linux/Documentation/filesystems/f2fs.rst
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   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.
 238whint_mode=%s            Control which write hints are passed down to block
 239                         layer. This supports "off", "user-based", and
 240                         "fs-based".  In "off" mode (default), f2fs does not pass
 241                         down hints. In "user-based" mode, f2fs tries to pass
 242                         down hints given by users. And in "fs-based" mode, f2fs
 243                         passes down hints with its policy.
 244alloc_mode=%s            Adjust block allocation policy, which supports "reuse"
 245                         and "default".
 246fsync_mode=%s            Control the policy of fsync. Currently supports "posix",
 247                         "strict", and "nobarrier". In "posix" mode, which is
 248                         default, fsync will follow POSIX semantics and does a
 249                         light operation to improve the filesystem performance.
 250                         In "strict" mode, fsync will be heavy and behaves in line
 251                         with xfs, ext4 and btrfs, where xfstest generic/342 will
 252                         pass, but the performance will regress. "nobarrier" is
 253                         based on "posix", but doesn't issue flush command for
 254                         non-atomic files likewise "nobarrier" mount option.
 255test_dummy_encryption
 256test_dummy_encryption=%s
 257                         Enable dummy encryption, which provides a fake fscrypt
 258                         context. The fake fscrypt context is used by xfstests.
 259                         The argument may be either "v1" or "v2", in order to
 260                         select the corresponding fscrypt policy version.
 261checkpoint=%s[:%u[%]]    Set to "disable" to turn off checkpointing. Set to "enable"
 262                         to reenable checkpointing. Is enabled by default. While
 263                         disabled, any unmounting or unexpected shutdowns will cause
 264                         the filesystem contents to appear as they did when the
 265                         filesystem was mounted with that option.
 266                         While mounting with checkpoint=disabled, the filesystem must
 267                         run garbage collection to ensure that all available space can
 268                         be used. If this takes too much time, the mount may return
 269                         EAGAIN. You may optionally add a value to indicate how much
 270                         of the disk you would be willing to temporarily give up to
 271                         avoid additional garbage collection. This can be given as a
 272                         number of blocks, or as a percent. For instance, mounting
 273                         with checkpoint=disable:100% would always succeed, but it may
 274                         hide up to all remaining free space. The actual space that
 275                         would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
 276                         This space is reclaimed once checkpoint=enable.
 277checkpoint_merge         When checkpoint is enabled, this can be used to create a kernel
 278                         daemon and make it to merge concurrent checkpoint requests as
 279                         much as possible to eliminate redundant checkpoint issues. Plus,
 280                         we can eliminate the sluggish issue caused by slow checkpoint
 281                         operation when the checkpoint is done in a process context in
 282                         a cgroup having low i/o budget and cpu shares. To make this
 283                         do better, we set the default i/o priority of the kernel daemon
 284                         to "3", to give one higher priority than other kernel threads.
 285                         This is the same way to give a I/O priority to the jbd2
 286                         journaling thread of ext4 filesystem.
 287nocheckpoint_merge       Disable checkpoint merge feature.
 288compress_algorithm=%s    Control compress algorithm, currently f2fs supports "lzo",
 289                         "lz4", "zstd" and "lzo-rle" algorithm.
 290compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
 291                         "lz4" and "zstd" support compress level config.
 292                         algorithm      level range
 293                         lz4            3 - 16
 294                         zstd           1 - 22
 295compress_log_size=%u     Support configuring compress cluster size, the size will
 296                         be 4KB * (1 << %u), 16KB is minimum size, also it's
 297                         default size.
 298compress_extension=%s    Support adding specified extension, so that f2fs can enable
 299                         compression on those corresponding files, e.g. if all files
 300                         with '.ext' has high compression rate, we can set the '.ext'
 301                         on compression extension list and enable compression on
 302                         these file by default rather than to enable it via ioctl.
