1.. SPDX-License-Identifier: GPL-2.0
2
3==========================================
4WHAT IS Flash-Friendly File System (F2FS)?
5==========================================
6
7NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
8been equipped on a variety systems ranging from mobile to server systems. Since
9they are known to have different characteristics from the conventional rotating
10disks, a file system, an upper layer to the storage device, should adapt to the
11changes from the sketch in the design level.
12
13F2FS is a file system exploiting NAND flash memory-based storage devices, which
14is based on Log-structured File System (LFS). The design has been focused on
15addressing the fundamental issues in LFS, which are snowball effect of wandering
16tree and high cleaning overhead.
17
18Since a NAND flash memory-based storage device shows different characteristic
19according to its internal geometry or flash memory management scheme, namely FTL,
20F2FS and its tools support various parameters not only for configuring on-disk
21layout, but also for selecting allocation and cleaning algorithms.
22
23The following git tree provides the file system formatting tool (mkfs.f2fs),
24a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
25
26- git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
27
28For reporting bugs and sending patches, please use the following mailing list:
29
30- linux-f2fs-devel@lists.sourceforge.net
31
32Background and Design issues
33============================
34
35Log-structured File System (LFS)
36--------------------------------
37"A log-structured file system writes all modifications to disk sequentially in
38a log-like structure, thereby speeding up  both file writing and crash recovery.
39The log is the only structure on disk; it contains indexing information so that
40files can be read back from the log efficiently. In order to maintain large free
41areas on disk for fast writing, we divide  the log into segments and use a
42segment cleaner to compress the live information from heavily fragmented
43segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
44implementation of a log-structured file system", ACM Trans. Computer Systems
4510, 1, 26–52.
46
47Wandering Tree Problem
48----------------------
49In LFS, when a file data is updated and written to the end of log, its direct
50pointer block is updated due to the changed location. Then the indirect pointer
51block is also updated due to the direct pointer block update. In this manner,
52the upper index structures such as inode, inode map, and checkpoint block are
53also updated recursively. This problem is called as wandering tree problem [1],
54and in order to enhance the performance, it should eliminate or relax the update
55propagation as much as possible.
56
57[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
58
59Cleaning Overhead
60-----------------
61Since LFS is based on out-of-place writes, it produces so many obsolete blocks
62scattered across the whole storage. In order to serve new empty log space, it
63needs to reclaim these obsolete blocks seamlessly to users. This job is called
64as a cleaning process.
65
66The process consists of three operations as follows.
67
681. A victim segment is selected through referencing segment usage table.
692. It loads parent index structures of all the data in the victim identified by
70   segment summary blocks.
713. It checks the cross-reference between the data and its parent index structure.
724. It moves valid data selectively.
73
74This cleaning job may cause unexpected long delays, so the most important goal
75is to hide the latencies to users. And also definitely, it should reduce the
76amount of valid data to be moved, and move them quickly as well.
77
78Key Features
79============
80
81Flash Awareness
82---------------
83- Enlarge the random write area for better performance, but provide the high
84  spatial locality
85- Align FS data structures to the operational units in FTL as best efforts
86
87Wandering 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.
92
93Cleaning Overhead
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
99
100Mount Options
101=============
102
103
104======================== ============================================================
105background_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.
113gc_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.
118nogc_merge		 Disable GC merge feature.
119disable_roll_forward	 Disable the roll-forward recovery routine
120norecovery		 Disable the roll-forward recovery routine, mounted read-
121			 only (i.e., -o ro,disable_roll_forward)
122discard/nodiscard	 Enable/disable real-time discard in f2fs, if discard is
123			 enabled, f2fs will issue discard/TRIM commands when a
124			 segment is cleaned.
125no_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.
128nouser_xattr		 Disable Extended User Attributes. Note: xattr is enabled
129			 by default if CONFIG_F2FS_FS_XATTR is selected.
130noacl			 Disable POSIX Access Control List. Note: acl is enabled
131			 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
132active_logs=%u		 Support configuring the number of active logs. In the
133			 current design, f2fs supports only 2, 4, and 6 logs.
134			 Default number is 6.
135disable_ext_identify	 Disable the extension list configured by mkfs, so f2fs
136			 is not aware of cold files such as media files.
137inline_xattr		 Enable the inline xattrs feature.
138noinline_xattr		 Disable the inline xattrs feature.
