1.. SPDX-License-Identifier: GPL-2.0 2 3========================================== 4WHAT IS Flash-Friendly File System (F2FS)? 5========================================== 6 7NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have 8been equipped on a variety systems ranging from mobile to server systems. Since 9they are known to have different characteristics from the conventional rotating 10disks, a file system, an upper layer to the storage device, should adapt to the 11changes from the sketch in the design level. 12 13F2FS is a file system exploiting NAND flash memory-based storage devices, which 14is based on Log-structured File System (LFS). The design has been focused on 15addressing the fundamental issues in LFS, which are snowball effect of wandering 16tree and high cleaning overhead. 17 18Since a NAND flash memory-based storage device shows different characteristic 19according to its internal geometry or flash memory management scheme, namely FTL, 20F2FS and its tools support various parameters not only for configuring on-disk 21layout, but also for selecting allocation and cleaning algorithms. 22 23The following git tree provides the file system formatting tool (mkfs.f2fs), 24a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs). 25 26- git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git 27 28For sending patches, please use the following mailing list: 29 30- linux-f2fs-devel@lists.sourceforge.net 31 32For reporting bugs, please use the following f2fs bug tracker link: 33 34- https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs 35 36Background and Design issues 37============================ 38 39Log-structured File System (LFS) 40-------------------------------- 41"A log-structured file system writes all modifications to disk sequentially in 42a log-like structure, thereby speeding up both file writing and crash recovery. 43The log is the only structure on disk; it contains indexing information so that 44files can be read back from the log efficiently. In order to maintain large free 45areas on disk for fast writing, we divide the log into segments and use a 46segment cleaner to compress the live information from heavily fragmented 47segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and 48implementation of a log-structured file system", ACM Trans. Computer Systems 4910, 1, 26���52. 50 51Wandering Tree Problem 52---------------------- 53In LFS, when a file data is updated and written to the end of log, its direct 54pointer block is updated due to the changed location. Then the indirect pointer 55block is also updated due to the direct pointer block update. In this manner, 56the upper index structures such as inode, inode map, and checkpoint block are 57also updated recursively. This problem is called as wandering tree problem [1], 58and in order to enhance the performance, it should eliminate or relax the update 59propagation as much as possible. 60 61[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ 62 63Cleaning Overhead 64----------------- 65Since LFS is based on out-of-place writes, it produces so many obsolete blocks 66scattered across the whole storage. In order to serve new empty log space, it 67needs to reclaim these obsolete blocks seamlessly to users. This job is called 68as a cleaning process. 69 70The process consists of three operations as follows. 71 721. A victim segment is selected through referencing segment usage table. 732. It loads parent index structures of all the data in the victim identified by 74 segment summary blocks. 753. It checks the cross-reference between the data and its parent index structure. 764. It moves valid data selectively. 77 78This cleaning job may cause unexpected long delays, so the most important goal 79is to hide the latencies to users. And also definitely, it should reduce the 80amount of valid data to be moved, and move them quickly as well. 81 82Key Features 83============ 84 85Flash Awareness 86--------------- 87- Enlarge the random write area for better performance, but provide the high 88 spatial locality 89- Align FS data structures to the operational units in FTL as best efforts 90 91Wandering Tree Problem 92---------------------- 93- Use a term, ���node���, that represents inodes as well as various pointer blocks 94- Introduce Node Address Table (NAT) containing the locations of all the ���node��� 95 blocks; this will cut off the update propagation. 96 97Cleaning Overhead 98----------------- 99- Support a background cleaning process 100- Support greedy and cost-benefit algorithms for victim selection policies 101- Support multi-head logs for static/dynamic hot and cold data separation 102- Introduce adaptive logging for efficient block allocation 103 104Mount Options 105============= 106 107 108======================== ============================================================ 109background_gc=%s Turn on/off cleaning operations, namely garbage 110 collection, triggered in background when I/O subsystem is 111 idle. If background_gc=on, it will turn on the garbage 112 collection and if background_gc=off, garbage collection 113 will be turned off. If background_gc=sync, it will turn 114 on synchronous garbage collection running in background. 