1.. SPDX-License-Identifier: GPL-2.0 2 3================================= 4Flash-Friendly File System (F2FS) 5================================= 6 7Overview 8======== 9 10NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have 11been equipped on a variety systems ranging from mobile to server systems. Since 12they are known to have different characteristics from the conventional rotating 13disks, a file system, an upper layer to the storage device, should adapt to the 14changes from the sketch in the design level. 15 16F2FS is a file system exploiting NAND flash memory-based storage devices, which 17is based on Log-structured File System (LFS). The design has been focused on 18addressing the fundamental issues in LFS, which are snowball effect of wandering 19tree and high cleaning overhead. 20 21Since a NAND flash memory-based storage device shows different characteristic 22according to its internal geometry or flash memory management scheme, namely FTL, 23F2FS and its tools support various parameters not only for configuring on-disk 24layout, but also for selecting allocation and cleaning algorithms. 25 26The following git tree provides the file system formatting tool (mkfs.f2fs), 27a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs). 28 29- git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git 30 31For sending patches, please use the following mailing list: 32 33- linux-f2fs-devel@lists.sourceforge.net 34 35For reporting bugs, please use the following f2fs bug tracker link: 36 37- https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs 38 39Background and Design issues 40============================ 41 42Log-structured File System (LFS) 43-------------------------------- 44"A log-structured file system writes all modifications to disk sequentially in 45a log-like structure, thereby speeding up both file writing and crash recovery. 46The log is the only structure on disk; it contains indexing information so that 47files can be read back from the log efficiently. In order to maintain large free 48areas on disk for fast writing, we divide the log into segments and use a 49segment cleaner to compress the live information from heavily fragmented 50segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and 51implementation of a log-structured file system", ACM Trans. Computer Systems 5210, 1, 26–52. 53 54Wandering Tree Problem 55---------------------- 56In LFS, when a file data is updated and written to the end of log, its direct 57pointer block is updated due to the changed location. Then the indirect pointer 58block is also updated due to the direct pointer block update. In this manner, 59the upper index structures such as inode, inode map, and checkpoint block are 60also updated recursively. This problem is called as wandering tree problem [1], 61and in order to enhance the performance, it should eliminate or relax the update 62propagation as much as possible. 63 64[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ 65 66Cleaning Overhead 67----------------- 68Since LFS is based on out-of-place writes, it produces so many obsolete blocks 69scattered across the whole storage. In order to serve new empty log space, it 70needs to reclaim these obsolete blocks seamlessly to users. This job is called 71as a cleaning process. 72 73The process consists of three operations as follows. 74 751. A victim segment is selected through referencing segment usage table. 762. It loads parent index structures of all the data in the victim identified by 77 segment summary blocks. 783. It checks the cross-reference between the data and its parent index structure. 794. It moves valid data selectively. 80 81This cleaning job may cause unexpected long delays, so the most important goal 82is to hide the latencies to users. And also definitely, it should reduce the 83amount of valid data to be moved, and move them quickly as well. 84 85Key Features 86============ 87 88Flash Awareness 89--------------- 90- Enlarge the random write area for better performance, but provide the high 91 spatial locality 92- Align FS data structures to the operational units in FTL as best efforts 93 94Wandering Tree Problem 95---------------------- 96- Use a term, “node”, that represents inodes as well as various pointer blocks 97- Introduce Node Address Table (NAT) containing the locations of all the “node” 98 blocks; this will cut off the update propagation. 99 100Cleaning Overhead 101----------------- 102- Support a background cleaning process 103- Support greedy and cost-benefit algorithms for victim selection policies 104- Support multi-head logs for static/dynamic hot and cold data separation 105- Introduce adaptive logging for efficient block allocation 106 107Mount Options 108============= 109 110 111======================== ============================================================ 112background_gc=%s Turn on/off cleaning operations, namely garbage 113 collection, triggered in background when I/O subsystem is 114 idle. If background_gc=on, it will turn on the garbage 115 collection and if background_gc=off, garbage collection 116 will be turned off. If background_gc=sync, it will turn 117 on synchronous garbage collection running in background. 118 Default value for this option is on. So garbage 119 collection is on by default. 120gc_merge When background_gc is on, this option can be enabled to 121 let background GC thread to handle foreground GC requests, 122 it can eliminate the sluggish issue caused by slow foreground 123 GC operation when GC is triggered from a process with limited 124 I/O and CPU resources. 125nogc_merge Disable GC merge feature. 126disable_roll_forward Disable the roll-forward recovery routine 127norecovery Disable the roll-forward recovery routine, mounted read- 128 only (i.e., -o ro,disable_roll_forward) 129discard/nodiscard Enable/disable real-time discard in f2fs, if discard is 130 enabled, f2fs will issue discard/TRIM commands when a 131 segment is cleaned. 