139inline_xattr_size=%u	 Support configuring inline xattr size, it depends on
140			 flexible inline xattr feature.
141inline_data		 Enable the inline data feature: Newly created small (<~3.4k)
142			 files can be written into inode block.
143inline_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.
147noinline_dentry		 Disable the inline dentry feature.
148flush_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.
152nobarrier		 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
156			 data writes.
157fastboot		 This option is used when a system wants to reduce mount
158			 time as much as possible, even though normal performance
159			 can be sacrificed.
160extent_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.
164noextent_cache		 Disable an extent cache based on rb-tree explicitly, see
165			 the above extent_cache mount option.
166noinline_data		 Disable the inline data feature, inline data feature is
167			 enabled by default.
168data_flush		 Enable data flushing before checkpoint in order to
169			 persist data of regular and symlink.
170reserve_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.
173resuid=%d		 The user ID which may use the reserved blocks.
174resgid=%d		 The group ID which may use the reserved blocks.
175fault_injection=%d	 Enable fault injection in all supported types with
176			 specified injection rate.
177fault_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.
180
181			 ===================	  ===========
182			 Type_Name		  Type_Value
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			 FAULT_DQUOT_INIT	  0x000010000
201			 FAULT_LOCK_OP		  0x000020000
202			 ===================	  ===========
203mode=%s			 Control block allocation mode which supports "adaptive"
204			 and "lfs". In "lfs" mode, there should be no random
205			 writes towards main area.
206			 "fragment:segment" and "fragment:block" are newly added here.
207			 These are developer options for experiments to simulate filesystem
208			 fragmentation/after-GC situation itself. The developers use these
209			 modes to understand filesystem fragmentation/after-GC condition well,
210			 and eventually get some insights to handle them better.
211			 In "fragment:segment", f2fs allocates a new segment in ramdom
212			 position. With this, we can simulate the after-GC condition.
213			 In "fragment:block", we can scatter block allocation with
214			 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes.
215			 We added some randomness to both chunk and hole size to make
216			 it close to realistic IO pattern. So, in this mode, f2fs will allocate
217			 1..<max_fragment_chunk> blocks in a chunk and make a hole in the
218			 length of 1..<max_fragment_hole> by turns. With this, the newly
219			 allocated blocks will be scattered throughout the whole partition.
220			 Note that "fragment:block" implicitly enables "fragment:segment"
221			 option for more randomness.
222			 Please, use these options for your experiments and we strongly
223			 recommend to re-format the filesystem after using these options.
224io_bits=%u		 Set the bit size of write IO requests. It should be set
225			 with "mode=lfs".
226usrquota		 Enable plain user disk quota accounting.
227grpquota		 Enable plain group disk quota accounting.
228prjquota		 Enable plain project quota accounting.
229usrjquota=<file>	 Appoint specified file and type during mount, so that quota
230grpjquota=<file>	 information can be properly updated during recovery flow,
231prjjquota=<file>	 <quota file>: must be in root directory;
232jqfmt=<quota type>	 <quota type>: [vfsold,vfsv0,vfsv1].
233offusrjquota		 Turn off user journalled quota.
234offgrpjquota		 Turn off group journalled quota.
235offprjjquota		 Turn off project journalled quota.
236quota			 Enable plain user disk quota accounting.
237noquota			 Disable all plain disk quota option.
238alloc_mode=%s		 Adjust block allocation policy, which supports "reuse"
239			 and "default".
240fsync_mode=%s		 Control the policy of fsync. Currently supports "posix",
241			 "strict", and "nobarrier". In "posix" mode, which is
242			 default, fsync will follow POSIX semantics and does a
243			 light operation to improve the filesystem performance.
244			 In "strict" mode, fsync will be heavy and behaves in line
245			 with xfs, ext4 and btrfs, where xfstest generic/342 will
246			 pass, but the performance will regress. "nobarrier" is
247			 based on "posix", but doesn't issue flush command for
248			 non-atomic files likewise "nobarrier" mount option.
249test_dummy_encryption
250test_dummy_encryption=%s
251			 Enable dummy encryption, which provides a fake fscrypt
252			 context. The fake fscrypt context is used by xfstests.
253			 The argument may be either "v1" or "v2", in order to
254			 select the corresponding fscrypt policy version.
255checkpoint=%s[:%u[%]]	 Set to "disable" to turn off checkpointing. Set to "enable"
256			 to reenable checkpointing. Is enabled by default. While
257			 disabled, any unmounting or unexpected shutdowns will cause
258			 the filesystem contents to appear as they did when the
259			 filesystem was mounted with that option.
