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 355 encrypted files using inline encryption hardware rather 356 than the CPU. For more details, see 357 Documentation/filesystems/fscrypt.rst. 358atgc Enable age-threshold garbage collection, it provides high 359 effectiveness and efficiency on background GC. 360discard_unit=%s Control discard unit, the argument can be "block", "segment" 361 and "section", issued discard command's offset/size will be 362 aligned to the unit, by default, "discard_unit=block" is set, 363 so that small discard functionality is enabled. 364 For blkzoned device, "discard_unit=section" will be set by 365 default, it is helpful for large sized SMR or ZNS devices to 366 reduce memory cost by getting rid of fs metadata supports small 367 discard. 368memory=%s Control memory mode. This supports "normal" and "low" modes. 369 "low" mode is introduced to support low memory devices. 370 Because of the nature of low memory devices, in this mode, f2fs 371 will try to save memory sometimes by sacrificing performance. 372 "normal" mode is the default mode and same as before. 373age_extent_cache Enable an age extent cache based on rb-tree. It records 374 data block update frequency of the extent per inode, in 375 order to provide better temperature hints for data block 376 allocation. 377errors=%s Specify f2fs behavior on critical errors. This supports modes: 378 "panic", "continue" and "remount-ro", respectively, trigger 379 panic immediately, continue without doing anything, and remount 380 the partition in read-only mode. By default it uses "continue" 381 mode. 382 383 .. code-block:: none 384 385 ====================== =============== =============== ======== 386 mode continue remount-ro panic 387 ====================== =============== =============== ======== 388 access ops normal normal N/A 389 syscall errors -EIO -EROFS N/A 390 mount option rw ro N/A 391 pending dir write keep keep N/A 392 pending non-dir write drop keep N/A 393 pending node write drop keep N/A 394 pending meta write keep keep N/A 395 ====================== =============== =============== ======== 396nat_bits Enable nat_bits feature to enhance full/empty nat blocks access, 397 by default it's disabled. 398lookup_mode=%s Control the directory lookup behavior for casefolded 399 directories. This option has no effect on directories 400 that do not have the casefold feature enabled. 401 402 .. code-block:: none 403 404 ================== ======================================== 405 Value Description 406 ================== ======================================== 407 perf (Default) Enforces a hash-only lookup. 408 The linear search fallback is always 409 disabled, ignoring the on-disk flag. 410 compat Enables the linear search fallback for 411 compatibility with directory entries 412 created by older kernel that used a 413 different case-folding algorithm. 414 This mode ignores the on-disk flag. 415 auto F2FS determines the mode based on the 416 on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL` 417 flag. 418resizable_tail_secno=%u Control the number of sections at the tail of the 419 filesystem reserved for online resizing. Pinned files 420 will only be allocated within sections 0 to 421 (MAIN_SECS - resizable_tail_secno) - 1. If set to 0 422 (default), there is no tail restriction unless running 423 on a zoned block device where conventional zones are 424 used. 425======================== ============================================================ 426 427Debugfs Entries 428=============== 429 430/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 431f2fs. Each file shows the whole f2fs information. 432 433/sys/kernel/debug/f2fs/status includes: 434 435 - major file system information managed by f2fs currently 436 - average SIT information about whole segments 437 - current memory footprint consumed by f2fs. 438 439Sysfs Entries 440============= 441 442Information about mounted f2fs file systems can be found in 443/sys/fs/f2fs. Each mounted filesystem will have a directory in 444/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 445The files in each per-device directory are shown in table below. 446 447Files in /sys/fs/f2fs/<devname> 448(see also Documentation/ABI/testing/sysfs-fs-f2fs) 449 450Usage 451===== 452 4531. Download userland tools and compile them. 454 4552. Skip, if f2fs was compiled statically inside kernel. 456 Otherwise, insert the f2fs.ko module:: 457 458 # insmod f2fs.ko 459 4603. Create a directory to use when mounting:: 461 462 # mkdir /mnt/f2fs 463 4644. Format the block device, and then mount as f2fs:: 465 466 # mkfs.f2fs -l label /dev/block_device 467 # mount -t f2fs /dev/block_device /mnt/f2fs 468 469mkfs.f2fs 470--------- 471The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 472which builds a basic on-disk layout. 473 474The quick options consist of: 475 476=============== =========================================================== 477``-l [label]`` Give a volume label, up to 512 unicode name. 478``-a [0 or 1]`` Split start location of each area for heap-based allocation. 