1 .. SPDX-License-Identifier: GPL-2.0
3 ==========================================
4 WHAT IS Flash-Friendly File System (F2FS)?
5 ==========================================
7 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
8 been equipped on a variety systems ranging from mobile to server systems. Since
9 they are known to have different characteristics from the conventional rotating
10 disks, a file system, an upper layer to the storage device, should adapt to the
11 changes from the sketch in the design level.
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
15 addressing the fundamental issues in LFS, which are snowball effect of wandering
16 tree and high cleaning overhead.
18 Since a NAND flash memory-based storage device shows different characteristic
19 according to its internal geometry or flash memory management scheme, namely FTL,
20 F2FS and its tools support various parameters not only for configuring on-disk
21 layout, but also for selecting allocation and cleaning algorithms.
23 The following git tree provides the file system formatting tool (mkfs.f2fs),
24 a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
26 - git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
28 For reporting bugs and sending patches, please use the following mailing list:
30 - linux-f2fs-devel@lists.sourceforge.net
32 Background and Design issues
33 ============================
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.
39 The log is the only structure on disk; it contains indexing information so that
40 files can be read back from the log efficiently. In order to maintain large free
41 areas on disk for fast writing, we divide the log into segments and use a
42 segment cleaner to compress the live information from heavily fragmented
43 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
44 implementation of a log-structured file system", ACM Trans. Computer Systems
47 Wandering Tree Problem
48 ----------------------
49 In LFS, when a file data is updated and written to the end of log, its direct
50 pointer block is updated due to the changed location. Then the indirect pointer
51 block is also updated due to the direct pointer block update. In this manner,
52 the upper index structures such as inode, inode map, and checkpoint block are
53 also updated recursively. This problem is called as wandering tree problem [1],
54 and in order to enhance the performance, it should eliminate or relax the update
55 propagation as much as possible.
57 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
61 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
62 scattered across the whole storage. In order to serve new empty log space, it
63 needs to reclaim these obsolete blocks seamlessly to users. This job is called
64 as a cleaning process.
66 The process consists of three operations as follows.
68 1. A victim segment is selected through referencing segment usage table.
69 2. It loads parent index structures of all the data in the victim identified by
70 segment summary blocks.
71 3. It checks the cross-reference between the data and its parent index structure.
72 4. It moves valid data selectively.
74 This cleaning job may cause unexpected long delays, so the most important goal
75 is to hide the latencies to users. And also definitely, it should reduce the
76 amount of valid data to be moved, and move them quickly as well.
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
87 Wandering Tree Problem
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”
91 blocks; this will cut off the update propagation.
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
104 ======================== ============================================================
105 background_gc=%s Turn on/off cleaning operations, namely garbage
106 collection, triggered in background when I/O subsystem is
107 idle. If background_gc=on, it will turn on the garbage
108 collection and if background_gc=off, garbage collection
109 will be turned off. If background_gc=sync, it will turn
110 on synchronous garbage collection running in background.
111 Default value for this option is on. So garbage
112 collection is on by default.
113 gc_merge When background_gc is on, this option can be enabled to
114 let background GC thread to handle foreground GC requests,
115 it can eliminate the sluggish issue caused by slow foreground
116 GC operation when GC is triggered from a process with limited
117 I/O and CPU resources.
118 nogc_merge Disable GC merge feature.
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
123 enabled, f2fs will issue discard/TRIM commands when a
125 no_heap Disable heap-style segment allocation which finds free
126 segments for data from the beginning of main area, while
127 for node from the end of main area.
128 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
129 by default if CONFIG_F2FS_FS_XATTR is selected.
130 noacl Disable POSIX Access Control List. Note: acl is enabled
131 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
132 active_logs=%u Support configuring the number of active logs. In the
133 current design, f2fs supports only 2, 4, and 6 logs.
135 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
136 is not aware of cold files such as media files.
137 inline_xattr Enable the inline xattrs feature.
138 noinline_xattr Disable the inline xattrs feature.
139 inline_xattr_size=%u Support configuring inline xattr size, it depends on
140 flexible inline xattr feature.