260			 While mounting with checkpoint=disabled, the filesystem must
261			 run garbage collection to ensure that all available space can
262			 be used. If this takes too much time, the mount may return
263			 EAGAIN. You may optionally add a value to indicate how much
264			 of the disk you would be willing to temporarily give up to
265			 avoid additional garbage collection. This can be given as a
266			 number of blocks, or as a percent. For instance, mounting
267			 with checkpoint=disable:100% would always succeed, but it may
268			 hide up to all remaining free space. The actual space that
269			 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
270			 This space is reclaimed once checkpoint=enable.
271checkpoint_merge	 When checkpoint is enabled, this can be used to create a kernel
272			 daemon and make it to merge concurrent checkpoint requests as
273			 much as possible to eliminate redundant checkpoint issues. Plus,
274			 we can eliminate the sluggish issue caused by slow checkpoint
275			 operation when the checkpoint is done in a process context in
276			 a cgroup having low i/o budget and cpu shares. To make this
277			 do better, we set the default i/o priority of the kernel daemon
278			 to "3", to give one higher priority than other kernel threads.
279			 This is the same way to give a I/O priority to the jbd2
280			 journaling thread of ext4 filesystem.
281nocheckpoint_merge	 Disable checkpoint merge feature.
282compress_algorithm=%s	 Control compress algorithm, currently f2fs supports "lzo",
283			 "lz4", "zstd" and "lzo-rle" algorithm.
284compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
285			 "lz4" and "zstd" support compress level config.
286			 algorithm	level range
287			 lz4		3 - 16
288			 zstd		1 - 22
289compress_log_size=%u	 Support configuring compress cluster size. The size will
290			 be 4KB * (1 << %u). The default and minimum sizes are 16KB.
291compress_extension=%s	 Support adding specified extension, so that f2fs can enable
292			 compression on those corresponding files, e.g. if all files
293			 with '.ext' has high compression rate, we can set the '.ext'
294			 on compression extension list and enable compression on
295			 these file by default rather than to enable it via ioctl.
296			 For other files, we can still enable compression via ioctl.
297			 Note that, there is one reserved special extension '*', it
298			 can be set to enable compression for all files.
299nocompress_extension=%s	 Support adding specified extension, so that f2fs can disable
300			 compression on those corresponding files, just contrary to compression extension.
301			 If you know exactly which files cannot be compressed, you can use this.
302			 The same extension name can't appear in both compress and nocompress
303			 extension at the same time.
304			 If the compress extension specifies all files, the types specified by the
305			 nocompress extension will be treated as special cases and will not be compressed.
306			 Don't allow use '*' to specifie all file in nocompress extension.
307			 After add nocompress_extension, the priority should be:
308			 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
309			 See more in compression sections.
310
311compress_chksum		 Support verifying chksum of raw data in compressed cluster.
312compress_mode=%s	 Control file compression mode. This supports "fs" and "user"
313			 modes. In "fs" mode (default), f2fs does automatic compression
314			 on the compression enabled files. In "user" mode, f2fs disables
315			 the automaic compression and gives the user discretion of
316			 choosing the target file and the timing. The user can do manual
317			 compression/decompression on the compression enabled files using
318			 ioctls.
319compress_cache		 Support to use address space of a filesystem managed inode to
320			 cache compressed block, in order to improve cache hit ratio of
321			 random read.
322inlinecrypt		 When possible, encrypt/decrypt the contents of encrypted
323			 files using the blk-crypto framework rather than
324			 filesystem-layer encryption. This allows the use of
325			 inline encryption hardware. The on-disk format is
326			 unaffected. For more details, see
327			 Documentation/block/inline-encryption.rst.
328atgc			 Enable age-threshold garbage collection, it provides high
329			 effectiveness and efficiency on background GC.
330discard_unit=%s		 Control discard unit, the argument can be "block", "segment"
331			 and "section", issued discard command's offset/size will be
332			 aligned to the unit, by default, "discard_unit=block" is set,
333			 so that small discard functionality is enabled.
334			 For blkzoned device, "discard_unit=section" will be set by
335			 default, it is helpful for large sized SMR or ZNS devices to
336			 reduce memory cost by getting rid of fs metadata supports small
337			 discard.