479 480 1 is set by default, which performs this. 481``-o [int]`` Set overprovision ratio in percent over volume size. 482 483 5 is set by default. 484``-s [int]`` Set the number of segments per section. 485 486 1 is set by default. 487``-z [int]`` Set the number of sections per zone. 488 489 1 is set by default. 490``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 491``-t [0 or 1]`` Disable discard command or not. 492 493 1 is set by default, which conducts discard. 494=============== =========================================================== 495 496Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 497 498fsck.f2fs 499--------- 500The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 501partition, which examines whether the filesystem metadata and user-made data 502are cross-referenced correctly or not. 503Note that, initial version of the tool does not fix any inconsistency. 504 505The quick options consist of:: 506 507 -d debug level [default:0] 508 509Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 510 511dump.f2fs 512--------- 513The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 514file. Each file is dump_ssa and dump_sit. 515 516The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 517It shows on-disk inode information recognized by a given inode number, and is 518able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 519./dump_sit respectively. 520 521The options consist of:: 522 523 -d debug level [default:0] 524 -i inode no (hex) 525 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 526 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 527 528Examples:: 529 530 # dump.f2fs -i [ino] /dev/sdx 531 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 532 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 533 534Note: please refer to the manpage of dump.f2fs(8) to get full option list. 535 536sload.f2fs 537---------- 538The sload.f2fs gives a way to insert files and directories in the existing disk 539image. This tool is useful when building f2fs images given compiled files. 540 541Note: please refer to the manpage of sload.f2fs(8) to get full option list. 542 543resize.f2fs 544----------- 545The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 546all the files and directories stored in the image. 547 548Note: please refer to the manpage of resize.f2fs(8) to get full option list. 549 550defrag.f2fs 551----------- 552The defrag.f2fs can be used to defragment scattered written data as well as 553filesystem metadata across the disk. This can improve the write speed by giving 554more free consecutive space. 555 556Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 557 558f2fs_io 559------- 560The f2fs_io is a simple tool to issue various filesystem APIs as well as 561f2fs-specific ones, which is very useful for QA tests. 562 563Note: please refer to the manpage of f2fs_io(8) to get full option list. 564 565Design 566====== 567 568On-disk Layout 569-------------- 570 571F2FS divides the whole volume into a number of segments, each of which is fixed 572to 2MB in size. A section is composed of consecutive segments, and a zone 573consists of a set of sections. By default, section and zone sizes are set to one 574segment size identically, but users can easily modify the sizes by mkfs. 575 576F2FS splits the entire volume into six areas, and all the areas except superblock 577consist of multiple segments as described below:: 578 579 align with the zone size <-| 580 |-> align with the segment size 581 _________________________________________________________________________ 582 | | | Segment | Node | Segment | | 583 | Superblock | Checkpoint | Info. | Address | Summary | Main | 584 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 585 |____________|_____2______|______N______|______N______|______N_____|__N___| 586 . . 587 . . 588 . . 589 ._________________________________________. 590 |_Segment_|_..._|_Segment_|_..._|_Segment_| 591 . . 592 ._________._________ 593 |_section_|__...__|_ 594 . . 595 .________. 596 |__zone__| 597 598- Superblock (SB) 599 It is located at the beginning of the partition, and there exist two copies 600 to avoid file system crash. It contains basic partition information and some 601 default parameters of f2fs. 602 603- Checkpoint (CP) 604 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 605 inode lists, and summary entries of current active segments. 606 607- Segment Information Table (SIT) 608 It contains segment information such as valid block count and bitmap for the 609 validity of all the blocks. 610 611- Node Address Table (NAT) 612 It is composed of a block address table for all the node blocks stored in 613 Main area. 614 615- Segment Summary Area (SSA) 616 It contains summary entries which contains the owner information of all the 617 data and node blocks stored in Main area. 618 619- Main Area 620 It contains file and directory data including their indices. 621 622In order to avoid misalignment between file system and flash-based storage, F2FS 623aligns the start block address of CP with the segment size. Also, it aligns the 624start block address of Main area with the zone size by reserving some segments 625in SSA area. 626 627Reference the following survey for additional technical details. 