141 inline_data Enable the inline data feature: Newly created small (<~3.4k)
142 files can be written into inode block.
143 inline_dentry Enable the inline dir feature: data in newly created
144 directory entries can be written into inode block. The
145 space of inode block which is used to store inline
146 dentries is limited to ~3.4k.
147 noinline_dentry Disable the inline dentry feature.
148 flush_merge Merge concurrent cache_flush commands as much as possible
149 to eliminate redundant command issues. If the underlying
150 device handles the cache_flush command relatively slowly,
151 recommend to enable this option.
152 nobarrier This option can be used if underlying storage guarantees
153 its cached data should be written to the novolatile area.
154 If this option is set, no cache_flush commands are issued
155 but f2fs still guarantees the write ordering of all the
157 fastboot This option is used when a system wants to reduce mount
158 time as much as possible, even though normal performance
160 extent_cache Enable an extent cache based on rb-tree, it can cache
161 as many as extent which map between contiguous logical
162 address and physical address per inode, resulting in
163 increasing the cache hit ratio. Set by default.
164 noextent_cache Disable an extent cache based on rb-tree explicitly, see
165 the above extent_cache mount option.
166 noinline_data Disable the inline data feature, inline data feature is
168 data_flush Enable data flushing before checkpoint in order to
169 persist data of regular and symlink.
170 reserve_root=%d Support configuring reserved space which is used for
171 allocation from a privileged user with specified uid or
172 gid, unit: 4KB, the default limit is 0.2% of user blocks.
173 resuid=%d The user ID which may use the reserved blocks.
174 resgid=%d The group ID which may use the reserved blocks.
175 fault_injection=%d Enable fault injection in all supported types with
176 specified injection rate.
177 fault_type=%d Support configuring fault injection type, should be
178 enabled with fault_injection option, fault type value
179 is shown below, it supports single or combined type.
181 =================== ===========
183 =================== ===========
184 FAULT_KMALLOC 0x000000001
185 FAULT_KVMALLOC 0x000000002
186 FAULT_PAGE_ALLOC 0x000000004
187 FAULT_PAGE_GET 0x000000008
188 FAULT_ALLOC_BIO 0x000000010 (obsolete)
189 FAULT_ALLOC_NID 0x000000020
190 FAULT_ORPHAN 0x000000040
191 FAULT_BLOCK 0x000000080
192 FAULT_DIR_DEPTH 0x000000100
193 FAULT_EVICT_INODE 0x000000200
194 FAULT_TRUNCATE 0x000000400
195 FAULT_READ_IO 0x000000800
196 FAULT_CHECKPOINT 0x000001000
197 FAULT_DISCARD 0x000002000
198 FAULT_WRITE_IO 0x000004000
199 FAULT_SLAB_ALLOC 0x000008000
200 =================== ===========
201 mode=%s Control block allocation mode which supports "adaptive"
202 and "lfs". In "lfs" mode, there should be no random
203 writes towards main area.
204 io_bits=%u Set the bit size of write IO requests. It should be set
206 usrquota Enable plain user disk quota accounting.
207 grpquota Enable plain group disk quota accounting.
208 prjquota Enable plain project quota accounting.
209 usrjquota=<file> Appoint specified file and type during mount, so that quota
210 grpjquota=<file> information can be properly updated during recovery flow,
211 prjjquota=<file> <quota file>: must be in root directory;
212 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
213 offusrjquota Turn off user journalled quota.
214 offgrpjquota Turn off group journalled quota.
215 offprjjquota Turn off project journalled quota.
216 quota Enable plain user disk quota accounting.
217 noquota Disable all plain disk quota option.
218 whint_mode=%s Control which write hints are passed down to block
219 layer. This supports "off", "user-based", and
220 "fs-based". In "off" mode (default), f2fs does not pass
221 down hints. In "user-based" mode, f2fs tries to pass
222 down hints given by users. And in "fs-based" mode, f2fs
223 passes down hints with its policy.