338memory=%s		 Control memory mode. This supports "normal" and "low" modes.
339			 "low" mode is introduced to support low memory devices.
340			 Because of the nature of low memory devices, in this mode, f2fs
341			 will try to save memory sometimes by sacrificing performance.
342			 "normal" mode is the default mode and same as before.
343======================== ============================================================
344
345Debugfs Entries
346===============
347
348/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
349f2fs. Each file shows the whole f2fs information.
350
351/sys/kernel/debug/f2fs/status includes:
352
353 - major file system information managed by f2fs currently
354 - average SIT information about whole segments
355 - current memory footprint consumed by f2fs.
356
357Sysfs Entries
358=============
359
360Information about mounted f2fs file systems can be found in
361/sys/fs/f2fs.  Each mounted filesystem will have a directory in
362/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
363The files in each per-device directory are shown in table below.
364
365Files in /sys/fs/f2fs/<devname>
366(see also Documentation/ABI/testing/sysfs-fs-f2fs)
367
368Usage
369=====
370
3711. Download userland tools and compile them.
372
3732. Skip, if f2fs was compiled statically inside kernel.
374   Otherwise, insert the f2fs.ko module::
375
376	# insmod f2fs.ko
377
3783. Create a directory to use when mounting::
379
380	# mkdir /mnt/f2fs
381
3824. Format the block device, and then mount as f2fs::
383
384	# mkfs.f2fs -l label /dev/block_device
385	# mount -t f2fs /dev/block_device /mnt/f2fs
386
387mkfs.f2fs
388---------
389The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
390which builds a basic on-disk layout.
391
392The quick options consist of:
393
394===============    ===========================================================
395``-l [label]``     Give a volume label, up to 512 unicode name.
396``-a [0 or 1]``    Split start location of each area for heap-based allocation.
397
398                   1 is set by default, which performs this.
399``-o [int]``       Set overprovision ratio in percent over volume size.
400
401                   5 is set by default.
402``-s [int]``       Set the number of segments per section.
403
404                   1 is set by default.
405``-z [int]``       Set the number of sections per zone.
406
407                   1 is set by default.
408``-e [str]``       Set basic extension list. e.g. "mp3,gif,mov"
409``-t [0 or 1]``    Disable discard command or not.
410
411                   1 is set by default, which conducts discard.
412===============    ===========================================================
413
414Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
415
416fsck.f2fs
417---------
418The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
419partition, which examines whether the filesystem metadata and user-made data
420are cross-referenced correctly or not.
421Note that, initial version of the tool does not fix any inconsistency.
422
423The quick options consist of::
424
425  -d debug level [default:0]
426
427Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
428
429dump.f2fs
430---------
431The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
432file. Each file is dump_ssa and dump_sit.
433
434The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
435It shows on-disk inode information recognized by a given inode number, and is
436able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
437./dump_sit respectively.
438
439The options consist of::
440
441  -d debug level [default:0]
442  -i inode no (hex)
443  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
444  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
445
446Examples::
447
448    # dump.f2fs -i [ino] /dev/sdx
449    # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
450    # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
451
452Note: please refer to the manpage of dump.f2fs(8) to get full option list.
453
454sload.f2fs
455----------
456The sload.f2fs gives a way to insert files and directories in the exisiting disk
457image. This tool is useful when building f2fs images given compiled files.
458
459Note: please refer to the manpage of sload.f2fs(8) to get full option list.
460
461resize.f2fs
462-----------
463The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
464all the files and directories stored in the image.
465
466Note: please refer to the manpage of resize.f2fs(8) to get full option list.
467
468defrag.f2fs
469-----------
470The defrag.f2fs can be used to defragment scattered written data as well as
471filesystem metadata across the disk. This can improve the write speed by giving
472more free consecutive space.
473
474Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
475
476f2fs_io
477-------
478The f2fs_io is a simple tool to issue various filesystem APIs as well as
479f2fs-specific ones, which is very useful for QA tests.
480
481Note: please refer to the manpage of f2fs_io(8) to get full option list.
482
483Design
484======
485
486On-disk Layout
487--------------
488
489F2FS divides the whole volume into a number of segments, each of which is fixed
490to 2MB in size. A section is composed of consecutive segments, and a zone
491consists of a set of sections. By default, section and zone sizes are set to one
492segment size identically, but users can easily modify the sizes by mkfs.