628https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 629 630File System Metadata Structure 631------------------------------ 632 633F2FS adopts the checkpointing scheme to maintain file system consistency. At 634mount time, F2FS first tries to find the last valid checkpoint data by scanning 635CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 636One of them always indicates the last valid data, which is called as shadow copy 637mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 638 639For file system consistency, each CP points to which NAT and SIT copies are 640valid, as shown as below:: 641 642 +--------+----------+---------+ 643 | CP | SIT | NAT | 644 +--------+----------+---------+ 645 . . . . 646 . . . . 647 . . . . 648 +-------+-------+--------+--------+--------+--------+ 649 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 650 +-------+-------+--------+--------+--------+--------+ 651 | ^ ^ 652 | | | 653 `----------------------------------------' 654 655Index Structure 656--------------- 657 658The key data structure to manage the data locations is a "node". Similar to 659traditional file structures, F2FS has three types of node: inode, direct node, 660indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 661indices, two direct node pointers, two indirect node pointers, and one double 662indirect node pointer as described below. One direct node block contains 1018 663data blocks, and one indirect node block contains also 1018 node blocks. Thus, 664one inode block (i.e., a file) covers:: 665 666 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 667 668 Inode block (4KB) 669 |- data (923) 670 |- direct node (2) 671 | `- data (1018) 672 |- indirect node (2) 673 | `- direct node (1018) 674 | `- data (1018) 675 `- double indirect node (1) 676 `- indirect node (1018) 677 `- direct node (1018) 678 `- data (1018) 679 680Note that all the node blocks are mapped by NAT which means the location of 681each node is translated by the NAT table. In the consideration of the wandering 682tree problem, F2FS is able to cut off the propagation of node updates caused by 683leaf data writes. 684 685Directory Structure 686------------------- 687 688A directory entry occupies 11 bytes, which consists of the following attributes. 689 690- hash hash value of the file name 691- ino inode number 692- len the length of file name 693- type file type such as directory, symlink, etc 694 695A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 696used to represent whether each dentry is valid or not. A dentry block occupies 6974KB with the following composition. 698 699:: 700 701 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 702 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 703 704 [Bucket] 705 +--------------------------------+ 706 |dentry block 1 | dentry block 2 | 707 +--------------------------------+ 708 . . 709 . . 710 . [Dentry Block Structure: 4KB] . 711 +--------+----------+----------+------------+ 712 | bitmap | reserved | dentries | file names | 713 +--------+----------+----------+------------+ 714 [Dentry Block: 4KB] . . 715 . . 716 . . 717 +------+------+-----+------+ 718 | hash | ino | len | type | 719 +------+------+-----+------+ 720 [Dentry Structure: 11 bytes] 721 722F2FS implements multi-level hash tables for directory structure. Each level has 723a hash table with dedicated number of hash buckets as shown below. Note that 724"A(2B)" means a bucket includes 2 data blocks. 725 726:: 727 728 ---------------------- 729 A : bucket 730 B : block 731 N : MAX_DIR_HASH_DEPTH 732 ---------------------- 733 734 level #0 | A(2B) 735 | 736 level #1 | A(2B) - A(2B) 737 | 738 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 739 . | . . . . 740 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 741 . | . . . . 742 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 743 744The number of blocks and buckets are determined by:: 745 746 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 747 # of blocks in level #n = | 748 `- 4, Otherwise 749 750 ,- 2^(n + dir_level), 751 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 752 # of buckets in level #n = | 753 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 754 Otherwise 755 756When F2FS finds a file name in a directory, at first a hash value of the file 757name is calculated. Then, F2FS scans the hash table in level #0 to find the 758dentry consisting of the file name and its inode number. If not found, F2FS 759scans the next hash table in level #1. In this way, F2FS scans hash tables in 760each levels incrementally from 1 to N. In each level F2FS needs to scan only 761one bucket determined by the following equation, which shows O(log(# of files)) 762complexity:: 763 764 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 765 766In the case of file creation, F2FS finds empty consecutive slots that cover the 767file name. F2FS searches the empty slots in the hash tables of whole levels from 7681 to N in the same way as the lookup operation. 