132heap/no_heap Deprecated. 133nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 134 by default if CONFIG_F2FS_FS_XATTR is selected. 135noacl Disable POSIX Access Control List. Note: acl is enabled 136 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 137active_logs=%u Support configuring the number of active logs. In the 138 current design, f2fs supports only 2, 4, and 6 logs. 139 Default number is 6. 140 When the underlying block device exposes write 141 streams, the default active_logs=6 configuration 142 maps hot, warm, and cold DATA writes to streams 1, 143 2, and 3, respectively. If only one or two write 144 streams are available, f2fs falls back to mapping 145 all DATA writes to stream 1 or mapping hot/warm 146 to stream 1 and cold to stream 2. If no write 147 streams are exposed, f2fs leaves the stream 148 unset. 149disable_ext_identify Disable the extension list configured by mkfs, so f2fs 150 is not aware of cold files such as media files. 151inline_xattr Enable the inline xattrs feature. 152noinline_xattr Disable the inline xattrs feature. 153inline_xattr_size=%u Support configuring inline xattr size, it depends on 154 flexible inline xattr feature. 155inline_data Enable the inline data feature: Newly created small (<~3.4k) 156 files can be written into inode block. 157inline_dentry Enable the inline dir feature: data in newly created 158 directory entries can be written into inode block. The 159 space of inode block which is used to store inline 160 dentries is limited to ~3.4k. 161noinline_dentry Disable the inline dentry feature. 162flush_merge Merge concurrent cache_flush commands as much as possible 163 to eliminate redundant command issues. If the underlying 164 device handles the cache_flush command relatively slowly, 165 recommend to enable this option. 166nobarrier This option can be used if underlying storage guarantees 167 its cached data should be written to the novolatile area. 168 If this option is set, no cache_flush commands are issued 169 but f2fs still guarantees the write ordering of all the 170 data writes. 171barrier If this option is set, cache_flush commands are allowed to be 172 issued. 173fastboot This option is used when a system wants to reduce mount 174 time as much as possible, even though normal performance 175 can be sacrificed. 176extent_cache Enable an extent cache based on rb-tree, it can cache 177 as many as extent which map between contiguous logical 178 address and physical address per inode, resulting in 179 increasing the cache hit ratio. Set by default. 180noextent_cache Disable an extent cache based on rb-tree explicitly, see 181 the above extent_cache mount option. 182noinline_data Disable the inline data feature, inline data feature is 183 enabled by default. 184data_flush Enable data flushing before checkpoint in order to 185 persist data of regular and symlink. 186reserve_root=%d Support configuring reserved space which is used for 187 allocation from a privileged user with specified uid or 188 gid, unit: 4KB, the default limit is 12.5% of user blocks. 189reserve_node=%d Support configuring reserved nodes which are used for 190 allocation from a privileged user with specified uid or 191 gid, the default limit is 12.5% of all nodes. 192resuid=%d The user ID which may use the reserved blocks and nodes. 193resgid=%d The group ID which may use the reserved blocks and nodes. 194fault_injection=%d Enable fault injection in all supported types with 195 specified injection rate. 196fault_type=%d Support configuring fault injection type, should be 197 enabled with fault_injection option, fault type value 198 is shown below, it supports single or combined type. 199 200 .. code-block:: none 201 202 =========================== ========== 203 Type_Name Type_Value 204 =========================== ========== 205 FAULT_KMALLOC 0x00000001 206 FAULT_KVMALLOC 0x00000002 207 FAULT_PAGE_ALLOC 0x00000004 208 FAULT_PAGE_GET 0x00000008 209 FAULT_ALLOC_BIO 0x00000010 (obsolete) 210 FAULT_ALLOC_NID 0x00000020 211 FAULT_ORPHAN 0x00000040 212 FAULT_BLOCK 0x00000080 213 FAULT_DIR_DEPTH 0x00000100 214 FAULT_EVICT_INODE 0x00000200 215 FAULT_TRUNCATE 0x00000400 216 FAULT_READ_IO 0x00000800 217 FAULT_CHECKPOINT 0x00001000 218 FAULT_DISCARD 0x00002000 (obsolete) 219 FAULT_WRITE_IO 0x00004000 220 FAULT_SLAB_ALLOC 0x00008000 221 FAULT_DQUOT_INIT 0x00010000 222 FAULT_LOCK_OP 0x00020000 223 FAULT_BLKADDR_VALIDITY 0x00040000 224 FAULT_BLKADDR_CONSISTENCE 0x00080000 225 FAULT_NO_SEGMENT 0x00100000 226 FAULT_INCONSISTENT_FOOTER 0x00200000 227 FAULT_ATOMIC_TIMEOUT 0x00400000 (1000ms) 228 FAULT_VMALLOC 0x00800000 229 FAULT_LOCK_TIMEOUT 0x01000000 (1000ms) 230 FAULT_SKIP_WRITE 0x02000000 231 =========================== ========== 232mode=%s Control block allocation mode which supports "adaptive" 233 and "lfs". In "lfs" mode, there should be no random 234 writes towards main area. 235 "fragment:segment" and "fragment:block" are newly added here. 236 These are developer options for experiments to simulate filesystem 237 fragmentation/after-GC situation itself. The developers use these 238 modes to understand filesystem fragmentation/after-GC condition well, 239 and eventually get some insights to handle them better. 240 In "fragment:segment", f2fs allocates a new segment in random 241 position. With this, we can simulate the after-GC condition. 242 In "fragment:block", we can scatter block allocation with 243 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. 244 We added some randomness to both chunk and hole size to make 245 it close to realistic IO pattern. So, in this mode, f2fs will allocate 246 1..<max_fragment_chunk> blocks in a chunk and make a hole in the 247 length of 1..