115 Default value for this option is on. So garbage 116 collection is on by default. 117gc_merge When background_gc is on, this option can be enabled to 118 let background GC thread to handle foreground GC requests, 119 it can eliminate the sluggish issue caused by slow foreground 120 GC operation when GC is triggered from a process with limited 121 I/O and CPU resources. 122nogc_merge Disable GC merge feature. 123disable_roll_forward Disable the roll-forward recovery routine 124norecovery Disable the roll-forward recovery routine, mounted read- 125 only (i.e., -o ro,disable_roll_forward) 126discard/nodiscard Enable/disable real-time discard in f2fs, if discard is 127 enabled, f2fs will issue discard/TRIM commands when a 128 segment is cleaned. 129heap/no_heap Deprecated. 130nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 131 by default if CONFIG_F2FS_FS_XATTR is selected. 132noacl Disable POSIX Access Control List. Note: acl is enabled 133 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 134active_logs=%u Support configuring the number of active logs. In the 135 current design, f2fs supports only 2, 4, and 6 logs. 136 Default number is 6. 137disable_ext_identify Disable the extension list configured by mkfs, so f2fs 138 is not aware of cold files such as media files. 139inline_xattr Enable the inline xattrs feature. 140noinline_xattr Disable the inline xattrs feature. 141inline_xattr_size=%u Support configuring inline xattr size, it depends on 142 flexible inline xattr feature. 143inline_data Enable the inline data feature: Newly created small (<~3.4k) 144 files can be written into inode block. 145inline_dentry Enable the inline dir feature: data in newly created 146 directory entries can be written into inode block. The 147 space of inode block which is used to store inline 148 dentries is limited to ~3.4k. 149noinline_dentry Disable the inline dentry feature. 150flush_merge Merge concurrent cache_flush commands as much as possible 151 to eliminate redundant command issues. If the underlying 152 device handles the cache_flush command relatively slowly, 153 recommend to enable this option. 154nobarrier This option can be used if underlying storage guarantees 155 its cached data should be written to the novolatile area. 156 If this option is set, no cache_flush commands are issued 157 but f2fs still guarantees the write ordering of all the 158 data writes. 159barrier If this option is set, cache_flush commands are allowed to be 160 issued. 161fastboot This option is used when a system wants to reduce mount 162 time as much as possible, even though normal performance 163 can be sacrificed. 164extent_cache Enable an extent cache based on rb-tree, it can cache 165 as many as extent which map between contiguous logical 166 address and physical address per inode, resulting in 167 increasing the cache hit ratio. Set by default. 168noextent_cache Disable an extent cache based on rb-tree explicitly, see 169 the above extent_cache mount option. 170noinline_data Disable the inline data feature, inline data feature is 171 enabled by default. 172data_flush Enable data flushing before checkpoint in order to 173 persist data of regular and symlink. 174reserve_root=%d Support configuring reserved space which is used for 175 allocation from a privileged user with specified uid or 176 gid, unit: 4KB, the default limit is 0.2% of user blocks. 177resuid=%d The user ID which may use the reserved blocks. 178resgid=%d The group ID which may use the reserved blocks. 179fault_injection=%d Enable fault injection in all supported types with 180 specified injection rate. 181fault_type=%d Support configuring fault injection type, should be 182 enabled with fault_injection option, fault type value 183 is shown below, it supports single or combined type. 184 185 =========================== =========== 186 Type_Name Type_Value 187 =========================== =========== 188 FAULT_KMALLOC 0x000000001 189 FAULT_KVMALLOC 0x000000002 190 FAULT_PAGE_ALLOC 0x000000004 191 FAULT_PAGE_GET 0x000000008 192 FAULT_ALLOC_BIO 0x000000010 (obsolete) 193 FAULT_ALLOC_NID 0x000000020 194 FAULT_ORPHAN 0x000000040 195 FAULT_BLOCK 0x000000080 196 FAULT_DIR_DEPTH 0x000000100 197 FAULT_EVICT_INODE 0x000000200 198 FAULT_TRUNCATE 0x000000400 199 FAULT_READ_IO 0x000000800 200 FAULT_CHECKPOINT 0x000001000 201 FAULT_DISCARD 0x000002000 202 FAULT_WRITE_IO 0x000004000 203 FAULT_SLAB_ALLOC 0x000008000 204 FAULT_DQUOT_INIT 0x000010000 205 FAULT_LOCK_OP 0x000020000 206 FAULT_BLKADDR_VALIDITY 0x000040000 207 FAULT_BLKADDR_CONSISTENCE 0x000080000 208 FAULT_NO_SEGMENT 0x000100000 209 =========================== =========== 210mode=%s Control block allocation mode which supports "adaptive" 211 and "lfs". In "lfs" mode, there should be no random 212 writes towards main area. 213 "fragment:segment" and "fragment:block" are newly added here. 214 These are developer options for experiments to simulate filesystem 215 fragmentation/after-GC situation itself. The developers use these 216 modes to understand filesystem fragmentation/after-GC condition well, 217 and eventually get some insights to handle them better. 