493
494F2FS splits the entire volume into six areas, and all the areas except superblock
495consist of multiple segments as described below::
496
497                                            align with the zone size <-|
498                 |-> align with the segment size
499     _________________________________________________________________________
500    |            |            |   Segment   |    Node     |   Segment  |      |
501    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
502    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
503    |____________|_____2______|______N______|______N______|______N_____|__N___|
504                                                                       .      .
505                                                             .                .
506                                                 .                            .
507                                    ._________________________________________.
508                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
509                                    .           .
510                                    ._________._________
511                                    |_section_|__...__|_
512                                    .            .
513		                    .________.
514	                            |__zone__|
515
516- Superblock (SB)
517   It is located at the beginning of the partition, and there exist two copies
518   to avoid file system crash. It contains basic partition information and some
519   default parameters of f2fs.
520
521- Checkpoint (CP)
522   It contains file system information, bitmaps for valid NAT/SIT sets, orphan
523   inode lists, and summary entries of current active segments.
524
525- Segment Information Table (SIT)
526   It contains segment information such as valid block count and bitmap for the
527   validity of all the blocks.
528
529- Node Address Table (NAT)
530   It is composed of a block address table for all the node blocks stored in
531   Main area.
532
533- Segment Summary Area (SSA)
534   It contains summary entries which contains the owner information of all the
535   data and node blocks stored in Main area.
536
537- Main Area
538   It contains file and directory data including their indices.
539
540In order to avoid misalignment between file system and flash-based storage, F2FS
541aligns the start block address of CP with the segment size. Also, it aligns the
542start block address of Main area with the zone size by reserving some segments
543in SSA area.
544
545Reference the following survey for additional technical details.
546https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
547
548File System Metadata Structure
549------------------------------
550
551F2FS adopts the checkpointing scheme to maintain file system consistency. At
552mount time, F2FS first tries to find the last valid checkpoint data by scanning
553CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
554One of them always indicates the last valid data, which is called as shadow copy
555mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
556
557For file system consistency, each CP points to which NAT and SIT copies are
558valid, as shown as below::
559
560  +--------+----------+---------+
561  |   CP   |    SIT   |   NAT   |
562  +--------+----------+---------+
563  .         .          .          .
564  .            .              .              .
565  .               .                 .                 .
566  +-------+-------+--------+--------+--------+--------+
567  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
568  +-------+-------+--------+--------+--------+--------+
569     |             ^                          ^
570     |             |                          |
571     `----------------------------------------'
572
573Index Structure
574---------------
575
576The key data structure to manage the data locations is a "node". Similar to
577traditional file structures, F2FS has three types of node: inode, direct node,
578indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
579indices, two direct node pointers, two indirect node pointers, and one double
580indirect node pointer as described below. One direct node block contains 1018
581data blocks, and one indirect node block contains also 1018 node blocks. Thus,
582one inode block (i.e., a file) covers::
583
584  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
585
586   Inode block (4KB)
587     |- data (923)
588     |- direct node (2)
589     |          `- data (1018)
590     |- indirect node (2)
591     |            `- direct node (1018)
592     |                       `- data (1018)
593     `- double indirect node (1)
594                         `- indirect node (1018)
595			              `- direct node (1018)
596	                                         `- data (1018)
597
598Note that all the node blocks are mapped by NAT which means the location of
599each node is translated by the NAT table. In the consideration of the wandering
600tree problem, F2FS is able to cut off the propagation of node updates caused by
601leaf data writes.
602
603Directory Structure
604-------------------
605
606A directory entry occupies 11 bytes, which consists of the following attributes.
607
608- hash		hash value of the file name
609- ino		inode number
610- len		the length of file name
611- type		file type such as directory, symlink, etc
612
613A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
614used to represent whether each dentry is valid or not. A dentry block occupies
6154KB with the following composition.
616
617::
618
619  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
620	              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
621
622                         [Bucket]
623             +--------------------------------+
624             |dentry block 1 | dentry block 2 |
625             +--------------------------------+
626             .               .
627       .                             .
628  .       [Dentry Block Structure: 4KB]       .
629  +--------+----------+----------+------------+
630  | bitmap | reserved | dentries | file names |
631  +--------+----------+----------+------------+
632  [Dentry Block: 4KB] .   .
633		 .               .
634            .                          .