<max_fragment_hole> by turns. With this, the newly 248 allocated blocks will be scattered throughout the whole partition. 249 Note that "fragment:block" implicitly enables "fragment:segment" 250 option for more randomness. 251 Please, use these options for your experiments and we strongly 252 recommend to re-format the filesystem after using these options. 253usrquota Enable plain user disk quota accounting. 254grpquota Enable plain group disk quota accounting. 255prjquota Enable plain project quota accounting. 256usrjquota=<file> Appoint specified file and type during mount, so that quota 257grpjquota=<file> information can be properly updated during recovery flow, 258prjjquota=<file> <quota file>: must be in root directory; 259jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 260usrjquota= Turn off user journalled quota. 261grpjquota= Turn off group journalled quota. 262prjjquota= Turn off project journalled quota. 263quota Enable plain user disk quota accounting. 264noquota Disable all plain disk quota option. 265alloc_mode=%s Adjust block allocation policy, which supports "reuse" 266 and "default". 267fsync_mode=%s Control the policy of fsync. Currently supports "posix", 268 "strict", and "nobarrier". In "posix" mode, which is 269 default, fsync will follow POSIX semantics and does a 270 light operation to improve the filesystem performance. 271 In "strict" mode, fsync will be heavy and behaves in line 272 with xfs, ext4 and btrfs, where xfstest generic/342 will 273 pass, but the performance will regress. "nobarrier" is 274 based on "posix", but doesn't issue flush command for 275 non-atomic files likewise "nobarrier" mount option. 276test_dummy_encryption 277test_dummy_encryption=%s 278 Enable dummy encryption, which provides a fake fscrypt 279 context. The fake fscrypt context is used by xfstests. 280 The argument may be either "v1" or "v2", in order to 281 select the corresponding fscrypt policy version. 282checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 283 to re-enable checkpointing. Is enabled by default. While 284 disabled, any unmounting or unexpected shutdowns will cause 285 the filesystem contents to appear as they did when the 286 filesystem was mounted with that option. 287 While mounting with checkpoint=disable, the filesystem must 288 run garbage collection to ensure that all available space can 289 be used. If this takes too much time, the mount may return 290 EAGAIN. You may optionally add a value to indicate how much 291 of the disk you would be willing to temporarily give up to 292 avoid additional garbage collection. This can be given as a 293 number of blocks, or as a percent. For instance, mounting 294 with checkpoint=disable:100% would always succeed, but it may 295 hide up to all remaining free space. The actual space that 296 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 297 This space is reclaimed once checkpoint=enable. 298checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 299 daemon and make it to merge concurrent checkpoint requests as 300 much as possible to eliminate redundant checkpoint issues. Plus, 301 we can eliminate the sluggish issue caused by slow checkpoint 302 operation when the checkpoint is done in a process context in 303 a cgroup having low i/o budget and cpu shares. To make this 304 do better, we set the default i/o priority of the kernel daemon 305 to "3", to give one higher priority than other kernel threads. 306 This is the same way to give a I/O priority to the jbd2 307 journaling thread of ext4 filesystem. 308nocheckpoint_merge Disable checkpoint merge feature. 309compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 310 "lz4", "zstd" and "lzo-rle" algorithm. 311compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 312 "lz4" and "zstd" support compress level config:: 313 314 ========= =========== 315 algorithm level range 316 ========= =========== 317 lz4 3 - 16 318 zstd 1 - 22 319 ========= =========== 320 321compress_log_size=%u Support configuring compress cluster size. The size will 322 be 4KB * (1 << %u). The default and minimum sizes are 16KB. 323compress_extension=%s Support adding specified extension, so that f2fs can enable 324 compression on those corresponding files, e.g. if all files 325 with '.ext' has high compression rate, we can set the '.ext' 326 on compression extension list and enable compression on 327 these file by default rather than to enable it via ioctl. 328 For other files, we can still enable compression via ioctl. 329 Note that, there is one reserved special extension '*', it 330 can be set to enable compression for all files. 331nocompress_extension=%s Support adding specified extension, so that f2fs can disable 332 compression on those corresponding files, just contrary to compression extension. 333 If you know exactly which files cannot be compressed, you can use this. 334 The same extension name can't appear in both compress and nocompress 335 extension at the same time. 336 If the compress extension specifies all files, the types specified by the 337 nocompress extension will be treated as special cases and will not be compressed. 338 Don't allow use '*' to specifie all file in nocompress extension. 339 After add nocompress_extension, the priority should be: 340 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 341 See more in compression sections. 342 343compress_chksum Support verifying chksum of raw data in compressed cluster. 344compress_mode=%s Control file compression mode. This supports "fs" and "user" 345 modes. In "fs" mode (default), f2fs does automatic compression 346 on the compression enabled files. In "user" mode, f2fs disables 347 the automaic compression and gives the user discretion of 348 choosing the target file and the timing. The user can do manual 349 compression/decompression on the compression enabled files using 350 ioctls. 