769 770The following figure shows an example of two cases holding children:: 771 772 --------------> Dir <-------------- 773 | | 774 child child 775 776 child - child [hole] - child 777 778 child - child - child [hole] - [hole] - child 779 780 Case 1: Case 2: 781 Number of children = 6, Number of children = 3, 782 File size = 7 File size = 7 783 784Default Block Allocation 785------------------------ 786 787At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 788and Hot/Warm/Cold data. 789 790- Hot node contains direct node blocks of directories. 791- Warm node contains direct node blocks except hot node blocks. 792- Cold node contains indirect node blocks 793- Hot data contains dentry blocks 794- Warm data contains data blocks except hot and cold data blocks 795- Cold data contains multimedia data or migrated data blocks 796 797LFS has two schemes for free space management: threaded log and copy-and-compac- 798tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 799for devices showing very good sequential write performance, since free segments 800are served all the time for writing new data. However, it suffers from cleaning 801overhead under high utilization. Contrarily, the threaded log scheme suffers 802from random writes, but no cleaning process is needed. F2FS adopts a hybrid 803scheme where the copy-and-compaction scheme is adopted by default, but the 804policy is dynamically changed to the threaded log scheme according to the file 805system status. 806 807In order to align F2FS with underlying flash-based storage, F2FS allocates a 808segment in a unit of section. F2FS expects that the section size would be the 809same as the unit size of garbage collection in FTL. Furthermore, with respect 810to the mapping granularity in FTL, F2FS allocates each section of the active 811logs from different zones as much as possible, since FTL can write the data in 812the active logs into one allocation unit according to its mapping granularity. 813 814Cleaning process 815---------------- 816 817F2FS does cleaning both on demand and in the background. On-demand cleaning is 818triggered when there are not enough free segments to serve VFS calls. Background 819cleaner is operated by a kernel thread, and triggers the cleaning job when the 820system is idle. 821 822F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 823In the greedy algorithm, F2FS selects a victim segment having the smallest number 824of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 825according to the segment age and the number of valid blocks in order to address 826log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 827algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 828algorithm. 829 830In order to identify whether the data in the victim segment are valid or not, 831F2FS manages a bitmap. Each bit represents the validity of a block, and the 832bitmap is composed of a bit stream covering whole blocks in main area. 833 834Write-hint Policy 835----------------- 836 837F2FS sets the whint all the time with the below policy. 838 839===================== ======================== =================== 840User F2FS Block 841===================== ======================== =================== 842N/A META WRITE_LIFE_NONE|REQ_META 843N/A HOT_NODE WRITE_LIFE_NONE 844N/A WARM_NODE WRITE_LIFE_MEDIUM 845N/A COLD_NODE WRITE_LIFE_LONG 846ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 847extension list " " 848 849-- buffered io 850------------------------------------------------------------------ 851N/A COLD_DATA WRITE_LIFE_EXTREME 852N/A HOT_DATA WRITE_LIFE_SHORT 853N/A WARM_DATA WRITE_LIFE_NOT_SET 854 855-- direct io 856------------------------------------------------------------------ 857WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 858WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 859WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 860WRITE_LIFE_NONE " WRITE_LIFE_NONE 861WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 862WRITE_LIFE_LONG " WRITE_LIFE_LONG 863===================== ======================== =================== 864 865Fallocate(2) Policy 866------------------- 867 868The default policy follows the below POSIX rule. 869 870Allocating disk space 871 The default operation (i.e., mode is zero) of fallocate() allocates 872 the disk space within the range specified by offset and len. The 873 file size (as reported by stat(2)) will be changed if offset+len is 874 greater than the file size. Any subregion within the range specified 875 by offset and len that did not contain data before the call will be 876 initialized to zero. This default behavior closely resembles the 877 behavior of the posix_fallocate(3) library function, and is intended 878 as a method of optimally implementing that function. 