224 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
226 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
227 "strict", and "nobarrier". In "posix" mode, which is
228 default, fsync will follow POSIX semantics and does a
229 light operation to improve the filesystem performance.
230 In "strict" mode, fsync will be heavy and behaves in line
231 with xfs, ext4 and btrfs, where xfstest generic/342 will
232 pass, but the performance will regress. "nobarrier" is
233 based on "posix", but doesn't issue flush command for
234 non-atomic files likewise "nobarrier" mount option.
235 test_dummy_encryption
236 test_dummy_encryption=%s
237 Enable dummy encryption, which provides a fake fscrypt
238 context. The fake fscrypt context is used by xfstests.
239 The argument may be either "v1" or "v2", in order to
240 select the corresponding fscrypt policy version.
241 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
242 to reenable checkpointing. Is enabled by default. While
243 disabled, any unmounting or unexpected shutdowns will cause
244 the filesystem contents to appear as they did when the
245 filesystem was mounted with that option.
246 While mounting with checkpoint=disabled, the filesystem must
247 run garbage collection to ensure that all available space can
248 be used. If this takes too much time, the mount may return
249 EAGAIN. You may optionally add a value to indicate how much
250 of the disk you would be willing to temporarily give up to
251 avoid additional garbage collection. This can be given as a
252 number of blocks, or as a percent. For instance, mounting
253 with checkpoint=disable:100% would always succeed, but it may
254 hide up to all remaining free space. The actual space that
255 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
256 This space is reclaimed once checkpoint=enable.
257 checkpoint_merge When checkpoint is enabled, this can be used to create a kernel
258 daemon and make it to merge concurrent checkpoint requests as
259 much as possible to eliminate redundant checkpoint issues. Plus,
260 we can eliminate the sluggish issue caused by slow checkpoint
261 operation when the checkpoint is done in a process context in
262 a cgroup having low i/o budget and cpu shares. To make this
263 do better, we set the default i/o priority of the kernel daemon
264 to "3", to give one higher priority than other kernel threads.
265 This is the same way to give a I/O priority to the jbd2
266 journaling thread of ext4 filesystem.
267 nocheckpoint_merge Disable checkpoint merge feature.
268 compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo",
269 "lz4", "zstd" and "lzo-rle" algorithm.
270 compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
271 "lz4" and "zstd" support compress level config.
272 algorithm level range
275 compress_log_size=%u Support configuring compress cluster size, the size will
276 be 4KB * (1 << %u), 16KB is minimum size, also it's
278 compress_extension=%s Support adding specified extension, so that f2fs can enable
279 compression on those corresponding files, e.g. if all files
280 with '.ext' has high compression rate, we can set the '.ext'
281 on compression extension list and enable compression on
282 these file by default rather than to enable it via ioctl.
283 For other files, we can still enable compression via ioctl.
284 Note that, there is one reserved special extension '*', it
285 can be set to enable compression for all files.
286 nocompress_extension=%s Support adding specified extension, so that f2fs can disable
287 compression on those corresponding files, just contrary to compression extension.
288 If you know exactly which files cannot be compressed, you can use this.
289 The same extension name can't appear in both compress and nocompress
290 extension at the same time.
291 If the compress extension specifies all files, the types specified by the
292 nocompress extension will be treated as special cases and will not be compressed.
293 Don't allow use '*' to specifie all file in nocompress extension.
294 After add nocompress_extension, the priority should be:
295 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
296 See more in compression sections.
298 compress_chksum Support verifying chksum of raw data in compressed cluster.
299 compress_mode=%s Control file compression mode. This supports "fs" and "user"
300 modes. In "fs" mode (default), f2fs does automatic compression
301 on the compression enabled files. In "user" mode, f2fs disables
302 the automaic compression and gives the user discretion of
303 choosing the target file and the timing. The user can do manual
304 compression/decompression on the compression enabled files using
306 compress_cache Support to use address space of a filesystem managed inode to
307 cache compressed block, in order to improve cache hit ratio of
309 inlinecrypt When possible, encrypt/decrypt the contents of encrypted
310 files using the blk-crypto framework rather than
311 filesystem-layer encryption. This allows the use of
312 inline encryption hardware. The on-disk format is
313 unaffected. For more details, see
314 Documentation/block/inline-encryption.rst.
315 atgc Enable age-threshold garbage collection, it provides high
316 effectiveness and efficiency on background GC.