 303                         For other files, we can still enable compression via ioctl.
 304                         Note that, there is one reserved special extension '*', it
 305                         can be set to enable compression for all files.
 306nocompress_extension=%s  Support adding specified extension, so that f2fs can disable
 307                         compression on those corresponding files, just contrary to compression extension.
 308                         If you know exactly which files cannot be compressed, you can use this.
 309                         The same extension name can't appear in both compress and nocompress
 310                         extension at the same time.
 311                         If the compress extension specifies all files, the types specified by the
 312                         nocompress extension will be treated as special cases and will not be compressed.
 313                         Don't allow use '*' to specifie all file in nocompress extension.
 314                         After add nocompress_extension, the priority should be:
 315                         dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
 316                         See more in compression sections.
 317
 318compress_chksum          Support verifying chksum of raw data in compressed cluster.
 319compress_mode=%s         Control file compression mode. This supports "fs" and "user"
 320                         modes. In "fs" mode (default), f2fs does automatic compression
 321                         on the compression enabled files. In "user" mode, f2fs disables
 322                         the automaic compression and gives the user discretion of
 323                         choosing the target file and the timing. The user can do manual
 324                         compression/decompression on the compression enabled files using
 325                         ioctls.
 326compress_cache           Support to use address space of a filesystem managed inode to
 327                         cache compressed block, in order to improve cache hit ratio of
 328                         random read.
 329inlinecrypt              When possible, encrypt/decrypt the contents of encrypted
 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
 333                         unaffected. For more details, see
 334                         Documentation/block/inline-encryption.rst.
 335atgc                     Enable age-threshold garbage collection, it provides high
 336                         effectiveness and efficiency on background GC.
 337discard_unit=%s          Control discard unit, the argument can be "block", "segment"
 338                         and "section", issued discard command's offset/size will be
 339                         aligned to the unit, by default, "discard_unit=block" is set,
 340                         so that small discard functionality is enabled.
 341                         For blkzoned device, "discard_unit=section" will be set by
 342                         default, it is helpful for large sized SMR or ZNS devices to
 343                         reduce memory cost by getting rid of fs metadata supports small
 344                         discard.
 345======================== ============================================================
 346
 347Debugfs Entries
 348===============
 349
 350/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
 351f2fs. Each file shows the whole f2fs information.
 352
 353/sys/kernel/debug/f2fs/status includes:
 354
 355 - major file system information managed by f2fs currently
 356 - average SIT information about whole segments
 357 - current memory footprint consumed by f2fs.
 358
 359Sysfs Entries
 360=============
 361
 362Information about mounted f2fs file systems can be found in
 363/sys/fs/f2fs.  Each mounted filesystem will have a directory in
 364/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
 365The files in each per-device directory are shown in table below.
 366
 367Files in /sys/fs/f2fs/<devname>
 368(see also Documentation/ABI/testing/sysfs-fs-f2fs)
 369
 370Usage
 371=====
 372
 3731. Download userland tools and compile them.
 374
 3752. Skip, if f2fs was compiled statically inside kernel.
 376   Otherwise, insert the f2fs.ko module::
 377
 378        # insmod f2fs.ko
 379
 3803. Create a directory to use when mounting::
 381
 382        # mkdir /mnt/f2fs
 383
 3844. Format the block device, and then mount as f2fs::
 385
 386        # mkfs.f2fs -l label /dev/block_device
 387        # mount -t f2fs /dev/block_device /mnt/f2fs
 388
 389mkfs.f2fs
 390---------
 391The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
 392which builds a basic on-disk layout.
 393
 394The quick options consist of:
 395
 396===============    ===========================================================
 397``-l [label]``     Give a volume label, up to 512 unicode name.
 398``-a [0 or 1]``    Split start location of each area for heap-based allocation.
 399
 400                   1 is set by default, which performs this.
 401``-o [int]``       Set overprovision ratio in percent over volume size.
 402
 403                   5 is set by default.