351compress_cache Support to use address space of a filesystem managed inode to 352 cache compressed block, in order to improve cache hit ratio of 353 random read. 354inlinecrypt When possible, encrypt/decrypt the contents of encrypted 355 files using the blk-crypto framework rather than 356 filesystem-layer encryption. This allows the use of 357 inline encryption hardware. The on-disk format is 358 unaffected. For more details, see 359 Documentation/block/inline-encryption.rst. 360atgc Enable age-threshold garbage collection, it provides high 361 effectiveness and efficiency on background GC. 362discard_unit=%s Control discard unit, the argument can be "block", "segment" 363 and "section", issued discard command's offset/size will be 364 aligned to the unit, by default, "discard_unit=block" is set, 365 so that small discard functionality is enabled. 366 For blkzoned device, "discard_unit=section" will be set by 367 default, it is helpful for large sized SMR or ZNS devices to 368 reduce memory cost by getting rid of fs metadata supports small 369 discard. 370memory=%s Control memory mode. This supports "normal" and "low" modes. 371 "low" mode is introduced to support low memory devices. 372 Because of the nature of low memory devices, in this mode, f2fs 373 will try to save memory sometimes by sacrificing performance. 374 "normal" mode is the default mode and same as before. 375age_extent_cache Enable an age extent cache based on rb-tree. It records 376 data block update frequency of the extent per inode, in 377 order to provide better temperature hints for data block 378 allocation. 379errors=%s Specify f2fs behavior on critical errors. This supports modes: 380 "panic", "continue" and "remount-ro", respectively, trigger 381 panic immediately, continue without doing anything, and remount 382 the partition in read-only mode. By default it uses "continue" 383 mode. 384 385 .. code-block:: none 386 387 ====================== =============== =============== ======== 388 mode continue remount-ro panic 389 ====================== =============== =============== ======== 390 access ops normal normal N/A 391 syscall errors -EIO -EROFS N/A 392 mount option rw ro N/A 393 pending dir write keep keep N/A 394 pending non-dir write drop keep N/A 395 pending node write drop keep N/A 396 pending meta write keep keep N/A 397 ====================== =============== =============== ======== 398nat_bits Enable nat_bits feature to enhance full/empty nat blocks access, 399 by default it's disabled. 400lookup_mode=%s Control the directory lookup behavior for casefolded 401 directories. This option has no effect on directories 402 that do not have the casefold feature enabled. 403 404 .. code-block:: none 405 406 ================== ======================================== 407 Value Description 408 ================== ======================================== 409 perf (Default) Enforces a hash-only lookup. 410 The linear search fallback is always 411 disabled, ignoring the on-disk flag. 412 compat Enables the linear search fallback for 413 compatibility with directory entries 414 created by older kernel that used a 415 different case-folding algorithm. 416 This mode ignores the on-disk flag. 417 auto F2FS determines the mode based on the 418 on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL` 419 flag. 420 ================== ======================================== 421======================== ============================================================ 422 423Debugfs Entries 424=============== 425 426/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 427f2fs. Each file shows the whole f2fs information. 428 429/sys/kernel/debug/f2fs/status includes: 430 431 - major file system information managed by f2fs currently 432 - average SIT information about whole segments 433 - current memory footprint consumed by f2fs. 434 435Sysfs Entries 436============= 437 438Information about mounted f2fs file systems can be found in 439/sys/fs/f2fs. Each mounted filesystem will have a directory in 440/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 441The files in each per-device directory are shown in table below. 442 443Files in /sys/fs/f2fs/<devname> 444(see also Documentation/ABI/testing/sysfs-fs-f2fs) 445 446Usage 447===== 448 4491. Download userland tools and compile them. 450 4512. Skip, if f2fs was compiled statically inside kernel. 452 Otherwise, insert the f2fs.ko module:: 453 454 # insmod f2fs.ko 455 4563. Create a directory to use when mounting:: 457 458 # mkdir /mnt/f2fs 459 4604. Format the block device, and then mount as f2fs:: 461 462 # mkfs.f2fs -l label /dev/block_device 463 # mount -t f2fs /dev/block_device /mnt/f2fs 464 465mkfs.f2fs 466--------- 467The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 468which builds a basic on-disk layout. 469 470The quick options consist of: 471 472=============== =========================================================== 473``-l [label]`` Give a volume label, up to 512 unicode name. 474``-a [0 or 1]`` Split start location of each area for heap-based allocation. 475 476 1 is set by default, which performs this. 477``-o [int]`` Set overprovision ratio in percent over volume size. 478 479 5 is set by default. 480``-s [int]`` Set the number of segments per section. 481 482 1 is set by default. 483``-z [int]`` Set the number of sections per zone. 484 485 1 is set by default. 486``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 487``-t [0 or 1]`` Disable discard command or not. 488 489 1 is set by default, which conducts discard. 490=============== =========================================================== 491 492Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 493 494fsck.f2fs 495--------- 496The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 497partition, which examines whether the filesystem metadata and user-made data 498are cross-referenced correctly or not. 499Note that, initial version of the tool does not fix any inconsistency. 