317 discard_unit=%s Control discard unit, the argument can be "block", "segment"
318 and "section", issued discard command's offset/size will be
319 aligned to the unit, by default, "discard_unit=block" is set,
320 so that small discard functionality is enabled.
321 For blkzoned device, "discard_unit=section" will be set by
322 default, it is helpful for large sized SMR or ZNS devices to
323 reduce memory cost by getting rid of fs metadata supports small
325 ======================== ============================================================
330 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
331 f2fs. Each file shows the whole f2fs information.
333 /sys/kernel/debug/f2fs/status includes:
335 - major file system information managed by f2fs currently
336 - average SIT information about whole segments
337 - current memory footprint consumed by f2fs.
342 Information about mounted f2fs file systems can be found in
343 /sys/fs/f2fs. Each mounted filesystem will have a directory in
344 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
345 The files in each per-device directory are shown in table below.
347 Files in /sys/fs/f2fs/<devname>
348 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
353 1. Download userland tools and compile them.
355 2. Skip, if f2fs was compiled statically inside kernel.
356 Otherwise, insert the f2fs.ko module::
360 3. Create a directory to use when mounting::
364 4. Format the block device, and then mount as f2fs::
366 # mkfs.f2fs -l label /dev/block_device
367 # mount -t f2fs /dev/block_device /mnt/f2fs
371 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
372 which builds a basic on-disk layout.
374 The quick options consist of:
376 =============== ===========================================================
377 ``-l [label]`` Give a volume label, up to 512 unicode name.
378 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
380 1 is set by default, which performs this.
381 ``-o [int]`` Set overprovision ratio in percent over volume size.
384 ``-s [int]`` Set the number of segments per section.
387 ``-z [int]`` Set the number of sections per zone.
390 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
391 ``-t [0 or 1]`` Disable discard command or not.
393 1 is set by default, which conducts discard.
394 =============== ===========================================================
396 Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
400 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
401 partition, which examines whether the filesystem metadata and user-made data
402 are cross-referenced correctly or not.
403 Note that, initial version of the tool does not fix any inconsistency.
405 The quick options consist of::
407 -d debug level [default:0]
409 Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
413 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
414 file. Each file is dump_ssa and dump_sit.
416 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
417 It shows on-disk inode information recognized by a given inode number, and is
418 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
419 ./dump_sit respectively.
421 The options consist of::
423 -d debug level [default:0]
425 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
426 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
430 # dump.f2fs -i [ino] /dev/sdx
431 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
432 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
434 Note: please refer to the manpage of dump.f2fs(8) to get full option list.
438 The sload.f2fs gives a way to insert files and directories in the exisiting disk
439 image. This tool is useful when building f2fs images given compiled files.
441 Note: please refer to the manpage of sload.f2fs(8) to get full option list.
445 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
446 all the files and directories stored in the image.
448 Note: please refer to the manpage of resize.f2fs(8) to get full option list.
452 The defrag.f2fs can be used to defragment scattered written data as well as
453 filesystem metadata across the disk. This can improve the write speed by giving
454 more free consecutive space.
456 Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
460 The f2fs_io is a simple tool to issue various filesystem APIs as well as
461 f2fs-specific ones, which is very useful for QA tests.
463 Note: please refer to the manpage of f2fs_io(8) to get full option list.
471 F2FS divides the whole volume into a number of segments, each of which is fixed
472 to 2MB in size. A section is composed of consecutive segments, and a zone
473 consists of a set of sections. By default, section and zone sizes are set to one
474 segment size identically, but users can easily modify the sizes by mkfs.