 404``-s [int]``       Set the number of segments per section.
 405
 406                   1 is set by default.
 407``-z [int]``       Set the number of sections per zone.
 408
 409                   1 is set by default.
 410``-e [str]``       Set basic extension list. e.g. "mp3,gif,mov"
 411``-t [0 or 1]``    Disable discard command or not.
 412
 413                   1 is set by default, which conducts discard.
 414===============    ===========================================================
 415
 416Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
 417
 418fsck.f2fs
 419---------
 420The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
 421partition, which examines whether the filesystem metadata and user-made data
 422are cross-referenced correctly or not.
 423Note that, initial version of the tool does not fix any inconsistency.
 424
 425The quick options consist of::
 426
 427  -d debug level [default:0]
 428
 429Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
 430
 431dump.f2fs
 432---------
 433The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
 434file. Each file is dump_ssa and dump_sit.
 435
 436The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
 437It shows on-disk inode information recognized by a given inode number, and is
 438able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
 439./dump_sit respectively.
 440
 441The options consist of::
 442
 443  -d debug level [default:0]
 444  -i inode no (hex)
 445  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
 446  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
 447
 448Examples::
 449
 450    # dump.f2fs -i [ino] /dev/sdx
 451    # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
 452    # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
 453
 454Note: please refer to the manpage of dump.f2fs(8) to get full option list.
 455
 456sload.f2fs
 457----------
 458The sload.f2fs gives a way to insert files and directories in the exisiting disk
 459image. This tool is useful when building f2fs images given compiled files.
 460
 461Note: please refer to the manpage of sload.f2fs(8) to get full option list.
 462
 463resize.f2fs
 464-----------
 465The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
 466all the files and directories stored in the image.
 467
 468Note: please refer to the manpage of resize.f2fs(8) to get full option list.
 469
 470defrag.f2fs
 471-----------
 472The defrag.f2fs can be used to defragment scattered written data as well as
 473filesystem metadata across the disk. This can improve the write speed by giving
 474more free consecutive space.
 475
 476Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
 477
 478f2fs_io
 479-------
 480The f2fs_io is a simple tool to issue various filesystem APIs as well as
 481f2fs-specific ones, which is very useful for QA tests.
 482
 483Note: please refer to the manpage of f2fs_io(8) to get full option list.
 484
 485Design
 486======
 487
 488On-disk Layout
 489--------------
 490
 491F2FS divides the whole volume into a number of segments, each of which is fixed
 492to 2MB in size. A section is composed of consecutive segments, and a zone
 493consists of a set of sections. By default, section and zone sizes are set to one
 494segment size identically, but users can easily modify the sizes by mkfs.
 495
 496F2FS splits the entire volume into six areas, and all the areas except superblock
 497consist of multiple segments as described below::
 498
 499                                            align with the zone size <-|
 500                 |-> align with the segment size
 501     _________________________________________________________________________
 502    |            |            |   Segment   |    Node     |   Segment  |      |
 503    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
 504    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
 505    |____________|_____2______|______N______|______N______|______N_____|__N___|
 506                                                                       .      .
 507                                                             .                .
 508                                                 .                            .
 509                                    ._________________________________________.
 510                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
 511                                    .           .
 512                                    ._________._________
 513                                    |_section_|__...__|_
 514                                    .            .
 515                                    .________.
 516                                    |__zone__|
 517
 518- Superblock (SB)
 519   It is located at the beginning of the partition, and there exist two copies
 520   to avoid file system crash. It contains basic partition information and some
 521   default parameters of f2fs.
 522
 523- Checkpoint (CP)
 524   It contains file system information, bitmaps for valid NAT/SIT sets, orphan
 525   inode lists, and summary entries of current active segments.
 526
 527- Segment Information Table (SIT)
 528   It contains segment information such as valid block count and bitmap for the
 529   validity of all the blocks.