635            +------+------+-----+------+
636            | hash | ino  | len | type |
637            +------+------+-----+------+
638            [Dentry Structure: 11 bytes]
639
640F2FS implements multi-level hash tables for directory structure. Each level has
641a hash table with dedicated number of hash buckets as shown below. Note that
642"A(2B)" means a bucket includes 2 data blocks.
643
644::
645
646    ----------------------
647    A : bucket
648    B : block
649    N : MAX_DIR_HASH_DEPTH
650    ----------------------
651
652    level #0   | A(2B)
653	    |
654    level #1   | A(2B) - A(2B)
655	    |
656    level #2   | A(2B) - A(2B) - A(2B) - A(2B)
657	.     |   .       .       .       .
658    level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
659	.     |   .       .       .       .
660    level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
661
662The number of blocks and buckets are determined by::
663
664                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
665  # of blocks in level #n = |
666                            `- 4, Otherwise
667
668                             ,- 2^(n + dir_level),
669			     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
670  # of buckets in level #n = |
671                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
672			              Otherwise
673
674When F2FS finds a file name in a directory, at first a hash value of the file
675name is calculated. Then, F2FS scans the hash table in level #0 to find the
676dentry consisting of the file name and its inode number. If not found, F2FS
677scans the next hash table in level #1. In this way, F2FS scans hash tables in
678each levels incrementally from 1 to N. In each level F2FS needs to scan only
679one bucket determined by the following equation, which shows O(log(# of files))
680complexity::
681
682  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
683
684In the case of file creation, F2FS finds empty consecutive slots that cover the
685file name. F2FS searches the empty slots in the hash tables of whole levels from
6861 to N in the same way as the lookup operation.
687
688The following figure shows an example of two cases holding children::
689
690       --------------> Dir <--------------
691       |                                 |
692    child                             child
693
694    child - child                     [hole] - child
695
696    child - child - child             [hole] - [hole] - child
697
698   Case 1:                           Case 2:
699   Number of children = 6,           Number of children = 3,
700   File size = 7                     File size = 7
701
702Default Block Allocation
703------------------------
704
705At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
706and Hot/Warm/Cold data.
707
708- Hot node	contains direct node blocks of directories.
709- Warm node	contains direct node blocks except hot node blocks.
710- Cold node	contains indirect node blocks
711- Hot data	contains dentry blocks
712- Warm data	contains data blocks except hot and cold data blocks
713- Cold data	contains multimedia data or migrated data blocks
714
715LFS has two schemes for free space management: threaded log and copy-and-compac-
716tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
717for devices showing very good sequential write performance, since free segments
718are served all the time for writing new data. However, it suffers from cleaning
719overhead under high utilization. Contrarily, the threaded log scheme suffers
720from random writes, but no cleaning process is needed. F2FS adopts a hybrid
721scheme where the copy-and-compaction scheme is adopted by default, but the
722policy is dynamically changed to the threaded log scheme according to the file
723system status.
724
725In order to align F2FS with underlying flash-based storage, F2FS allocates a
726segment in a unit of section. F2FS expects that the section size would be the
727same as the unit size of garbage collection in FTL. Furthermore, with respect
728to the mapping granularity in FTL, F2FS allocates each section of the active
729logs from different zones as much as possible, since FTL can write the data in
730the active logs into one allocation unit according to its mapping granularity.
731
732Cleaning process
733----------------
734
735F2FS does cleaning both on demand and in the background. On-demand cleaning is
736triggered when there are not enough free segments to serve VFS calls. Background
737cleaner is operated by a kernel thread, and triggers the cleaning job when the
738system is idle.
739
740F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
741In the greedy algorithm, F2FS selects a victim segment having the smallest number
742of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
743according to the segment age and the number of valid blocks in order to address
744log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
745algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
746algorithm.
747
748In order to identify whether the data in the victim segment are valid or not,
749F2FS manages a bitmap. Each bit represents the validity of a block, and the
750bitmap is composed of a bit stream covering whole blocks in main area.
751
752Fallocate(2) Policy
753-------------------
754
755The default policy follows the below POSIX rule.
756
757Allocating disk space
758    The default operation (i.e., mode is zero) of fallocate() allocates
759    the disk space within the range specified by offset and len.  The
760    file size (as reported by stat(2)) will be changed if offset+len is
761    greater than the file size.  Any subregion within the range specified
762    by offset and len that did not contain data before the call will be
763    initialized to zero.  This default behavior closely resembles the
764    behavior of the posix_fallocate(3) library function, and is intended
765    as a method of optimally implementing that function.