476 F2FS splits the entire volume into six areas, and all the areas except superblock
477 consist of multiple segments as described below::
479 align with the zone size <-|
480 |-> align with the segment size
481 _________________________________________________________________________
482 | | | Segment | Node | Segment | |
483 | Superblock | Checkpoint | Info. | Address | Summary | Main |
484 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
485 |____________|_____2______|______N______|______N______|______N_____|__N___|
489 ._________________________________________.
490 |_Segment_|_..._|_Segment_|_..._|_Segment_|
499 It is located at the beginning of the partition, and there exist two copies
500 to avoid file system crash. It contains basic partition information and some
501 default parameters of f2fs.
504 It contains file system information, bitmaps for valid NAT/SIT sets, orphan
505 inode lists, and summary entries of current active segments.
507 - Segment Information Table (SIT)
508 It contains segment information such as valid block count and bitmap for the
509 validity of all the blocks.
511 - Node Address Table (NAT)
512 It is composed of a block address table for all the node blocks stored in
515 - Segment Summary Area (SSA)
516 It contains summary entries which contains the owner information of all the
517 data and node blocks stored in Main area.
520 It contains file and directory data including their indices.
522 In order to avoid misalignment between file system and flash-based storage, F2FS
523 aligns the start block address of CP with the segment size. Also, it aligns the
524 start block address of Main area with the zone size by reserving some segments
527 Reference the following survey for additional technical details.
528 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
530 File System Metadata Structure
531 ------------------------------
533 F2FS adopts the checkpointing scheme to maintain file system consistency. At
534 mount time, F2FS first tries to find the last valid checkpoint data by scanning
535 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
536 One of them always indicates the last valid data, which is called as shadow copy
537 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
539 For file system consistency, each CP points to which NAT and SIT copies are
540 valid, as shown as below::
542 +--------+----------+---------+
544 +--------+----------+---------+
548 +-------+-------+--------+--------+--------+--------+
549 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
550 +-------+-------+--------+--------+--------+--------+
553 `----------------------------------------'
558 The key data structure to manage the data locations is a "node". Similar to
559 traditional file structures, F2FS has three types of node: inode, direct node,
560 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
561 indices, two direct node pointers, two indirect node pointers, and one double
562 indirect node pointer as described below. One direct node block contains 1018
563 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
564 one inode block (i.e., a file) covers::
566 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
573 | `- direct node (1018)
575 `- double indirect node (1)
576 `- indirect node (1018)
577 `- direct node (1018)
580 Note that all the node blocks are mapped by NAT which means the location of
581 each node is translated by the NAT table. In the consideration of the wandering
582 tree problem, F2FS is able to cut off the propagation of node updates caused by
588 A directory entry occupies 11 bytes, which consists of the following attributes.
590 - hash hash value of the file name
592 - len the length of file name
593 - type file type such as directory, symlink, etc
595 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
596 used to represent whether each dentry is valid or not. A dentry block occupies
597 4KB with the following composition.
601 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
602 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
605 +--------------------------------+
606 |dentry block 1 | dentry block 2 |
607 +--------------------------------+
610 . [Dentry Block Structure: 4KB] .
611 +--------+----------+----------+------------+
612 | bitmap | reserved | dentries | file names |
613 +--------+----------+----------+------------+
614 [Dentry Block: 4KB] . .
617 +------+------+-----+------+
618 | hash | ino | len | type |
619 +------+------+-----+------+
620 [Dentry Structure: 11 bytes]
622 F2FS implements multi-level hash tables for directory structure. Each level has
623 a hash table with dedicated number of hash buckets as shown below. Note that
624 "A(2B)" means a bucket includes 2 data blocks.
628 ----------------------
631 N : MAX_DIR_HASH_DEPTH
632 ----------------------
636 level #1 | A(2B) - A(2B)
638 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
640 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
642 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
644 The number of blocks and buckets are determined by::
646 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
647 # of blocks in level #n = |
650 ,- 2^(n + dir_level),
651 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
652 # of buckets in level #n = |
653 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
656 When F2FS finds a file name in a directory, at first a hash value of the file
657 name is calculated. Then, F2FS scans the hash table in level #0 to find the
658 dentry consisting of the file name and its inode number. If not found, F2FS
659 scans the next hash table in level #1. In this way, F2FS scans hash tables in
660 each levels incrementally from 1 to N. In each level F2FS needs to scan only
661 one bucket determined by the following equation, which shows O(log(# of files))
664 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
666 In the case of file creation, F2FS finds empty consecutive slots that cover the
667 file name. F2FS searches the empty slots in the hash tables of whole levels from
668 1 to N in the same way as the lookup operation.
