - type file type such as directory, symlink, etc
650 +--------------------------------+
652 +--------------------------------+
656 +--------+----------+----------+------------+
658 +--------+----------+----------+------------+
662 +------+------+-----+------+
664 +------+------+-----+------+
667 F2FS implements multi-level hash tables for directory structure. Each level has
673 ----------------------
677 ----------------------
681 level #1 | A(2B) - A(2B)
683 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
685 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
687 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
691 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
693 `- 4, Otherwise
695 ,- 2^(n + dir_level),
698 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
717 --------------> Dir <--------------
721 child - child [hole] - child
723 child - child - child [hole] - [hole] - child
730 ------------------------
735 - Hot node contains direct node blocks of directories.
736 - Warm node contains direct node blocks except hot node blocks.
737 - Cold node contains indirect node blocks
738 - Hot data contains dentry blocks
739 - Warm data contains data blocks except hot and cold data blocks
740 - Cold data contains multimedia data or migrated data blocks
742 LFS has two schemes for free space management: threaded log and copy-and-compac-
743 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
748 scheme where the copy-and-compaction scheme is adopted by default, but the
752 In order to align F2FS with underlying flash-based storage, F2FS allocates a
760 ----------------
762 F2FS does cleaning both on demand and in the background. On-demand cleaning is
767 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
769 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
772 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
780 -------------------
795 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having
806 --------------------------
808 - New term named cluster is defined as basic unit of compression, file can
813 - In cluster metadata layout, one special block address is used to indicate
815 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
818 - In order to eliminate write amplification during overwrite, F2FS only
819 support compression on write-once file, data can be compressed only when
823 - To enable compression on regular inode, there are four ways:
827 * mount w/ -o compress_extension=ext; touch file.ext
828 * mount w/ -o compress_extension=*; touch any_file
830 - To disable compression on regular inode, there are two ways:
832 * chattr -c file
833 * mount w/ -o nocompress_extension=ext; touch file.ext
835 - Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
839 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
841 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
843 compresse, bar.zip and baz.txt should be non-compressed.
847 - At this point, compression feature doesn't expose compressed space to user
861 +-----------------------------------------------+
863 +-----------------------------------------------+
867 +----------+---------+---------+---------+ +---------+---------+---------+---------+
869 +----------+---------+---------+---------+ +---------+---------+---------+---------+
873 +-------------+-------------+----------+----------------------------+
875 +-------------+-------------+----------+----------------------------+
878 --------------------------
906 ----------------------------
908 - ZNS defines a per-zone capacity which can be equal or less than the
909 zone-size. Zone-capacity is the number of usable blocks in the zone.
910 F2FS checks if zone-capacity is less than zone-size, if it is, then any
911 segment which starts after the zone-capacity is marked as not-free in
915 zone-capacity is not aligned to default segment size(2MB), then a segment
916 can start before the zone-capacity and span across zone-capacity boundary.
918 past the zone-capacity are considered unusable in these segments.