218 In "fragment:segment", f2fs allocates a new segment in ramdom 219 position. With this, we can simulate the after-GC condition. 220 In "fragment:block", we can scatter block allocation with 221 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. 222 We added some randomness to both chunk and hole size to make 223 it close to realistic IO pattern. So, in this mode, f2fs will allocate 224 1..<max_fragment_chunk> blocks in a chunk and make a hole in the 225 length of 1..<max_fragment_hole> by turns. With this, the newly 226 allocated blocks will be scattered throughout the whole partition. 227 Note that "fragment:block" implicitly enables "fragment:segment" 228 option for more randomness. 229 Please, use these options for your experiments and we strongly 230 recommend to re-format the filesystem after using these options. 231usrquota Enable plain user disk quota accounting. 232grpquota Enable plain group disk quota accounting. 233prjquota Enable plain project quota accounting. 234usrjquota=<file> Appoint specified file and type during mount, so that quota 235grpjquota=<file> information can be properly updated during recovery flow, 236prjjquota=<file> <quota file>: must be in root directory; 237jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 238offusrjquota Turn off user journalled quota. 239offgrpjquota Turn off group journalled quota. 240offprjjquota Turn off project journalled quota. 241quota Enable plain user disk quota accounting. 242noquota Disable all plain disk quota option. 243alloc_mode=%s Adjust block allocation policy, which supports "reuse" 244 and "default". 245fsync_mode=%s Control the policy of fsync. Currently supports "posix", 246 "strict", and "nobarrier". In "posix" mode, which is 247 default, fsync will follow POSIX semantics and does a 248 light operation to improve the filesystem performance. 249 In "strict" mode, fsync will be heavy and behaves in line 250 with xfs, ext4 and btrfs, where xfstest generic/342 will 251 pass, but the performance will regress. "nobarrier" is 252 based on "posix", but doesn't issue flush command for 253 non-atomic files likewise "nobarrier" mount option. 254test_dummy_encryption 255test_dummy_encryption=%s 256 Enable dummy encryption, which provides a fake fscrypt 257 context. The fake fscrypt context is used by xfstests. 258 The argument may be either "v1" or "v2", in order to 259 select the corresponding fscrypt policy version. 260checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 261 to reenable checkpointing. Is enabled by default. While 262 disabled, any unmounting or unexpected shutdowns will cause 263 the filesystem contents to appear as they did when the 264 filesystem was mounted with that option. 265 While mounting with checkpoint=disable, the filesystem must 266 run garbage collection to ensure that all available space can 267 be used. If this takes too much time, the mount may return 268 EAGAIN. You may optionally add a value to indicate how much 269 of the disk you would be willing to temporarily give up to 270 avoid additional garbage collection. This can be given as a 271 number of blocks, or as a percent. For instance, mounting 272 with checkpoint=disable:100% would always succeed, but it may 273 hide up to all remaining free space. The actual space that 274 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 275 This space is reclaimed once checkpoint=enable. 276checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 277 daemon and make it to merge concurrent checkpoint requests as 278 much as possible to eliminate redundant checkpoint issues. Plus, 279 we can eliminate the sluggish issue caused by slow checkpoint 280 operation when the checkpoint is done in a process context in 281 a cgroup having low i/o budget and cpu shares. To make this 282 do better, we set the default i/o priority of the kernel daemon 283 to "3", to give one higher priority than other kernel threads. 284 This is the same way to give a I/O priority to the jbd2 285 journaling thread of ext4 filesystem. 286nocheckpoint_merge Disable checkpoint merge feature. 287compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 288 "lz4", "zstd" and "lzo-rle" algorithm. 289compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 290 "lz4" and "zstd" support compress level config. 291 algorithm level range 292 lz4 3 - 16 293 zstd 1 - 22 294compress_log_size=%u Support configuring compress cluster size. The size will 295 be 4KB * (1 << %u). The default and minimum sizes are 16KB. 296compress_extension=%s Support adding specified extension, so that f2fs can enable 297 compression on those corresponding files, e.g. if all files 298 with '.ext' has high compression rate, we can set the '.ext' 299 on compression extension list and enable compression on 300 these file by default rather than to enable it via ioctl. 