 530
 531- Node Address Table (NAT)
 532   It is composed of a block address table for all the node blocks stored in
 533   Main area.
 534
 535- Segment Summary Area (SSA)
 536   It contains summary entries which contains the owner information of all the
 537   data and node blocks stored in Main area.
 538
 539- Main Area
 540   It contains file and directory data including their indices.
 541
 542In order to avoid misalignment between file system and flash-based storage, F2FS
 543aligns the start block address of CP with the segment size. Also, it aligns the
 544start block address of Main area with the zone size by reserving some segments
 545in SSA area.
 546
 547Reference the following survey for additional technical details.
 548https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
 549
 550File System Metadata Structure
 551------------------------------
 552
 553F2FS adopts the checkpointing scheme to maintain file system consistency. At
 554mount time, F2FS first tries to find the last valid checkpoint data by scanning
 555CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
 556One of them always indicates the last valid data, which is called as shadow copy
 557mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
 558
 559For file system consistency, each CP points to which NAT and SIT copies are
 560valid, as shown as below::
 561
 562  +--------+----------+---------+
 563  |   CP   |    SIT   |   NAT   |
 564  +--------+----------+---------+
 565  .         .          .          .
 566  .            .              .              .
 567  .               .                 .                 .
 568  +-------+-------+--------+--------+--------+--------+
 569  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
 570  +-------+-------+--------+--------+--------+--------+
 571     |             ^                          ^
 572     |             |                          |
 573     `----------------------------------------'
 574
 575Index Structure
 576---------------
 577
 578The key data structure to manage the data locations is a "node". Similar to
 579traditional file structures, F2FS has three types of node: inode, direct node,
 580indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
 581indices, two direct node pointers, two indirect node pointers, and one double
 582indirect node pointer as described below. One direct node block contains 1018
 583data blocks, and one indirect node block contains also 1018 node blocks. Thus,
 584one inode block (i.e., a file) covers::
 585
 586  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
 587
 588   Inode block (4KB)
 589     |- data (923)
 590     |- direct node (2)
 591     |          `- data (1018)
 592     |- indirect node (2)
 593     |            `- direct node (1018)
 594     |                       `- data (1018)
 595     `- double indirect node (1)
 596                         `- indirect node (1018)
 597                                      `- direct node (1018)
 598                                                 `- data (1018)
 599
 600Note that all the node blocks are mapped by NAT which means the location of
 601each node is translated by the NAT table. In the consideration of the wandering
 602tree problem, F2FS is able to cut off the propagation of node updates caused by
 603leaf data writes.
 604
 605Directory Structure
 606-------------------
 607
 608A directory entry occupies 11 bytes, which consists of the following attributes.
 609
 610- hash          hash value of the file name
 611- ino           inode number
 612- len           the length of file name
 613- type          file type such as directory, symlink, etc
 614
 615A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
 616used to represent whether each dentry is valid or not. A dentry block occupies
 6174KB with the following composition.
 618
 619::
 620
 621  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
 622                      dentries(11 * 214 bytes) + file name (8 * 214 bytes)
 623
 624                         [Bucket]
 625             +--------------------------------+
 626             |dentry block 1 | dentry block 2 |
 627             +--------------------------------+
 628             .               .
 629       .                             .
 630  .       [Dentry Block Structure: 4KB]       .
 631  +--------+----------+----------+------------+
 632  | bitmap | reserved | dentries | file names |
 633  +--------+----------+----------+------------+
 634  [Dentry Block: 4KB] .   .
 635                 .               .
 636            .                          .
 637            +------+------+-----+------+
 638            | hash | ino  | len | type |
 639            +------+------+-----+------+
 640            [Dentry Structure: 11 bytes]
 641
 642F2FS implements multi-level hash tables for directory structure. Each level has
 643a hash table with dedicated number of hash buckets as shown below. Note that
 644"A(2B)" means a bucket includes 2 data blocks.
