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. 129no_heap Disable heap-style segment allocation which finds free 130 segments for data from the beginning of main area, while 131 for node from the end of main area. 132nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 133 by default if CONFIG_F2FS_FS_XATTR is selected. 134noacl Disable POSIX Access Control List. Note: acl is enabled 135 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 136active_logs=%u Support configuring the number of active logs. In the 137 current design, f2fs supports only 2, 4, and 6 logs. 138 Default number is 6. 139disable_ext_identify Disable the extension list configured by mkfs, so f2fs 140 is not aware of cold files such as media files. 141inline_xattr Enable the inline xattrs feature. 142noinline_xattr Disable the inline xattrs feature. 143inline_xattr_size=%u Support configuring inline xattr size, it depends on 144 flexible inline xattr feature. 145inline_data Enable the inline data feature: Newly created small (<~3.4k) 146 files can be written into inode block. 147inline_dentry Enable the inline dir feature: data in newly created 148 directory entries can be written into inode block. The 149 space of inode block which is used to store inline 150 dentries is limited to ~3.4k. 151noinline_dentry Disable the inline dentry feature. 152flush_merge Merge concurrent cache_flush commands as much as possible 153 to eliminate redundant command issues. If the underlying 154 device handles the cache_flush command relatively slowly, 155 recommend to enable this option. 156nobarrier This option can be used if underlying storage guarantees 157 its cached data should be written to the novolatile area. 158 If this option is set, no cache_flush commands are issued 159 but f2fs still guarantees the write ordering of all the 160 data writes. 161barrier If this option is set, cache_flush commands are allowed to be 162 issued. 163fastboot This option is used when a system wants to reduce mount 164 time as much as possible, even though normal performance 165 can be sacrificed. 166extent_cache Enable an extent cache based on rb-tree, it can cache 167 as many as extent which map between contiguous logical 168 address and physical address per inode, resulting in 169 increasing the cache hit ratio. Set by default. 170noextent_cache Disable an extent cache based on rb-tree explicitly, see 171 the above extent_cache mount option. 172noinline_data Disable the inline data feature, inline data feature is 173 enabled by default. 174data_flush Enable data flushing before checkpoint in order to 175 persist data of regular and symlink. 176reserve_root=%d Support configuring reserved space which is used for 177 allocation from a privileged user with specified uid or 178 gid, unit: 4KB, the default limit is 0.2% of user blocks. 179resuid=%d The user ID which may use the reserved blocks. 180resgid=%d The group ID which may use the reserved blocks. 181fault_injection=%d Enable fault injection in all supported types with 182 specified injection rate. 183fault_type=%d Support configuring fault injection type, should be 184 enabled with fault_injection option, fault type value 185 is shown below, it supports single or combined type. 186 187 =================== =========== 188 Type_Name Type_Value 189 =================== =========== 190 FAULT_KMALLOC 0x000000001 191 FAULT_KVMALLOC 0x000000002 192 FAULT_PAGE_ALLOC 0x000000004 193 FAULT_PAGE_GET 0x000000008 194 FAULT_ALLOC_BIO 0x000000010 (obsolete) 195 FAULT_ALLOC_NID 0x000000020 196 FAULT_ORPHAN 0x000000040 197 FAULT_BLOCK 0x000000080 198 FAULT_DIR_DEPTH 0x000000100 199 FAULT_EVICT_INODE 0x000000200 200 FAULT_TRUNCATE 0x000000400 201 FAULT_READ_IO 0x000000800 202 FAULT_CHECKPOINT 0x000001000 203 FAULT_DISCARD 0x000002000 204 FAULT_WRITE_IO 0x000004000 205 FAULT_SLAB_ALLOC 0x000008000 206 FAULT_DQUOT_INIT 0x000010000 207 FAULT_LOCK_OP 0x000020000 208 FAULT_BLKADDR 0x000040000 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. 231io_bits=%u Set the bit size of write IO requests. It should be set 232 with "mode=lfs". 233usrquota Enable plain user disk quota accounting. 234grpquota Enable plain group disk quota accounting. 235prjquota Enable plain project quota accounting. 236usrjquota=<file> Appoint specified file and type during mount, so that quota 237grpjquota=<file> information can be properly updated during recovery flow, 238prjjquota=<file> <quota file>: must be in root directory; 239jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 240offusrjquota Turn off user journalled quota. 241offgrpjquota Turn off group journalled quota. 242offprjjquota Turn off project journalled quota. 243quota Enable plain user disk quota accounting. 244noquota Disable all plain disk quota option. 245alloc_mode=%s Adjust block allocation policy, which supports "reuse" 246 and "default". 247fsync_mode=%s Control the policy of fsync. Currently supports "posix", 248 "strict", and "nobarrier". In "posix" mode, which is 249 default, fsync will follow POSIX semantics and does a 250 light operation to improve the filesystem performance. 251 In "strict" mode, fsync will be heavy and behaves in line 252 with xfs, ext4 and btrfs, where xfstest generic/342 will 253 pass, but the performance will regress. "nobarrier" is 254 based on "posix", but doesn't issue flush command for 255 non-atomic files likewise "nobarrier" mount option. 