879 880However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 881fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having 882zero or random data, which is useful to the below scenario where: 883 884 1. create(fd) 885 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 886 3. fallocate(fd, 0, 0, size) 887 4. address = fibmap(fd, offset) 888 5. open(blkdev) 889 6. write(blkdev, address) 890 891Compression implementation 892-------------------------- 893 894- New term named cluster is defined as basic unit of compression, file can 895 be divided into multiple clusters logically. One cluster includes 4 << n 896 (n >= 0) logical pages, compression size is also cluster size, each of 897 cluster can be compressed or not. 898 899- In cluster metadata layout, one special block address is used to indicate 900 a cluster is a compressed one or normal one; for compressed cluster, following 901 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 902 stores data including compress header and compressed data. 903 904- In order to eliminate write amplification during overwrite, F2FS only 905 support compression on write-once file, data can be compressed only when 906 all logical blocks in cluster contain valid data and compress ratio of 907 cluster data is lower than specified threshold. 908 909- To enable compression on regular inode, there are four ways: 910 911 * chattr +c file 912 * chattr +c dir; touch dir/file 913 * mount w/ -o compress_extension=ext; touch file.ext 914 * mount w/ -o compress_extension=*; touch any_file 915 916- To disable compression on regular inode, there are two ways: 917 918 * chattr -c file 919 * mount w/ -o nocompress_extension=ext; touch file.ext 920 921- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 922 923 * 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 and baz.txt 925 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 926 can enable compress on bar.zip. 927 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 928 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 929 compresse, bar.zip and baz.txt should be non-compressed. 930 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 931 and baz.txt. 932 933- At this point, compression feature doesn't expose compressed space to user 934 directly in order to guarantee potential data updates later to the space. 935 Instead, the main goal is to reduce data writes to flash disk as much as 936 possible, resulting in extending disk life time as well as relaxing IO 937 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 938 interface to reclaim compressed space and show it to user after setting a 939 special flag to the inode. Once the compressed space is released, the flag 940 will block writing data to the file until either the compressed space is 941 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 942 truncated to zero. 943 944Compress metadata layout:: 945 946 [Dnode Structure] 947 +-----------------------------------------------+ 948 | cluster 1 | cluster 2 | ......... | cluster N | 949 +-----------------------------------------------+ 950 . . . . 951 . . . . 952 . Compressed Cluster . . Normal Cluster . 953 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 954 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 955 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 956 . . 957 . . 958 . . 959 +-------------+-------------+----------+----------------------------+ 960 | data length | data chksum | reserved | compressed data | 961 +-------------+-------------+----------+----------------------------+ 962 963Compression mode 964-------------------------- 965 966f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 967With this option, f2fs provides a choice to select the way how to compress the 968compression enabled files (refer to "Compression implementation" section for how to 969enable compression on a regular inode). 970 9711) compress_mode=fs 972 973 This is the default option. f2fs does automatic compression in the writeback of the 974 compression enabled files. 975 9762) compress_mode=user 977 978 This disables the automatic compression and gives the user discretion of choosing the 979 target file and the timing. The user can do manual compression/decompression on the 980 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 981 ioctls like the below. 982 983To decompress a file:: 984 985 fd = open(filename, O_WRONLY, 0); 986 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 987 988To compress a file:: 989 990 fd = open(filename, O_WRONLY, 0); 991 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 992 993NVMe Zoned Namespace devices 994---------------------------- 995 996- ZNS defines a per-zone capacity which can be equal or less than the 997 zone-size. Zone-capacity is the number of usable blocks in the zone. 998 F2FS checks if zone-capacity is less than zone-size, if it is, then any 999 segment which starts after the zone-capacity is marked as not-free in 1000 the free segment bitmap at initial mount time. These segments are marked 1001 as permanently used so they are not allocated for writes and 1002 consequently are not needed to be garbage collected. In case the 1003 zone-capacity is not aligned to default segment size(2MB), then a segment 1004 can start before the zone-capacity and span across zone-capacity boundary. 1005 Such spanning segments are also considered as usable segments. All blocks 1006 past the zone-capacity are considered unusable in these segments. 