 645
 646::
 647
 648    ----------------------
 649    A : bucket
 650    B : block
 651    N : MAX_DIR_HASH_DEPTH
 652    ----------------------
 653
 654    level #0   | A(2B)
 655            |
 656    level #1   | A(2B) - A(2B)
 657            |
 658    level #2   | A(2B) - A(2B) - A(2B) - A(2B)
 659        .     |   .       .       .       .
 660    level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
 661        .     |   .       .       .       .
 662    level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
 663
 664The number of blocks and buckets are determined by::
 665
 666                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
 667  # of blocks in level #n = |
 668                            `- 4, Otherwise
 669
 670                             ,- 2^(n + dir_level),
 671                             |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
 672  # of buckets in level #n = |
 673                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
 674                                      Otherwise
 675
 676When F2FS finds a file name in a directory, at first a hash value of the file
 677name is calculated. Then, F2FS scans the hash table in level #0 to find the
 678dentry consisting of the file name and its inode number. If not found, F2FS
 679scans the next hash table in level #1. In this way, F2FS scans hash tables in
 680each levels incrementally from 1 to N. In each level F2FS needs to scan only
 681one bucket determined by the following equation, which shows O(log(# of files))
 682complexity::
 683
 684  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
 685
 686In the case of file creation, F2FS finds empty consecutive slots that cover the
 687file name. F2FS searches the empty slots in the hash tables of whole levels from
 6881 to N in the same way as the lookup operation.
 689
 690The following figure shows an example of two cases holding children::
 691
 692       --------------> Dir <--------------
 693       |                                 |
 694    child                             child
 695
 696    child - child                     [hole] - child
 697
 698    child - child - child             [hole] - [hole] - child
 699
 700   Case 1:                           Case 2:
 701   Number of children = 6,           Number of children = 3,
 702   File size = 7                     File size = 7
 703
 704Default Block Allocation
 705------------------------
 706
 707At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
 708and Hot/Warm/Cold data.
 709
 710- Hot node      contains direct node blocks of directories.
 711- Warm node     contains direct node blocks except hot node blocks.
 712- Cold node     contains indirect node blocks
 713- Hot data      contains dentry blocks
 714- Warm data     contains data blocks except hot and cold data blocks
 715- Cold data     contains multimedia data or migrated data blocks
 716
 717LFS has two schemes for free space management: threaded log and copy-and-compac-
 718tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
 719for devices showing very good sequential write performance, since free segments
 720are served all the time for writing new data. However, it suffers from cleaning
 721overhead under high utilization. Contrarily, the threaded log scheme suffers
 722from random writes, but no cleaning process is needed. F2FS adopts a hybrid
 723scheme where the copy-and-compaction scheme is adopted by default, but the
 724policy is dynamically changed to the threaded log scheme according to the file
 725system status.
 726
 727In order to align F2FS with underlying flash-based storage, F2FS allocates a
 728segment in a unit of section. F2FS expects that the section size would be the
 729same as the unit size of garbage collection in FTL. Furthermore, with respect
 730to the mapping granularity in FTL, F2FS allocates each section of the active
 731logs from different zones as much as possible, since FTL can write the data in
 732the active logs into one allocation unit according to its mapping granularity.
 733
 734Cleaning process
 735----------------
 736
 737F2FS does cleaning both on demand and in the background. On-demand cleaning is
 738triggered when there are not enough free segments to serve VFS calls. Background
 739cleaner is operated by a kernel thread, and triggers the cleaning job when the
 740system is idle.
 741
 742F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
 743In the greedy algorithm, F2FS selects a victim segment having the smallest number
 744of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
 745according to the segment age and the number of valid blocks in order to address
 746log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
 747algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
 748algorithm.
 749
 750In order to identify whether the data in the victim segment are valid or not,
 751F2FS manages a bitmap. Each bit represents the validity of a block, and the
 752bitmap is composed of a bit stream covering whole blocks in main area.