256test_dummy_encryption 257test_dummy_encryption=%s 258 Enable dummy encryption, which provides a fake fscrypt 259 context. The fake fscrypt context is used by xfstests. 260 The argument may be either "v1" or "v2", in order to 261 select the corresponding fscrypt policy version. 262checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 263 to reenable checkpointing. Is enabled by default. While 264 disabled, any unmounting or unexpected shutdowns will cause 265 the filesystem contents to appear as they did when the 266 filesystem was mounted with that option. 267 While mounting with checkpoint=disabled, the filesystem must 268 run garbage collection to ensure that all available space can 269 be used. If this takes too much time, the mount may return 270 EAGAIN. You may optionally add a value to indicate how much 271 of the disk you would be willing to temporarily give up to 272 avoid additional garbage collection. This can be given as a 273 number of blocks, or as a percent. For instance, mounting 274 with checkpoint=disable:100% would always succeed, but it may 275 hide up to all remaining free space. The actual space that 276 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 277 This space is reclaimed once checkpoint=enable. 278checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 279 daemon and make it to merge concurrent checkpoint requests as 280 much as possible to eliminate redundant checkpoint issues. Plus, 281 we can eliminate the sluggish issue caused by slow checkpoint 282 operation when the checkpoint is done in a process context in 283 a cgroup having low i/o budget and cpu shares. To make this 284 do better, we set the default i/o priority of the kernel daemon 285 to "3", to give one higher priority than other kernel threads. 286 This is the same way to give a I/O priority to the jbd2 287 journaling thread of ext4 filesystem. 288nocheckpoint_merge Disable checkpoint merge feature. 289compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 290 "lz4", "zstd" and "lzo-rle" algorithm. 291compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 292 "lz4" and "zstd" support compress level config. 293 algorithm level range 294 lz4 3 - 16 295 zstd 1 - 22 296compress_log_size=%u Support configuring compress cluster size. The size will 297 be 4KB * (1 << %u). The default and minimum sizes are 16KB. 298compress_extension=%s Support adding specified extension, so that f2fs can enable 299 compression on those corresponding files, e.g. if all files 300 with '.ext' has high compression rate, we can set the '.ext' 301 on compression extension list and enable compression on 302 these file by default rather than to enable it via ioctl. 303 For other files, we can still enable compression via ioctl. 304 Note that, there is one reserved special extension '*', it 305 can be set to enable compression for all files. 306nocompress_extension=%s Support adding specified extension, so that f2fs can disable 307 compression on those corresponding files, just contrary to compression extension. 308 If you know exactly which files cannot be compressed, you can use this. 309 The same extension name can't appear in both compress and nocompress 310 extension at the same time. 311 If the compress extension specifies all files, the types specified by the 312 nocompress extension will be treated as special cases and will not be compressed. 313 Don't allow use '*' to specifie all file in nocompress extension. 314 After add nocompress_extension, the priority should be: 315 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 316 See more in compression sections. 317 318compress_chksum Support verifying chksum of raw data in compressed cluster. 319compress_mode=%s Control file compression mode. This supports "fs" and "user" 320 modes. In "fs" mode (default), f2fs does automatic compression 321 on the compression enabled files. In "user" mode, f2fs disables 322 the automaic compression and gives the user discretion of 323 choosing the target file and the timing. The user can do manual 324 compression/decompression on the compression enabled files using 325 ioctls. 326compress_cache Support to use address space of a filesystem managed inode to 327 cache compressed block, in order to improve cache hit ratio of 328 random read. 329inlinecrypt When possible, encrypt/decrypt the contents of encrypted 330 files using the blk-crypto framework rather than 331 filesystem-layer encryption. This allows the use of 332 inline encryption hardware. The on-disk format is 333 unaffected. For more details, see 334 Documentation/block/inline-encryption.rst. 335atgc Enable age-threshold garbage collection, it provides high 336 effectiveness and efficiency on background GC. 337discard_unit=%s Control discard unit, the argument can be "block", "segment" 338 and "section", issued discard command's offset/size will be 339 aligned to the unit, by default, "discard_unit=block" is set, 340 so that small discard functionality is enabled. 341 For blkzoned device, "discard_unit=section" will be set by 342 default, it is helpful for large sized SMR or ZNS devices to 343 reduce memory cost by getting rid of fs metadata supports small 344 discard. 345memory=%s Control memory mode. This supports "normal" and "low" modes. 346 "low" mode is introduced to support low memory devices. 347 Because of the nature of low memory devices, in this mode, f2fs 348 will try to save memory sometimes by sacrificing performance. 