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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
35 Log-structured File System (LFS)
36 --------------------------------
37 "A log-structured file system writes all modifications to disk sequentially in
38 a log-like structure, thereby speeding up both file writing and crash recovery.
44 implementation of a log-structured file system", ACM Trans. Computer Systems
48 ----------------------
57 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
60 -----------------
61 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
71 3. It checks the cross-reference between the data and its parent index structure.
82 ---------------
83 - Enlarge the random write area for better performance, but provide the high
85 - Align FS data structures to the operational units in FTL as best efforts
88 ----------------------
89 - Use a term, “node”, that represents inodes as well as various pointer blocks
90 - Introduce Node Address Table (NAT) containing the locations of all the “node”
94 -----------------
95 - Support a background cleaning process
96 - Support greedy and cost-benefit algorithms for victim selection policies
97 - Support multi-head logs for static/dynamic hot and cold data separation
98 - Introduce adaptive logging for efficient block allocation
119 disable_roll_forward Disable the roll-forward recovery routine
120 norecovery Disable the roll-forward recovery routine, mounted read-
121 only (i.e., -o ro,disable_roll_forward)
122 discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
125 no_heap Disable heap-style segment allocation which finds free
160 extent_cache Enable an extent cache based on rb-tree, it can cache
163 increasing the cache hit ratio. Set by default.
164 noextent_cache Disable an extent cache based on rb-tree explicitly, see
218 layer. This supports "off", "user-based", and
219 "fs-based". In "off" mode (default), f2fs does not pass
220 down hints. In "user-based" mode, f2fs tries to pass
221 down hints given by users. And in "fs-based" mode, f2fs
233 non-atomic files likewise "nobarrier" mount option.
257 "lz4", "zstd" and "lzo-rle" algorithm.
270 files using the blk-crypto framework rather than
271 filesystem-layer encryption. This allows the use of
272 inline encryption hardware. The on-disk format is
274 Documentation/block/inline-encryption.rst.
275 atgc Enable age-threshold garbage collection, it provides high
287 - major file system information managed by f2fs currently
288 - average SIT information about whole segments
289 - current memory footprint consumed by f2fs.
297 The files in each per-device directory are shown in table below.
300 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
305 1. Download userland tools and compile them.
318 # mkfs.f2fs -l label /dev/block_device
319 # mount -t f2fs /dev/block_device /mnt/f2fs
322 ---------
324 which builds a basic on-disk layout.
329 ``-l [label]`` Give a volume label, up to 512 unicode name.
330 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
333 ``-o [int]`` Set overprovision ratio in percent over volume size.
336 ``-s [int]`` Set the number of segments per section.
339 ``-z [int]`` Set the number of sections per zone.
342 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
343 ``-t [0 or 1]`` Disable discard command or not.
351 ---------
352 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
353 partition, which examines whether the filesystem metadata and user-made data
354 are cross-referenced correctly or not.
359 -d debug level [default:0]
364 ---------
368 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
369 It shows on-disk inode information recognized by a given inode number, and is
375 -d debug level [default:0]
376 -i inode no (hex)
377 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
378 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
382 # dump.f2fs -i [ino] /dev/sdx
383 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
384 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
389 ----------
396 -----------
397 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
403 -----------
411 -------
413 f2fs-specific ones, which is very useful for QA tests.
420 On-disk Layout
421 --------------
431 align with the zone size <-|
432 |-> align with the segment size
450 - Superblock (SB)
455 - Checkpoint (CP)
459 - Segment Information Table (SIT)
463 - Node Address Table (NAT)
467 - Segment Summary Area (SSA)
471 - Main Area
474 In order to avoid misalignment between file system and flash-based storage, F2FS
483 ------------------------------
494 +--------+----------+---------+
496 +--------+----------+---------+
500 +-------+-------+--------+--------+--------+--------+
502 +-------+-------+--------+--------+--------+--------+
505 `----------------------------------------'
508 ---------------
521 |- data (923)
522 |- direct node (2)
523 | `- data (1018)
524 |- indirect node (2)
525 | `- direct node (1018)
526 | `- data (1018)
527 `- double indirect node (1)
528 `- indirect node (1018)
529 `- direct node (1018)
530 `- data (1018)
538 -------------------
542 - hash hash value of the file name
543 - ino inode number
544 - len the length of file name
545 - type file type such as directory, symlink, etc
557 +--------------------------------+
559 +--------------------------------+
563 +--------+----------+----------+------------+
565 +--------+----------+----------+------------+
569 +------+------+-----+------+
571 +------+------+-----+------+
574 F2FS implements multi-level hash tables for directory structure. Each level has
580 ----------------------
584 ----------------------
588 level #1 | A(2B) - A(2B)
590 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
592 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
594 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
598 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
600 `- 4, Otherwise
602 ,- 2^(n + dir_level),
605 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
624 --------------> Dir <--------------
628 child - child [hole] - child
630 child - child - child [hole] - [hole] - child
637 ------------------------
642 - Hot node contains direct node blocks of directories.
643 - Warm node contains direct node blocks except hot node blocks.
644 - Cold node contains indirect node blocks
645 - Hot data contains dentry blocks
646 - Warm data contains data blocks except hot and cold data blocks
647 - Cold data contains multimedia data or migrated data blocks
649 LFS has two schemes for free space management: threaded log and copy-and-compac-
650 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
655 scheme where the copy-and-compaction scheme is adopted by default, but the
659 In order to align F2FS with underlying flash-based storage, F2FS allocates a
667 ----------------
669 F2FS does cleaning both on demand and in the background. On-demand cleaning is
674 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
676 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
679 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
686 Write-hint Policy
687 -----------------
691 2) whint_mode=user-based. F2FS tries to pass down hints given by
704 -- buffered io
712 -- direct io
721 3) whint_mode=fs-based. F2FS passes down hints with its policy.
733 -- buffered io
741 -- direct io
751 -------------------
766 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
777 --------------------------
779 - New term named cluster is defined as basic unit of compression, file can
784 - In cluster metadata layout, one special block address is used to indicate
786 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
789 - In order to eliminate write amplification during overwrite, F2FS only
790 support compression on write-once file, data can be compressed only when
794 - To enable compression on regular inode, there are three ways:
798 * mount w/ -o compress_extension=ext; touch file.ext
803 +-----------------------------------------------+
805 +-----------------------------------------------+
809 +----------+---------+---------+---------+ +---------+---------+---------+---------+
811 +----------+---------+---------+---------+ +---------+---------+---------+---------+
815 +-------------+-------------+----------+----------------------------+
817 +-------------+-------------+----------+----------------------------+
820 ----------------------------
822 - ZNS defines a per-zone capacity which can be equal or less than the
823 zone-size. Zone-capacity is the number of usable blocks in the zone.
824 F2FS checks if zone-capacity is less than zone-size, if it is, then any
825 segment which starts after the zone-capacity is marked as not-free in
829 zone-capacity is not aligned to default segment size(2MB), then a segment
830 can start before the zone-capacity and span across zone-capacity boundary.
832 past the zone-capacity are considered unusable in these segments.