301 For other files, we can still enable compression via ioctl. 302 Note that, there is one reserved special extension '*', it 303 can be set to enable compression for all files. 304nocompress_extension=%s Support adding specified extension, so that f2fs can disable 305 compression on those corresponding files, just contrary to compression extension. 306 If you know exactly which files cannot be compressed, you can use this. 307 The same extension name can't appear in both compress and nocompress 308 extension at the same time. 309 If the compress extension specifies all files, the types specified by the 310 nocompress extension will be treated as special cases and will not be compressed. 311 Don't allow use '*' to specifie all file in nocompress extension. 312 After add nocompress_extension, the priority should be: 313 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 314 See more in compression sections. 315 316compress_chksum Support verifying chksum of raw data in compressed cluster. 317compress_mode=%s Control file compression mode. This supports "fs" and "user" 318 modes. In "fs" mode (default), f2fs does automatic compression 319 on the compression enabled files. In "user" mode, f2fs disables 320 the automaic compression and gives the user discretion of 321 choosing the target file and the timing. The user can do manual 322 compression/decompression on the compression enabled files using 323 ioctls. 324compress_cache Support to use address space of a filesystem managed inode to 325 cache compressed block, in order to improve cache hit ratio of 326 random read. 327inlinecrypt When possible, encrypt/decrypt the contents of encrypted 328 files using the blk-crypto framework rather than 329 filesystem-layer encryption. This allows the use of 330 inline encryption hardware. The on-disk format is 331 unaffected. For more details, see 332 Documentation/block/inline-encryption.rst. 333atgc Enable age-threshold garbage collection, it provides high 334 effectiveness and efficiency on background GC. 335discard_unit=%s Control discard unit, the argument can be "block", "segment" 336 and "section", issued discard command's offset/size will be 337 aligned to the unit, by default, "discard_unit=block" is set, 338 so that small discard functionality is enabled. 339 For blkzoned device, "discard_unit=section" will be set by 340 default, it is helpful for large sized SMR or ZNS devices to 341 reduce memory cost by getting rid of fs metadata supports small 342 discard. 343memory=%s Control memory mode. This supports "normal" and "low" modes. 344 "low" mode is introduced to support low memory devices. 345 Because of the nature of low memory devices, in this mode, f2fs 346 will try to save memory sometimes by sacrificing performance. 347 "normal" mode is the default mode and same as before. 348age_extent_cache Enable an age extent cache based on rb-tree. It records 349 data block update frequency of the extent per inode, in 350 order to provide better temperature hints for data block 351 allocation. 352errors=%s Specify f2fs behavior on critical errors. This supports modes: 353 "panic", "continue" and "remount-ro", respectively, trigger 354 panic immediately, continue without doing anything, and remount 355 the partition in read-only mode. By default it uses "continue" 356 mode. 357 ====================== =============== =============== ======== 358 mode continue remount-ro panic 359 ====================== =============== =============== ======== 360 access ops normal normal N/A 361 syscall errors -EIO -EROFS N/A 362 mount option rw ro N/A 363 pending dir write keep keep N/A 364 pending non-dir write drop keep N/A 365 pending node write drop keep N/A 366 pending meta write keep keep N/A 367 ====================== =============== =============== ======== 368======================== ============================================================ 369 370Debugfs Entries 371=============== 372 373/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 374f2fs. Each file shows the whole f2fs information. 375 376/sys/kernel/debug/f2fs/status includes: 377 378 - major file system information managed by f2fs currently 379 - average SIT information about whole segments 380 - current memory footprint consumed by f2fs. 381 382Sysfs Entries 383============= 384 385Information about mounted f2fs file systems can be found in 386/sys/fs/f2fs. Each mounted filesystem will have a directory in 387/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 388The files in each per-device directory are shown in table below. 389 390Files in /sys/fs/f2fs/<devname> 391(see also Documentation/ABI/testing/sysfs-fs-f2fs) 392 393Usage 394===== 395 3961. Download userland tools and compile them. 397 3982. Skip, if f2fs was compiled statically inside kernel. 