500 501The quick options consist of:: 502 503 -d debug level [default:0] 504 505Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 506 507dump.f2fs 508--------- 509The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 510file. Each file is dump_ssa and dump_sit. 511 512The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 513It shows on-disk inode information recognized by a given inode number, and is 514able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 515./dump_sit respectively. 516 517The options consist of:: 518 519 -d debug level [default:0] 520 -i inode no (hex) 521 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 522 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 523 524Examples:: 525 526 # dump.f2fs -i [ino] /dev/sdx 527 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 528 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 529 530Note: please refer to the manpage of dump.f2fs(8) to get full option list. 531 532sload.f2fs 533---------- 534The sload.f2fs gives a way to insert files and directories in the existing disk 535image. This tool is useful when building f2fs images given compiled files. 536 537Note: please refer to the manpage of sload.f2fs(8) to get full option list. 538 539resize.f2fs 540----------- 541The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 542all the files and directories stored in the image. 543 544Note: please refer to the manpage of resize.f2fs(8) to get full option list. 545 546defrag.f2fs 547----------- 548The defrag.f2fs can be used to defragment scattered written data as well as 549filesystem metadata across the disk. This can improve the write speed by giving 550more free consecutive space. 551 552Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 553 554f2fs_io 555------- 556The f2fs_io is a simple tool to issue various filesystem APIs as well as 557f2fs-specific ones, which is very useful for QA tests. 558 559Note: please refer to the manpage of f2fs_io(8) to get full option list. 560 561Design 562====== 563 564On-disk Layout 565-------------- 566 567F2FS divides the whole volume into a number of segments, each of which is fixed 568to 2MB in size. A section is composed of consecutive segments, and a zone 569consists of a set of sections. By default, section and zone sizes are set to one 570segment size identically, but users can easily modify the sizes by mkfs. 571 572F2FS splits the entire volume into six areas, and all the areas except superblock 573consist of multiple segments as described below:: 574 575 align with the zone size <-| 576 |-> align with the segment size 577 _________________________________________________________________________ 578 | | | Segment | Node | Segment | | 579 | Superblock | Checkpoint | Info. | Address | Summary | Main | 580 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 581 |____________|_____2______|______N______|______N______|______N_____|__N___| 582 . . 583 . . 584 . . 585 ._________________________________________. 586 |_Segment_|_..._|_Segment_|_..._|_Segment_| 587 . . 588 ._________._________ 589 |_section_|__...__|_ 590 . . 591 .________. 592 |__zone__| 593 594- Superblock (SB) 595 It is located at the beginning of the partition, and there exist two copies 596 to avoid file system crash. It contains basic partition information and some 597 default parameters of f2fs. 598 599- Checkpoint (CP) 600 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 601 inode lists, and summary entries of current active segments. 602 603- Segment Information Table (SIT) 604 It contains segment information such as valid block count and bitmap for the 605 validity of all the blocks. 606 607- Node Address Table (NAT) 608 It is composed of a block address table for all the node blocks stored in 609 Main area. 610 611- Segment Summary Area (SSA) 612 It contains summary entries which contains the owner information of all the 613 data and node blocks stored in Main area. 614 615- Main Area 616 It contains file and directory data including their indices. 617 618In order to avoid misalignment between file system and flash-based storage, F2FS 619aligns the start block address of CP with the segment size. Also, it aligns the 620start block address of Main area with the zone size by reserving some segments 621in SSA area. 622 623Reference the following survey for additional technical details. 624https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 625 626File System Metadata Structure 627------------------------------ 628 629F2FS adopts the checkpointing scheme to maintain file system consistency. At 630mount time, F2FS first tries to find the last valid checkpoint data by scanning 631CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 632One of them always indicates the last valid data, which is called as shadow copy 633mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 634 635For file system consistency, each CP points to which NAT and SIT copies are 636valid, as shown as below:: 637 638 +--------+----------+---------+ 639 | CP | SIT | NAT | 640 +--------+----------+---------+ 641 . . . . 642 . . . . 643 . . . . 