670 The following figure shows an example of two cases holding children::
672 --------------> Dir <--------------
676 child - child [hole] - child
678 child - child - child [hole] - [hole] - child
681 Number of children = 6, Number of children = 3,
682 File size = 7 File size = 7
684 Default Block Allocation
685 ------------------------
687 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
688 and Hot/Warm/Cold data.
690 - Hot node contains direct node blocks of directories.
691 - Warm node contains direct node blocks except hot node blocks.
692 - Cold node contains indirect node blocks
693 - Hot data contains dentry blocks
694 - Warm data contains data blocks except hot and cold data blocks
695 - Cold data contains multimedia data or migrated data blocks
697 LFS has two schemes for free space management: threaded log and copy-and-compac-
698 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
699 for devices showing very good sequential write performance, since free segments
700 are served all the time for writing new data. However, it suffers from cleaning
701 overhead under high utilization. Contrarily, the threaded log scheme suffers
702 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
703 scheme where the copy-and-compaction scheme is adopted by default, but the
704 policy is dynamically changed to the threaded log scheme according to the file
707 In order to align F2FS with underlying flash-based storage, F2FS allocates a
708 segment in a unit of section. F2FS expects that the section size would be the
709 same as the unit size of garbage collection in FTL. Furthermore, with respect
710 to the mapping granularity in FTL, F2FS allocates each section of the active
711 logs from different zones as much as possible, since FTL can write the data in
712 the active logs into one allocation unit according to its mapping granularity.
717 F2FS does cleaning both on demand and in the background. On-demand cleaning is
718 triggered when there are not enough free segments to serve VFS calls. Background
719 cleaner is operated by a kernel thread, and triggers the cleaning job when the
722 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
723 In the greedy algorithm, F2FS selects a victim segment having the smallest number
724 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
725 according to the segment age and the number of valid blocks in order to address
726 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
727 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
730 In order to identify whether the data in the victim segment are valid or not,
731 F2FS manages a bitmap. Each bit represents the validity of a block, and the
732 bitmap is composed of a bit stream covering whole blocks in main area.
737 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
739 2) whint_mode=user-based. F2FS tries to pass down hints given by
742 ===================== ======================== ===================
744 ===================== ======================== ===================
745 N/A META WRITE_LIFE_NOT_SET
749 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
753 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
754 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
755 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
757 WRITE_LIFE_MEDIUM " "
761 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
762 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
763 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
764 WRITE_LIFE_NONE " WRITE_LIFE_NONE
765 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
766 WRITE_LIFE_LONG " WRITE_LIFE_LONG
767 ===================== ======================== ===================
769 3) whint_mode=fs-based. F2FS passes down hints with its policy.
771 ===================== ======================== ===================
773 ===================== ======================== ===================
774 N/A META WRITE_LIFE_MEDIUM;
775 N/A HOT_NODE WRITE_LIFE_NOT_SET
777 N/A COLD_NODE WRITE_LIFE_NONE
778 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
782 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
783 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
784 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
786 WRITE_LIFE_MEDIUM " "
790 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
791 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
792 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
793 WRITE_LIFE_NONE " WRITE_LIFE_NONE
794 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
795 WRITE_LIFE_LONG " WRITE_LIFE_LONG
796 ===================== ======================== ===================
801 The default policy follows the below POSIX rule.
803 Allocating disk space
804 The default operation (i.e., mode is zero) of fallocate() allocates
805 the disk space within the range specified by offset and len. The
806 file size (as reported by stat(2)) will be changed if offset+len is
807 greater than the file size. Any subregion within the range specified
808 by offset and len that did not contain data before the call will be
809 initialized to zero. This default behavior closely resembles the
810 behavior of the posix_fallocate(3) library function, and is intended
811 as a method of optimally implementing that function.