399 Otherwise, insert the f2fs.ko module:: 400 401 # insmod f2fs.ko 402 4033. Create a directory to use when mounting:: 404 405 # mkdir /mnt/f2fs 406 4074. Format the block device, and then mount as f2fs:: 408 409 # mkfs.f2fs -l label /dev/block_device 410 # mount -t f2fs /dev/block_device /mnt/f2fs 411 412mkfs.f2fs 413--------- 414The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 415which builds a basic on-disk layout. 416 417The quick options consist of: 418 419=============== =========================================================== 420``-l [label]`` Give a volume label, up to 512 unicode name. 421``-a [0 or 1]`` Split start location of each area for heap-based allocation. 422 423 1 is set by default, which performs this. 424``-o [int]`` Set overprovision ratio in percent over volume size. 425 426 5 is set by default. 427``-s [int]`` Set the number of segments per section. 428 429 1 is set by default. 430``-z [int]`` Set the number of sections per zone. 431 432 1 is set by default. 433``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 434``-t [0 or 1]`` Disable discard command or not. 435 436 1 is set by default, which conducts discard. 437=============== =========================================================== 438 439Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 440 441fsck.f2fs 442--------- 443The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 444partition, which examines whether the filesystem metadata and user-made data 445are cross-referenced correctly or not. 446Note that, initial version of the tool does not fix any inconsistency. 447 448The quick options consist of:: 449 450 -d debug level [default:0] 451 452Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 453 454dump.f2fs 455--------- 456The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 457file. Each file is dump_ssa and dump_sit. 458 459The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 460It shows on-disk inode information recognized by a given inode number, and is 461able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 462./dump_sit respectively. 463 464The options consist of:: 465 466 -d debug level [default:0] 467 -i inode no (hex) 468 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 469 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 470 471Examples:: 472 473 # dump.f2fs -i [ino] /dev/sdx 474 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 475 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 476 477Note: please refer to the manpage of dump.f2fs(8) to get full option list. 478 479sload.f2fs 480---------- 481The sload.f2fs gives a way to insert files and directories in the existing disk 482image. This tool is useful when building f2fs images given compiled files. 483 484Note: please refer to the manpage of sload.f2fs(8) to get full option list. 485 486resize.f2fs 487----------- 488The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 489all the files and directories stored in the image. 490 491Note: please refer to the manpage of resize.f2fs(8) to get full option list. 492 493defrag.f2fs 494----------- 495The defrag.f2fs can be used to defragment scattered written data as well as 496filesystem metadata across the disk. This can improve the write speed by giving 497more free consecutive space. 498 499Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 500 501f2fs_io 502------- 503The f2fs_io is a simple tool to issue various filesystem APIs as well as 504f2fs-specific ones, which is very useful for QA tests. 505 506Note: please refer to the manpage of f2fs_io(8) to get full option list. 507 508Design 509====== 510 511On-disk Layout 512-------------- 513 514F2FS divides the whole volume into a number of segments, each of which is fixed 515to 2MB in size. A section is composed of consecutive segments, and a zone 516consists of a set of sections. By default, section and zone sizes are set to one 517segment size identically, but users can easily modify the sizes by mkfs. 518 519F2FS splits the entire volume into six areas, and all the areas except superblock 520consist of multiple segments as described below:: 521 522 align with the zone size <-| 523 |-> align with the segment size 524 _________________________________________________________________________ 525 | | | Segment | Node | Segment | | 526 | Superblock | Checkpoint | Info. | Address | Summary | Main | 527 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 528 |____________|_____2______|______N______|______N______|______N_____|__N___| 529 . . 530 . . 531 . . 532 ._________________________________________. 533 |_Segment_|_..._|_Segment_|_..._|_Segment_| 534 . . 535 ._________._________ 536 |_section_|__...__|_ 537 . . 538 .________. 539 |__zone__| 540 541- Superblock (SB) 542 It is located at the beginning of the partition, and there exist two copies 543 to avoid file system crash. It contains basic partition information and some 544 default parameters of f2fs. 545 546- Checkpoint (CP) 547 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 548 inode lists, and summary entries of current active segments. 