644 +-------+-------+--------+--------+--------+--------+ 645 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 646 +-------+-------+--------+--------+--------+--------+ 647 | ^ ^ 648 | | | 649 `----------------------------------------' 650 651Index Structure 652--------------- 653 654The key data structure to manage the data locations is a "node". Similar to 655traditional file structures, F2FS has three types of node: inode, direct node, 656indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 657indices, two direct node pointers, two indirect node pointers, and one double 658indirect node pointer as described below. One direct node block contains 1018 659data blocks, and one indirect node block contains also 1018 node blocks. Thus, 660one inode block (i.e., a file) covers:: 661 662 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 663 664 Inode block (4KB) 665 |- data (923) 666 |- direct node (2) 667 | `- data (1018) 668 |- indirect node (2) 669 | `- direct node (1018) 670 | `- data (1018) 671 `- double indirect node (1) 672 `- indirect node (1018) 673 `- direct node (1018) 674 `- data (1018) 675 676Note that all the node blocks are mapped by NAT which means the location of 677each node is translated by the NAT table. In the consideration of the wandering 678tree problem, F2FS is able to cut off the propagation of node updates caused by 679leaf data writes. 680 681Directory Structure 682------------------- 683 684A directory entry occupies 11 bytes, which consists of the following attributes. 685 686- hash hash value of the file name 687- ino inode number 688- len the length of file name 689- type file type such as directory, symlink, etc 690 691A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 692used to represent whether each dentry is valid or not. A dentry block occupies 6934KB with the following composition. 694 695:: 696 697 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 698 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 699 700 [Bucket] 701 +--------------------------------+ 702 |dentry block 1 | dentry block 2 | 703 +--------------------------------+ 704 . . 705 . . 706 . [Dentry Block Structure: 4KB] . 707 +--------+----------+----------+------------+ 708 | bitmap | reserved | dentries | file names | 709 +--------+----------+----------+------------+ 710 [Dentry Block: 4KB] . . 711 . . 712 . . 713 +------+------+-----+------+ 714 | hash | ino | len | type | 715 +------+------+-----+------+ 716 [Dentry Structure: 11 bytes] 717 718F2FS implements multi-level hash tables for directory structure. Each level has 719a hash table with dedicated number of hash buckets as shown below. Note that 720"A(2B)" means a bucket includes 2 data blocks. 721 722:: 723 724 ---------------------- 725 A : bucket 726 B : block 727 N : MAX_DIR_HASH_DEPTH 728 ---------------------- 729 730 level #0 | A(2B) 731 | 732 level #1 | A(2B) - A(2B) 733 | 734 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 735 . | . . . . 736 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 737 . | . . . . 738 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 739 740The number of blocks and buckets are determined by:: 741 742 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 743 # of blocks in level #n = | 744 `- 4, Otherwise 745 746 ,- 2^(n + dir_level), 747 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 748 # of buckets in level #n = | 749 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 750 Otherwise 751 752When F2FS finds a file name in a directory, at first a hash value of the file 753name is calculated. Then, F2FS scans the hash table in level #0 to find the 754dentry consisting of the file name and its inode number. If not found, F2FS 755scans the next hash table in level #1. In this way, F2FS scans hash tables in 756each levels incrementally from 1 to N. In each level F2FS needs to scan only 757one bucket determined by the following equation, which shows O(log(# of files)) 758complexity:: 759 760 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 761 762In the case of file creation, F2FS finds empty consecutive slots that cover the 763file name. F2FS searches the empty slots in the hash tables of whole levels from 7641 to N in the same way as the lookup operation. 765 766The following figure shows an example of two cases holding children:: 767 768 --------------> Dir <-------------- 769 | | 770 child child 771 772 child - child [hole] - child 773 774 child - child - child [hole] - [hole] - child 775 776 Case 1: Case 2: 777 Number of children = 6, Number of children = 3, 778 File size = 7 File size = 7 779 780Default Block Allocation 781------------------------ 782 783At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 784and Hot/Warm/Cold data. 785 786- Hot node contains direct node blocks of directories. 787- Warm node contains direct node blocks except hot node blocks. 788- Cold node contains indirect node blocks 789- Hot data contains dentry blocks 790- Warm data contains data blocks except hot and cold data blocks 791- Cold data contains multimedia data or migrated data blocks 792 793LFS has two schemes for free space management: threaded log and copy-and-compac- 794tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 795for devices showing very good sequential write performance, since free segments 796are served all the time for writing new data. However, it suffers from cleaning 797overhead under high utilization. Contrarily, the threaded log scheme suffers 798from random writes, but no cleaning process is needed. F2FS adopts a hybrid 799scheme where the copy-and-compaction scheme is adopted by default, but the 800policy is dynamically changed to the threaded log scheme according to the file 801system status. 802 803In order to align F2FS with underlying flash-based storage, F2FS allocates a 804segment in a unit of section. F2FS expects that the section size would be the 805same as the unit size of garbage collection in FTL. Furthermore, with respect 806to the mapping granularity in FTL, F2FS allocates each section of the active 807logs from different zones as much as possible, since FTL can write the data in 808the active logs into one allocation unit according to its mapping granularity. 