813 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
814 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
815 zero or random data, which is useful to the below scenario where:
818 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
819 3. fallocate(fd, 0, 0, size)
820 4. address = fibmap(fd, offset)
822 6. write(blkdev, address)
824 Compression implementation
825 --------------------------
827 - New term named cluster is defined as basic unit of compression, file can
828 be divided into multiple clusters logically. One cluster includes 4 << n
829 (n >= 0) logical pages, compression size is also cluster size, each of
830 cluster can be compressed or not.
832 - In cluster metadata layout, one special block address is used to indicate
833 a cluster is a compressed one or normal one; for compressed cluster, following
834 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
835 stores data including compress header and compressed data.
837 - In order to eliminate write amplification during overwrite, F2FS only
838 support compression on write-once file, data can be compressed only when
839 all logical blocks in cluster contain valid data and compress ratio of
840 cluster data is lower than specified threshold.
842 - To enable compression on regular inode, there are four ways:
845 * chattr +c dir; touch dir/file
846 * mount w/ -o compress_extension=ext; touch file.ext
847 * mount w/ -o compress_extension=*; touch any_file
849 - To disable compression on regular inode, there are two ways:
852 * mount w/ -o nocompress_extension=ext; touch file.ext
854 - Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
856 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
857 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
858 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
859 can enable compress on bar.zip.
860 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
861 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
862 compresse, bar.zip and baz.txt should be non-compressed.
863 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
866 - At this point, compression feature doesn't expose compressed space to user
867 directly in order to guarantee potential data updates later to the space.
868 Instead, the main goal is to reduce data writes to flash disk as much as
869 possible, resulting in extending disk life time as well as relaxing IO
870 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
871 interface to reclaim compressed space and show it to user after putting the
872 immutable bit. Immutable bit, after release, it doesn't allow writing/mmaping
873 on the file, until reserving compressed space via
874 ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or truncating filesize to zero.
876 Compress metadata layout::
879 +-----------------------------------------------+
880 | cluster 1 | cluster 2 | ......... | cluster N |
881 +-----------------------------------------------+
884 . Compressed Cluster . . Normal Cluster .
885 +----------+---------+---------+---------+ +---------+---------+---------+---------+
886 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
887 +----------+---------+---------+---------+ +---------+---------+---------+---------+
891 +-------------+-------------+----------+----------------------------+
892 | data length | data chksum | reserved | compressed data |
893 +-------------+-------------+----------+----------------------------+
896 --------------------------
898 f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
899 With this option, f2fs provides a choice to select the way how to compress the
900 compression enabled files (refer to "Compression implementation" section for how to
901 enable compression on a regular inode).
904 This is the default option. f2fs does automatic compression in the writeback of the
905 compression enabled files.
907 2) compress_mode=user
908 This disables the automatic compression and gives the user discretion of choosing the
909 target file and the timing. The user can do manual compression/decompression on the
910 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
911 ioctls like the below.
913 To decompress a file,
915 fd = open(filename, O_WRONLY, 0);
916 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
920 fd = open(filename, O_WRONLY, 0);
921 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
923 NVMe Zoned Namespace devices
924 ----------------------------
926 - ZNS defines a per-zone capacity which can be equal or less than the
927 zone-size. Zone-capacity is the number of usable blocks in the zone.
928 F2FS checks if zone-capacity is less than zone-size, if it is, then any
929 segment which starts after the zone-capacity is marked as not-free in
930 the free segment bitmap at initial mount time. These segments are marked
931 as permanently used so they are not allocated for writes and
932 consequently are not needed to be garbage collected. In case the
933 zone-capacity is not aligned to default segment size(2MB), then a segment
934 can start before the zone-capacity and span across zone-capacity boundary.
935 Such spanning segments are also considered as usable segments. All blocks
936 past the zone-capacity are considered unusable in these segments.
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