549 550- Segment Information Table (SIT) 551 It contains segment information such as valid block count and bitmap for the 552 validity of all the blocks. 553 554- Node Address Table (NAT) 555 It is composed of a block address table for all the node blocks stored in 556 Main area. 557 558- Segment Summary Area (SSA) 559 It contains summary entries which contains the owner information of all the 560 data and node blocks stored in Main area. 561 562- Main Area 563 It contains file and directory data including their indices. 564 565In order to avoid misalignment between file system and flash-based storage, F2FS 566aligns the start block address of CP with the segment size. Also, it aligns the 567start block address of Main area with the zone size by reserving some segments 568in SSA area. 569 570Reference the following survey for additional technical details. 571https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 572 573File System Metadata Structure 574------------------------------ 575 576F2FS adopts the checkpointing scheme to maintain file system consistency. At 577mount time, F2FS first tries to find the last valid checkpoint data by scanning 578CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 579One of them always indicates the last valid data, which is called as shadow copy 580mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 581 582For file system consistency, each CP points to which NAT and SIT copies are 583valid, as shown as below:: 584 585 +--------+----------+---------+ 586 | CP | SIT | NAT | 587 +--------+----------+---------+ 588 . . . . 589 . . . . 590 . . . . 591 +-------+-------+--------+--------+--------+--------+ 592 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 593 +-------+-------+--------+--------+--------+--------+ 594 | ^ ^ 595 | | | 596 `----------------------------------------' 597 598Index Structure 599--------------- 600 601The key data structure to manage the data locations is a "node". Similar to 602traditional file structures, F2FS has three types of node: inode, direct node, 603indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 604indices, two direct node pointers, two indirect node pointers, and one double 605indirect node pointer as described below. One direct node block contains 1018 606data blocks, and one indirect node block contains also 1018 node blocks. Thus, 607one inode block (i.e., a file) covers:: 608 609 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 610 611 Inode block (4KB) 612 |- data (923) 613 |- direct node (2) 614 | `- data (1018) 615 |- indirect node (2) 616 | `- direct node (1018) 617 | `- data (1018) 618 `- double indirect node (1) 619 `- indirect node (1018) 620 `- direct node (1018) 621 `- data (1018) 622 623Note that all the node blocks are mapped by NAT which means the location of 624each node is translated by the NAT table. In the consideration of the wandering 625tree problem, F2FS is able to cut off the propagation of node updates caused by 626leaf data writes. 627 628Directory Structure 629------------------- 630 631A directory entry occupies 11 bytes, which consists of the following attributes. 632 633- hash hash value of the file name 634- ino inode number 635- len the length of file name 636- type file type such as directory, symlink, etc 637 638A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 639used to represent whether each dentry is valid or not. A dentry block occupies 6404KB with the following composition. 641 642:: 643 644 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 645 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 646 647 [Bucket] 648 +--------------------------------+ 649 |dentry block 1 | dentry block 2 | 650 +--------------------------------+ 651 . . 652 . . 653 . [Dentry Block Structure: 4KB] . 654 +--------+----------+----------+------------+ 655 | bitmap | reserved | dentries | file names | 656 +--------+----------+----------+------------+ 657 [Dentry Block: 4KB] . . 658 . . 659 . . 660 +------+------+-----+------+ 661 | hash | ino | len | type | 662 +------+------+-----+------+ 663 [Dentry Structure: 11 bytes] 664 665F2FS implements multi-level hash tables for directory structure. Each level has 666a hash table with dedicated number of hash buckets as shown below. Note that 667"A(2B)" means a bucket includes 2 data blocks. 668 669:: 670 671 ---------------------- 672 A : bucket 673 B : block 674 N : MAX_DIR_HASH_DEPTH 675 ---------------------- 676 677 level #0 | A(2B) 678 | 679 level #1 | A(2B) - A(2B) 680 | 681 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 682 . | . . . . 683 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 684 . | . . . . 