809 810Cleaning process 811---------------- 812 813F2FS does cleaning both on demand and in the background. On-demand cleaning is 814triggered when there are not enough free segments to serve VFS calls. Background 815cleaner is operated by a kernel thread, and triggers the cleaning job when the 816system is idle. 817 818F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 819In the greedy algorithm, F2FS selects a victim segment having the smallest number 820of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 821according to the segment age and the number of valid blocks in order to address 822log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 823algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 824algorithm. 825 826In order to identify whether the data in the victim segment are valid or not, 827F2FS manages a bitmap. Each bit represents the validity of a block, and the 828bitmap is composed of a bit stream covering whole blocks in main area. 829 830Write-hint Policy 831----------------- 832 833F2FS sets the whint all the time with the below policy. 834 835===================== ======================== =================== 836User F2FS Block 837===================== ======================== =================== 838N/A META WRITE_LIFE_NONE|REQ_META 839N/A HOT_NODE WRITE_LIFE_NONE 840N/A WARM_NODE WRITE_LIFE_MEDIUM 841N/A COLD_NODE WRITE_LIFE_LONG 842ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 843extension list " " 844 845-- buffered io 846------------------------------------------------------------------ 847N/A COLD_DATA WRITE_LIFE_EXTREME 848N/A HOT_DATA WRITE_LIFE_SHORT 849N/A WARM_DATA WRITE_LIFE_NOT_SET 850 851-- direct io 852------------------------------------------------------------------ 853WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 854WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 855WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 856WRITE_LIFE_NONE " WRITE_LIFE_NONE 857WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 858WRITE_LIFE_LONG " WRITE_LIFE_LONG 859===================== ======================== =================== 860 861Fallocate(2) Policy 862------------------- 863 864The default policy follows the below POSIX rule. 865 866Allocating disk space 867 The default operation (i.e., mode is zero) of fallocate() allocates 868 the disk space within the range specified by offset and len. The 869 file size (as reported by stat(2)) will be changed if offset+len is 870 greater than the file size. Any subregion within the range specified 871 by offset and len that did not contain data before the call will be 872 initialized to zero. This default behavior closely resembles the 873 behavior of the posix_fallocate(3) library function, and is intended 874 as a method of optimally implementing that function. 875 876However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 877fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having 878zero or random data, which is useful to the below scenario where: 879 880 1. create(fd) 881 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 882 3. fallocate(fd, 0, 0, size) 883 4. address = fibmap(fd, offset) 884 5. open(blkdev) 885 6. write(blkdev, address) 886 887Compression implementation 888-------------------------- 889 890- New term named cluster is defined as basic unit of compression, file can 891 be divided into multiple clusters logically. One cluster includes 4 << n 892 (n >= 0) logical pages, compression size is also cluster size, each of 893 cluster can be compressed or not. 894 895- In cluster metadata layout, one special block address is used to indicate 896 a cluster is a compressed one or normal one; for compressed cluster, following 897 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 898 stores data including compress header and compressed data. 899 900- In order to eliminate write amplification during overwrite, F2FS only 901 support compression on write-once file, data can be compressed only when 902 all logical blocks in cluster contain valid data and compress ratio of 903 cluster data is lower than specified threshold. 904 905- To enable compression on regular inode, there are four ways: 906 907 * chattr +c file 908 * chattr +c dir; touch dir/file 909 * mount w/ -o compress_extension=ext; touch file.ext 910 * mount w/ -o compress_extension=*; touch any_file 911 912- To disable compression on regular inode, there are two ways: 913 914 * chattr -c file 915 * mount w/ -o nocompress_extension=ext; touch file.ext 916 917- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 918 919 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 920 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 921 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 922 can enable compress on bar.zip. 923 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 924 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 925 compresse, bar.zip and baz.txt should be non-compressed. 926 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 927 and baz.txt. 928 929- At this point, compression feature doesn't expose compressed space to user 930 directly in order to guarantee potential data updates later to the space. 931 Instead, the main goal is to reduce data writes to flash disk as much as 932 possible, resulting in extending disk life time as well as relaxing IO 933 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 934 interface to reclaim compressed space and show it to user after setting a 935 special flag to the inode. Once the compressed space is released, the flag 936 will block writing data to the file until either the compressed space is 937 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 938 truncated to zero. 939 940Compress metadata layout:: 941 942 [Dnode Structure] 943 +-----------------------------------------------+ 944 | cluster 1 | cluster 2 | ......... | cluster N | 945 +-----------------------------------------------+ 946 . . . . 947 . . . . 948 . Compressed Cluster . . Normal Cluster . 