 753
 754Write-hint Policy
 755-----------------
 756
 7571) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
 758
 7592) whint_mode=user-based. F2FS tries to pass down hints given by
 760users.
 761
 762===================== ======================== ===================
 763User                  F2FS                     Block
 764===================== ======================== ===================
 765N/A                   META                     WRITE_LIFE_NOT_SET
 766N/A                   HOT_NODE                 "
 767N/A                   WARM_NODE                "
 768N/A                   COLD_NODE                "
 769ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
 770extension list        "                        "
 771
 772-- buffered io
 773WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
 774WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
 775WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
 776WRITE_LIFE_NONE       "                        "
 777WRITE_LIFE_MEDIUM     "                        "
 778WRITE_LIFE_LONG       "                        "
 779
 780-- direct io
 781WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
 782WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
 783WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
 784WRITE_LIFE_NONE       "                        WRITE_LIFE_NONE
 785WRITE_LIFE_MEDIUM     "                        WRITE_LIFE_MEDIUM
 786WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG
 787===================== ======================== ===================
 788
 7893) whint_mode=fs-based. F2FS passes down hints with its policy.
 790
 791===================== ======================== ===================
 792User                  F2FS                     Block
 793===================== ======================== ===================
 794N/A                   META                     WRITE_LIFE_MEDIUM;
 795N/A                   HOT_NODE                 WRITE_LIFE_NOT_SET
 796N/A                   WARM_NODE                "
 797N/A                   COLD_NODE                WRITE_LIFE_NONE
 798ioctl(COLD)           COLD_DATA                WRITE_LIFE_EXTREME
 799extension list        "                        "
 800
 801-- buffered io
 802WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
 803WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
 804WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_LONG
 805WRITE_LIFE_NONE       "                        "
 806WRITE_LIFE_MEDIUM     "                        "
 807WRITE_LIFE_LONG       "                        "
 808
 809-- direct io
 810WRITE_LIFE_EXTREME    COLD_DATA                WRITE_LIFE_EXTREME
 811WRITE_LIFE_SHORT      HOT_DATA                 WRITE_LIFE_SHORT
 812WRITE_LIFE_NOT_SET    WARM_DATA                WRITE_LIFE_NOT_SET
 813WRITE_LIFE_NONE       "                        WRITE_LIFE_NONE
 814WRITE_LIFE_MEDIUM     "                        WRITE_LIFE_MEDIUM
 815WRITE_LIFE_LONG       "                        WRITE_LIFE_LONG
 816===================== ======================== ===================
 817
 818Fallocate(2) Policy
 819-------------------
 820
 821The default policy follows the below POSIX rule.
 822
 823Allocating disk space
 824    The default operation (i.e., mode is zero) of fallocate() allocates
 825    the disk space within the range specified by offset and len.  The
 826    file size (as reported by stat(2)) will be changed if offset+len is
 827    greater than the file size.  Any subregion within the range specified
 828    by offset and len that did not contain data before the call will be
 829    initialized to zero.  This default behavior closely resembles the
 830    behavior of the posix_fallocate(3) library function, and is intended
 831    as a method of optimally implementing that function.
 832
 833However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
 834fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
 835zero or random data, which is useful to the below scenario where:
 836
 837 1. create(fd)
 838 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
 839 3. fallocate(fd, 0, 0, size)
 840 4. address = fibmap(fd, offset)
 841 5. open(blkdev)
 842 6. write(blkdev, address)
 843
 844Compression implementation
 845--------------------------
 846
 847- New term named cluster is defined as basic unit of compression, file can
 848  be divided into multiple clusters logically. One cluster includes 4 << n
 849  (n >= 0) logical pages, compression size is also cluster size, each of
 850  cluster can be compressed or not.
 851
 852- In cluster metadata layout, one special block address is used to indicate
 853  a cluster is a compressed one or normal one; for compressed cluster, following
 854  metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
 855  stores data including compress header and compressed data.