349 "normal" mode is the default mode and same as before. 350age_extent_cache Enable an age extent cache based on rb-tree. It records 351 data block update frequency of the extent per inode, in 352 order to provide better temperature hints for data block 353 allocation. 354======================== ============================================================ 355 356Debugfs Entries 357=============== 358 359/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 360f2fs. Each file shows the whole f2fs information. 361 362/sys/kernel/debug/f2fs/status includes: 363 364 - major file system information managed by f2fs currently 365 - average SIT information about whole segments 366 - current memory footprint consumed by f2fs. 367 368Sysfs Entries 369============= 370 371Information about mounted f2fs file systems can be found in 372/sys/fs/f2fs. Each mounted filesystem will have a directory in 373/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 374The files in each per-device directory are shown in table below. 375 376Files in /sys/fs/f2fs/<devname> 377(see also Documentation/ABI/testing/sysfs-fs-f2fs) 378 379Usage 380===== 381 3821. Download userland tools and compile them. 383 3842. Skip, if f2fs was compiled statically inside kernel. 385 Otherwise, insert the f2fs.ko module:: 386 387 # insmod f2fs.ko 388 3893. Create a directory to use when mounting:: 390 391 # mkdir /mnt/f2fs 392 3934. Format the block device, and then mount as f2fs:: 394 395 # mkfs.f2fs -l label /dev/block_device 396 # mount -t f2fs /dev/block_device /mnt/f2fs 397 398mkfs.f2fs 399--------- 400The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 401which builds a basic on-disk layout. 402 403The quick options consist of: 404 405=============== =========================================================== 406``-l [label]`` Give a volume label, up to 512 unicode name. 407``-a [0 or 1]`` Split start location of each area for heap-based allocation. 408 409 1 is set by default, which performs this. 410``-o [int]`` Set overprovision ratio in percent over volume size. 411 412 5 is set by default. 413``-s [int]`` Set the number of segments per section. 414 415 1 is set by default. 416``-z [int]`` Set the number of sections per zone. 417 418 1 is set by default. 419``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 420``-t [0 or 1]`` Disable discard command or not. 421 422 1 is set by default, which conducts discard. 423=============== =========================================================== 424 425Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 426 427fsck.f2fs 428--------- 429The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 430partition, which examines whether the filesystem metadata and user-made data 431are cross-referenced correctly or not. 432Note that, initial version of the tool does not fix any inconsistency. 433 434The quick options consist of:: 435 436 -d debug level [default:0] 437 438Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 439 440dump.f2fs 441--------- 442The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 443file. Each file is dump_ssa and dump_sit. 444 445The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 446It shows on-disk inode information recognized by a given inode number, and is 447able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 448./dump_sit respectively. 449 450The options consist of:: 451 452 -d debug level [default:0] 453 -i inode no (hex) 454 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 455 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 456 457Examples:: 458 459 # dump.f2fs -i [ino] /dev/sdx 460 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 461 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 462 463Note: please refer to the manpage of dump.f2fs(8) to get full option list. 464 465sload.f2fs 466---------- 467The sload.f2fs gives a way to insert files and directories in the exisiting disk 468image. This tool is useful when building f2fs images given compiled files. 469 470Note: please refer to the manpage of sload.f2fs(8) to get full option list. 471 472resize.f2fs 473----------- 474The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 475all the files and directories stored in the image. 476 477Note: please refer to the manpage of resize.f2fs(8) to get full option list. 478 479defrag.f2fs 480----------- 481The defrag.f2fs can be used to defragment scattered written data as well as 482filesystem metadata across the disk. This can improve the write speed by giving 483more free consecutive space. 484 485Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 486 487f2fs_io 488------- 489The f2fs_io is a simple tool to issue various filesystem APIs as well as 490f2fs-specific ones, which is very useful for QA tests. 491 492Note: please refer to the manpage of f2fs_io(8) to get full option list. 493 494Design 495====== 496 497On-disk Layout 498-------------- 499 500F2FS divides the whole volume into a number of segments, each of which is fixed 501to 2MB in size. A section is composed of consecutive segments, and a zone 502consists of a set of sections. By default, section and zone sizes are set to one 503segment size identically, but users can easily modify the sizes by mkfs. 504 505F2FS splits the entire volume into six areas, and all the areas except superblock 506consist of multiple segments as described below:: 507 508 align with the zone size <-| 509 |-> align with the segment size 510 _________________________________________________________________________ 511 | | | Segment | Node | Segment | | 512 | Superblock | Checkpoint | Info. | Address | Summary | Main | 513 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 514 |____________|_____2______|______N______|______N______|______N_____|__N___| 515 . . 