949 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 950 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 951 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 952 . . 953 . . 954 . . 955 +-------------+-------------+----------+----------------------------+ 956 | data length | data chksum | reserved | compressed data | 957 +-------------+-------------+----------+----------------------------+ 958 959Compression mode 960-------------------------- 961 962f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 963With this option, f2fs provides a choice to select the way how to compress the 964compression enabled files (refer to "Compression implementation" section for how to 965enable compression on a regular inode). 966 9671) compress_mode=fs 968 969 This is the default option. f2fs does automatic compression in the writeback of the 970 compression enabled files. 971 9722) compress_mode=user 973 974 This disables the automatic compression and gives the user discretion of choosing the 975 target file and the timing. The user can do manual compression/decompression on the 976 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 977 ioctls like the below. 978 979To decompress a file:: 980 981 fd = open(filename, O_WRONLY, 0); 982 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 983 984To compress a file:: 985 986 fd = open(filename, O_WRONLY, 0); 987 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 988 989NVMe Zoned Namespace devices 990---------------------------- 991 992- ZNS defines a per-zone capacity which can be equal or less than the 993 zone-size. Zone-capacity is the number of usable blocks in the zone. 994 F2FS checks if zone-capacity is less than zone-size, if it is, then any 995 segment which starts after the zone-capacity is marked as not-free in 996 the free segment bitmap at initial mount time. These segments are marked 997 as permanently used so they are not allocated for writes and 998 consequently are not needed to be garbage collected. In case the 999 zone-capacity is not aligned to default segment size(2MB), then a segment 1000 can start before the zone-capacity and span across zone-capacity boundary. 1001 Such spanning segments are also considered as usable segments. All blocks 1002 past the zone-capacity are considered unusable in these segments. 1003 1004Device aliasing feature 1005----------------------- 1006 1007f2fs can utilize a special file called a "device aliasing file." This file allows 1008the entire storage device to be mapped with a single, large extent, not using 1009the usual f2fs node structures. This mapped area is pinned and primarily intended 1010for holding the space. 1011 1012Essentially, this mechanism allows a portion of the f2fs area to be temporarily 1013reserved and used by another filesystem or for different purposes. Once that 1014external usage is complete, the device aliasing file can be deleted, releasing 1015the reserved space back to F2FS for its own use. 1016 1017.. code-block:: 1018 1019 # ls /dev/vd* 1020 /dev/vdb (32GB) /dev/vdc (32GB) 1021 # mkfs.ext4 /dev/vdc 1022 # mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb 1023 # mount /dev/vdb /mnt/f2fs 1024 # ls -l /mnt/f2fs 1025 vdc.file 1026 # df -h 1027 /dev/vdb 64G 33G 32G 52% /mnt/f2fs 1028 1029 # mount -o loop /dev/vdc /mnt/ext4 1030 # df -h 1031 /dev/vdb 64G 33G 32G 52% /mnt/f2fs 1032 /dev/loop7 32G 24K 30G 1% /mnt/ext4 1033 # umount /mnt/ext4 1034 1035 # f2fs_io getflags /mnt/f2fs/vdc.file 1036 get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable 1037 # f2fs_io setflags noimmutable /mnt/f2fs/vdc.file 1038 get a flag on noimmutable ret=0, flags=800010 1039 set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable 1040 # rm /mnt/f2fs/vdc.file 1041 # df -h 1042 /dev/vdb 64G 753M 64G 2% /mnt/f2fs 1043 1044So, the key idea is, user can do any file operations on /dev/vdc, and 1045reclaim the space after the use, while the space is counted as /data. 1046That doesn't require modifying partition size and filesystem format. 1047 1048Per-file Read-Only Large Folio Support 1049-------------------------------------- 1050 1051F2FS implements large folio support on the read path to leverage high-order 1052page allocation for significant performance gains. To minimize code complexity, 1053this support is currently excluded from the write path, which requires handling 1054complex optimizations such as compression and block allocation modes. 1055 1056This optional feature is triggered only when a file's immutable bit is set. 1057Consequently, F2FS will return EOPNOTSUPP if a user attempts to open a cached 1058file with write permissions, even immediately after clearing the bit. Write 1059access is only restored once the cached inode is dropped. The usage flow is 1060demonstrated below: 1061 1062.. code-block:: 1063 1064 # f2fs_io setflags immutable /data/testfile_read_seq 1065 1066 /* flush and reload the inode to enable the large folio */ 1067 # sync && echo 3 > /proc/sys/vm/drop_caches 1068 1069 /* mmap(MAP_POPULATE) + mlock() */ 1070 # f2fs_io read 128 0 1024 mmap 1 0 /data/testfile_read_seq 1071 1072 /* mmap() + fadvise(POSIX_FADV_WILLNEED) + mlock() */ 1073 # f2fs_io read 128 0 1024 fadvise 1 0 /data/testfile_read_seq 1074 1075 /* mmap() + mlock2(MLOCK_ONFAULT) + madvise(MADV_POPULATE_READ) */ 1076 # f2fs_io read 128 0 1024 madvise 1 0 /data/testfile_read_seq 1077 1078 # f2fs_io clearflags immutable /data/testfile_read_seq 1079 1080 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq 1081 Failed to open /mnt/test/test: Operation not supported 1082 1083 /* flush and reload the inode to disable the large folio */ 1084 # sync && echo 3 > /proc/sys/vm/drop_caches 1085 1086 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq 1087 Written 4096 bytes with pattern = zero, total_time = 29 us, max_latency = 28 us 1088 1089 # rm /data/testfile_read_seq 1090