766
767However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
768fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
769zero or random data, which is useful to the below scenario where:
770
771 1. create(fd)
772 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
773 3. fallocate(fd, 0, 0, size)
774 4. address = fibmap(fd, offset)
775 5. open(blkdev)
776 6. write(blkdev, address)
777
778Compression implementation
779--------------------------
780
781- New term named cluster is defined as basic unit of compression, file can
782  be divided into multiple clusters logically. One cluster includes 4 << n
783  (n >= 0) logical pages, compression size is also cluster size, each of
784  cluster can be compressed or not.
785
786- In cluster metadata layout, one special block address is used to indicate
787  a cluster is a compressed one or normal one; for compressed cluster, following
788  metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
789  stores data including compress header and compressed data.
790
791- In order to eliminate write amplification during overwrite, F2FS only
792  support compression on write-once file, data can be compressed only when
793  all logical blocks in cluster contain valid data and compress ratio of
794  cluster data is lower than specified threshold.
795
796- To enable compression on regular inode, there are four ways:
797
798  * chattr +c file
799  * chattr +c dir; touch dir/file
800  * mount w/ -o compress_extension=ext; touch file.ext
801  * mount w/ -o compress_extension=*; touch any_file
802
803- To disable compression on regular inode, there are two ways:
804
805  * chattr -c file
806  * mount w/ -o nocompress_extension=ext; touch file.ext
807
808- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
809
810  * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
811    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
812    should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
813    can enable compress on bar.zip.
814  * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
815    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
816    compresse, bar.zip and baz.txt should be non-compressed.
817    chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
818    and baz.txt.
819
820- At this point, compression feature doesn't expose compressed space to user
821  directly in order to guarantee potential data updates later to the space.
822  Instead, the main goal is to reduce data writes to flash disk as much as
823  possible, resulting in extending disk life time as well as relaxing IO
824  congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
825  interface to reclaim compressed space and show it to user after setting a
826  special flag to the inode. Once the compressed space is released, the flag
827  will block writing data to the file until either the compressed space is
828  reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is
829  truncated to zero.
830
831Compress metadata layout::
832
833				[Dnode Structure]
834		+-----------------------------------------------+
835		| cluster 1 | cluster 2 | ......... | cluster N |
836		+-----------------------------------------------+
837		.           .                       .           .
838	  .                      .                .                      .
839    .         Compressed Cluster       .        .        Normal Cluster            .
840    +----------+---------+---------+---------+  +---------+---------+---------+---------+
841    |compr flag| block 1 | block 2 | block 3 |  | block 1 | block 2 | block 3 | block 4 |
842    +----------+---------+---------+---------+  +---------+---------+---------+---------+
843	       .                             .
844	    .                                           .
845	.                                                           .
846	+-------------+-------------+----------+----------------------------+
847	| data length | data chksum | reserved |      compressed data       |
848	+-------------+-------------+----------+----------------------------+
849
850Compression mode
851--------------------------
852
853f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
854With this option, f2fs provides a choice to select the way how to compress the
855compression enabled files (refer to "Compression implementation" section for how to
856enable compression on a regular inode).
857
8581) compress_mode=fs
859This is the default option. f2fs does automatic compression in the writeback of the
860compression enabled files.
861
8622) compress_mode=user
863This disables the automatic compression and gives the user discretion of choosing the
864target file and the timing. The user can do manual compression/decompression on the
865compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
866ioctls like the below.
867
868To decompress a file,
869
870fd = open(filename, O_WRONLY, 0);
871ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
872
873To compress a file,
874
875fd = open(filename, O_WRONLY, 0);
876ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
877
878NVMe Zoned Namespace devices
879----------------------------
880
881- ZNS defines a per-zone capacity which can be equal or less than the
882  zone-size. Zone-capacity is the number of usable blocks in the zone.
883  F2FS checks if zone-capacity is less than zone-size, if it is, then any
884  segment which starts after the zone-capacity is marked as not-free in
885  the free segment bitmap at initial mount time. These segments are marked
886  as permanently used so they are not allocated for writes and
887  consequently are not needed to be garbage collected. In case the
888  zone-capacity is not aligned to default segment size(2MB), then a segment
889  can start before the zone-capacity and span across zone-capacity boundary.
890  Such spanning segments are also considered as usable segments. All blocks
891  past the zone-capacity are considered unusable in these segments.
892