685 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 686 687The number of blocks and buckets are determined by:: 688 689 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 690 # of blocks in level #n = | 691 `- 4, Otherwise 692 693 ,- 2^(n + dir_level), 694 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 695 # of buckets in level #n = | 696 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 697 Otherwise 698 699When F2FS finds a file name in a directory, at first a hash value of the file 700name is calculated. Then, F2FS scans the hash table in level #0 to find the 701dentry consisting of the file name and its inode number. If not found, F2FS 702scans the next hash table in level #1. In this way, F2FS scans hash tables in 703each levels incrementally from 1 to N. In each level F2FS needs to scan only 704one bucket determined by the following equation, which shows O(log(# of files)) 705complexity:: 706 707 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 708 709In the case of file creation, F2FS finds empty consecutive slots that cover the 710file name. F2FS searches the empty slots in the hash tables of whole levels from 7111 to N in the same way as the lookup operation. 712 713The following figure shows an example of two cases holding children:: 714 715 --------------> Dir <-------------- 716 | | 717 child child 718 719 child - child [hole] - child 720 721 child - child - child [hole] - [hole] - child 722 723 Case 1: Case 2: 724 Number of children = 6, Number of children = 3, 725 File size = 7 File size = 7 726 727Default Block Allocation 728------------------------ 729 730At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 731and Hot/Warm/Cold data. 732 733- Hot node contains direct node blocks of directories. 734- Warm node contains direct node blocks except hot node blocks. 735- Cold node contains indirect node blocks 736- Hot data contains dentry blocks 737- Warm data contains data blocks except hot and cold data blocks 738- Cold data contains multimedia data or migrated data blocks 739 740LFS has two schemes for free space management: threaded log and copy-and-compac- 741tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 742for devices showing very good sequential write performance, since free segments 743are served all the time for writing new data. However, it suffers from cleaning 744overhead under high utilization. Contrarily, the threaded log scheme suffers 745from random writes, but no cleaning process is needed. F2FS adopts a hybrid 746scheme where the copy-and-compaction scheme is adopted by default, but the 747policy is dynamically changed to the threaded log scheme according to the file 748system status. 749 750In order to align F2FS with underlying flash-based storage, F2FS allocates a 751segment in a unit of section. F2FS expects that the section size would be the 752same as the unit size of garbage collection in FTL. Furthermore, with respect 753to the mapping granularity in FTL, F2FS allocates each section of the active 754logs from different zones as much as possible, since FTL can write the data in 755the active logs into one allocation unit according to its mapping granularity. 756 757Cleaning process 758---------------- 759 760F2FS does cleaning both on demand and in the background. On-demand cleaning is 761triggered when there are not enough free segments to serve VFS calls. Background 762cleaner is operated by a kernel thread, and triggers the cleaning job when the 763system is idle. 764 765F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 766In the greedy algorithm, F2FS selects a victim segment having the smallest number 767of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 768according to the segment age and the number of valid blocks in order to address 769log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 770algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 771algorithm. 772 773In order to identify whether the data in the victim segment are valid or not, 774F2FS manages a bitmap. Each bit represents the validity of a block, and the 775bitmap is composed of a bit stream covering whole blocks in main area. 776 777Fallocate(2) Policy 778------------------- 779 780The default policy follows the below POSIX rule. 781 782Allocating disk space 783 The default operation (i.e., mode is zero) of fallocate() allocates 784 the disk space within the range specified by offset and len. The 785 file size (as reported by stat(2)) will be changed if offset+len is 786 greater than the file size. Any subregion within the range specified 787 by offset and len that did not contain data before the call will be 788 initialized to zero. This default behavior closely resembles the 789 behavior of the posix_fallocate(3) library function, and is intended 790 as a method of optimally implementing that function. 