 856
 857- In order to eliminate write amplification during overwrite, F2FS only
 858  support compression on write-once file, data can be compressed only when
 859  all logical blocks in cluster contain valid data and compress ratio of
 860  cluster data is lower than specified threshold.
 861
 862- To enable compression on regular inode, there are four ways:
 863
 864  * chattr +c file
 865  * chattr +c dir; touch dir/file
 866  * mount w/ -o compress_extension=ext; touch file.ext
 867  * mount w/ -o compress_extension=*; touch any_file
 868
 869- To disable compression on regular inode, there are two ways:
 870
 871  * chattr -c file
 872  * mount w/ -o nocompress_extension=ext; touch file.ext
 873
 874- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
 875
 876  * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
 877    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
 878    should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
 879    can enable compress on bar.zip.
 880  * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
 881    dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
 882    compresse, bar.zip and baz.txt should be non-compressed.
 883    chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
 884    and baz.txt.
 885
 886- At this point, compression feature doesn't expose compressed space to user
 887  directly in order to guarantee potential data updates later to the space.
 888  Instead, the main goal is to reduce data writes to flash disk as much as
 889  possible, resulting in extending disk life time as well as relaxing IO
 890  congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
 891  interface to reclaim compressed space and show it to user after putting the
 892  immutable bit. Immutable bit, after release, it doesn't allow writing/mmaping
 893  on the file, until reserving compressed space via
 894  ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or truncating filesize to zero.
 895
 896Compress metadata layout::
 897
 898                                [Dnode Structure]
 899                +-----------------------------------------------+
 900                | cluster 1 | cluster 2 | ......... | cluster N |
 901                +-----------------------------------------------+
 902                .           .                       .           .
 903        .                       .                .                      .
 904    .         Compressed Cluster       .        .        Normal Cluster            .
 905    +----------+---------+---------+---------+  +---------+---------+---------+---------+
 906    |compr flag| block 1 | block 2 | block 3 |  | block 1 | block 2 | block 3 | block 4 |
 907    +----------+---------+---------+---------+  +---------+---------+---------+---------+
 908            .                             .
 909            .                                           .
 910        .                                                           .
 911        +-------------+-------------+----------+----------------------------+
 912        | data length | data chksum | reserved |      compressed data       |
 913        +-------------+-------------+----------+----------------------------+
 914
 915Compression mode
 916--------------------------
 917
 918f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
 919With this option, f2fs provides a choice to select the way how to compress the
 920compression enabled files (refer to "Compression implementation" section for how to
 921enable compression on a regular inode).
 922
 9231) compress_mode=fs
 924This is the default option. f2fs does automatic compression in the writeback of the
 925compression enabled files.
 926
 9272) compress_mode=user
 928This disables the automatic compression and gives the user discretion of choosing the
 929target file and the timing. The user can do manual compression/decompression on the
 930compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
 931ioctls like the below.
 932
 933To decompress a file,
 934
 935fd = open(filename, O_WRONLY, 0);
 936ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
 937
 938To compress a file,
 939
 940fd = open(filename, O_WRONLY, 0);
 941ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
 942
 943NVMe Zoned Namespace devices
 944----------------------------
 945
 946- ZNS defines a per-zone capacity which can be equal or less than the
 947  zone-size. Zone-capacity is the number of usable blocks in the zone.
 948  F2FS checks if zone-capacity is less than zone-size, if it is, then any
 949  segment which starts after the zone-capacity is marked as not-free in
 950  the free segment bitmap at initial mount time. These segments are marked
 951  as permanently used so they are not allocated for writes and
 952  consequently are not needed to be garbage collected. In case the
 953  zone-capacity is not aligned to default segment size(2MB), then a segment
 954  can start before the zone-capacity and span across zone-capacity boundary.
 955  Such spanning segments are also considered as usable segments. All blocks
 956  past the zone-capacity are considered unusable in these segments.
 957