1007 1008Device aliasing feature 1009----------------------- 1010 1011f2fs can utilize a special file called a "device aliasing file." This file allows 1012the entire storage device to be mapped with a single, large extent, not using 1013the usual f2fs node structures. This mapped area is pinned and primarily intended 1014for holding the space. 1015 1016Essentially, this mechanism allows a portion of the f2fs area to be temporarily 1017reserved and used by another filesystem or for different purposes. Once that 1018external usage is complete, the device aliasing file can be deleted, releasing 1019the reserved space back to F2FS for its own use. 1020 1021.. code-block:: 1022 1023 # ls /dev/vd* 1024 /dev/vdb (32GB) /dev/vdc (32GB) 1025 # mkfs.ext4 /dev/vdc 1026 # mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb 1027 # mount /dev/vdb /mnt/f2fs 1028 # ls -l /mnt/f2fs 1029 vdc.file 1030 # df -h 1031 /dev/vdb 64G 33G 32G 52% /mnt/f2fs 1032 1033 # mount -o loop /dev/vdc /mnt/ext4 1034 # df -h 1035 /dev/vdb 64G 33G 32G 52% /mnt/f2fs 1036 /dev/loop7 32G 24K 30G 1% /mnt/ext4 1037 # umount /mnt/ext4 1038 1039 # f2fs_io getflags /mnt/f2fs/vdc.file 1040 get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable 1041 # f2fs_io setflags noimmutable /mnt/f2fs/vdc.file 1042 get a flag on noimmutable ret=0, flags=800010 1043 set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable 1044 # rm /mnt/f2fs/vdc.file 1045 # df -h 1046 /dev/vdb 64G 753M 64G 2% /mnt/f2fs 1047 1048So, the key idea is, user can do any file operations on /dev/vdc, and 1049reclaim the space after the use, while the space is counted as /data. 1050That doesn't require modifying partition size and filesystem format. 1051 1052Dynamic Device Aliasing Management 1053---------------------------------- 1054 1055In addition to static device aliasing by deleting the aliasing file, F2FS 1056supports dynamic management of device aliasing. This mechanism allows the system 1057to dynamically transition partition ownership between F2FS userdata and external 1058entities (e.g., zRAM, raw partition) based on system requirements without 1059deleting the master aliasing file or requiring unmount/remount. 1060 1061The master aliasing file is created during the initial format of the file system 1062and remains as a persistent control entity (ioctl gateway) in the root directory. 1063 1064- Partition Reservation (In-service to Aliased) 1065 When a specific partition needs to be dedicated to external services (e.g., zRAM), 1066 a user can reserve the device alias range via ioctl. The kernel resets GC victim 1067 information for the target range, marks segments as in-use to prevent new 1068 allocations, and triggers forced GC to migrate existing valid data out of the 1069 range. Finally, it reserves these blocks in the SIT to effectively exclude the 1070 device from the usable capacity. 1071 1072- Partition Release (Aliased to In-service) 1073 When external usage concludes, the space is reclaimed not by deleting the file, 1074 but through the release ioctl. The kernel truncates blocks associated with 1075 the file, releasing them back to general filesystem allocation. 1076 1077.. code-block:: 1078 1079 # f2fs_io dev_alias release /mnt/f2fs/vdc.file 1080 # df -h 1081 /dev/vdb 64G 753M 64G 2% /mnt/f2fs 1082 1083 # f2fs_io dev_alias reserve /mnt/f2fs/vdc.file 1084 # df -h 1085 /dev/vdb 64G 33G 32G 52% /mnt/f2fs 1086 1087Per-file Read-Only Large Folio Support 1088-------------------------------------- 1089 1090F2FS implements large folio support on the read path to leverage high-order 1091page allocation for significant performance gains. To minimize code complexity, 1092this support is currently excluded from the write path, which requires handling 1093complex optimizations such as compression and block allocation modes. 1094 1095This optional feature is triggered only when a file's immutable bit is set. 1096Consequently, F2FS will return EOPNOTSUPP if a user attempts to open a cached 1097file with write permissions, even immediately after clearing the bit. Write 1098access is only restored once the cached inode is dropped. The usage flow is 1099demonstrated below: 1100 1101.. code-block:: 1102 1103 # f2fs_io setflags immutable /data/testfile_read_seq 1104 1105 /* flush and reload the inode to enable the large folio */ 1106 # sync && echo 3 > /proc/sys/vm/drop_caches 1107 1108 /* mmap(MAP_POPULATE) + mlock() */ 1109 # f2fs_io read 128 0 1024 mmap 1 0 /data/testfile_read_seq 1110 1111 /* mmap() + fadvise(POSIX_FADV_WILLNEED) + mlock() */ 1112 # f2fs_io read 128 0 1024 fadvise 1 0 /data/testfile_read_seq 1113 1114 /* mmap() + mlock2(MLOCK_ONFAULT) + madvise(MADV_POPULATE_READ) */ 1115 # f2fs_io read 128 0 1024 madvise 1 0 /data/testfile_read_seq 1116 1117 # f2fs_io clearflags immutable /data/testfile_read_seq 1118 1119 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq 1120 Failed to open /mnt/test/test: Operation not supported 1121 1122 /* flush and reload the inode to disable the large folio */ 1123 # sync && echo 3 > /proc/sys/vm/drop_caches 1124 1125 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq 1126 Written 4096 bytes with pattern = zero, total_time = 29 us, max_latency = 28 us 1127 1128 # rm /data/testfile_read_seq 1129