791 792However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 793fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having 794zero or random data, which is useful to the below scenario where: 795 796 1. create(fd) 797 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 798 3. fallocate(fd, 0, 0, size) 799 4. address = fibmap(fd, offset) 800 5. open(blkdev) 801 6. write(blkdev, address) 802 803Compression implementation 804-------------------------- 805 806- New term named cluster is defined as basic unit of compression, file can 807 be divided into multiple clusters logically. One cluster includes 4 << n 808 (n >= 0) logical pages, compression size is also cluster size, each of 809 cluster can be compressed or not. 810 811- In cluster metadata layout, one special block address is used to indicate 812 a cluster is a compressed one or normal one; for compressed cluster, following 813 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 814 stores data including compress header and compressed data. 815 816- In order to eliminate write amplification during overwrite, F2FS only 817 support compression on write-once file, data can be compressed only when 818 all logical blocks in cluster contain valid data and compress ratio of 819 cluster data is lower than specified threshold. 820 821- To enable compression on regular inode, there are four ways: 822 823 * chattr +c file 824 * chattr +c dir; touch dir/file 825 * mount w/ -o compress_extension=ext; touch file.ext 826 * mount w/ -o compress_extension=*; touch any_file 827 828- To disable compression on regular inode, there are two ways: 829 830 * chattr -c file 831 * mount w/ -o nocompress_extension=ext; touch file.ext 832 833- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 834 835 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 836 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 837 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 838 can enable compress on bar.zip. 839 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 840 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 841 compresse, bar.zip and baz.txt should be non-compressed. 842 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 843 and baz.txt. 844 845- At this point, compression feature doesn't expose compressed space to user 846 directly in order to guarantee potential data updates later to the space. 847 Instead, the main goal is to reduce data writes to flash disk as much as 848 possible, resulting in extending disk life time as well as relaxing IO 849 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 850 interface to reclaim compressed space and show it to user after setting a 851 special flag to the inode. Once the compressed space is released, the flag 852 will block writing data to the file until either the compressed space is 853 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 854 truncated to zero. 855 856Compress metadata layout:: 857 858 [Dnode Structure] 859 +-----------------------------------------------+ 860 | cluster 1 | cluster 2 | ......... | cluster N | 861 +-----------------------------------------------+ 862 . . . . 863 . . . . 864 . Compressed Cluster . . Normal Cluster . 865 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 866 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 867 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 868 . . 869 . . 870 . . 871 +-------------+-------------+----------+----------------------------+ 872 | data length | data chksum | reserved | compressed data | 873 +-------------+-------------+----------+----------------------------+ 874 875Compression mode 876-------------------------- 877 878f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 879With this option, f2fs provides a choice to select the way how to compress the 880compression enabled files (refer to "Compression implementation" section for how to 881enable compression on a regular inode). 882 8831) compress_mode=fs 884This is the default option. f2fs does automatic compression in the writeback of the 885compression enabled files. 886 8872) compress_mode=user 888This disables the automatic compression and gives the user discretion of choosing the 889target file and the timing. The user can do manual compression/decompression on the 890compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 891ioctls like the below. 892 893To decompress a file, 894 895fd = open(filename, O_WRONLY, 0); 896ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 897 898To compress a file, 899 900fd = open(filename, O_WRONLY, 0); 901ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 902 903NVMe Zoned Namespace devices 904---------------------------- 905 906- ZNS defines a per-zone capacity which can be equal or less than the 907 zone-size. Zone-capacity is the number of usable blocks in the zone. 908 F2FS checks if zone-capacity is less than zone-size, if it is, then any 909 segment which starts after the zone-capacity is marked as not-free in 910 the free segment bitmap at initial mount time. These segments are marked 911 as permanently used so they are not allocated for writes and 912 consequently are not needed to be garbage collected. In case the 913 zone-capacity is not aligned to default segment size(2MB), then a segment 914 can start before the zone-capacity and span across zone-capacity boundary. 915 Such spanning segments are also considered as usable segments. All blocks 916 past the zone-capacity are considered unusable in these segments. 917