516 . . 517 . . 518 ._________________________________________. 519 |_Segment_|_..._|_Segment_|_..._|_Segment_| 520 . . 521 ._________._________ 522 |_section_|__...__|_ 523 . . 524 .________. 525 |__zone__| 526 527- Superblock (SB) 528 It is located at the beginning of the partition, and there exist two copies 529 to avoid file system crash. It contains basic partition information and some 530 default parameters of f2fs. 531 532- Checkpoint (CP) 533 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 534 inode lists, and summary entries of current active segments. 535 536- Segment Information Table (SIT) 537 It contains segment information such as valid block count and bitmap for the 538 validity of all the blocks. 539 540- Node Address Table (NAT) 541 It is composed of a block address table for all the node blocks stored in 542 Main area. 543 544- Segment Summary Area (SSA) 545 It contains summary entries which contains the owner information of all the 546 data and node blocks stored in Main area. 547 548- Main Area 549 It contains file and directory data including their indices. 550 551In order to avoid misalignment between file system and flash-based storage, F2FS 552aligns the start block address of CP with the segment size. Also, it aligns the 553start block address of Main area with the zone size by reserving some segments 554in SSA area. 555 556Reference the following survey for additional technical details. 557https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 558 559File System Metadata Structure 560------------------------------ 561 562F2FS adopts the checkpointing scheme to maintain file system consistency. At 563mount time, F2FS first tries to find the last valid checkpoint data by scanning 564CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 565One of them always indicates the last valid data, which is called as shadow copy 566mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 567 568For file system consistency, each CP points to which NAT and SIT copies are 569valid, as shown as below:: 570 571 +--------+----------+---------+ 572 | CP | SIT | NAT | 573 +--------+----------+---------+ 574 . . . . 575 . . . . 576 . . . . 577 +-------+-------+--------+--------+--------+--------+ 578 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 579 +-------+-------+--------+--------+--------+--------+ 580 | ^ ^ 581 | | | 582 `----------------------------------------' 583 584Index Structure 585--------------- 586 587The key data structure to manage the data locations is a "node". Similar to 588traditional file structures, F2FS has three types of node: inode, direct node, 589indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 590indices, two direct node pointers, two indirect node pointers, and one double 591indirect node pointer as described below. One direct node block contains 1018 592data blocks, and one indirect node block contains also 1018 node blocks. Thus, 593one inode block (i.e., a file) covers:: 594 595 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 596 597 Inode block (4KB) 598 |- data (923) 599 |- direct node (2) 600 | `- data (1018) 601 |- indirect node (2) 602 | `- direct node (1018) 603 | `- data (1018) 604 `- double indirect node (1) 605 `- indirect node (1018) 606 `- direct node (1018) 607 `- data (1018) 608 609Note that all the node blocks are mapped by NAT which means the location of 610each node is translated by the NAT table. In the consideration of the wandering 611tree problem, F2FS is able to cut off the propagation of node updates caused by 612leaf data writes. 613 614Directory Structure 615------------------- 616 617A directory entry occupies 11 bytes, which consists of the following attributes. 618 619- hash hash value of the file name 620- ino inode number 621- len the length of file name 622- type file type such as directory, symlink, etc 623 624A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 625used to represent whether each dentry is valid or not. A dentry block occupies 6264KB with the following composition. 627 628:: 629 630 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 631 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 632 633 [Bucket] 634 +--------------------------------+ 635 |dentry block 1 | dentry block 2 | 636 +--------------------------------+ 637 . . 638 . . 639 . [Dentry Block Structure: 4KB] . 640 +--------+----------+----------+------------+ 641 | bitmap | reserved | dentries | file names | 642 +--------+----------+----------+------------+ 643 [Dentry Block: 4KB] . . 644 . . 645 . . 646 +------+------+-----+------+ 647 | hash | ino | len | type | 648 +------+------+-----+------+ 649 [Dentry Structure: 11 bytes] 650 651F2FS implements multi-level hash tables for directory structure. Each level has 652a hash table with dedicated number of hash buckets as shown below. Note that 653"A(2B)" means a bucket includes 2 data blocks. 654 655:: 656 657 ---------------------- 658 A : bucket 659 B : block 660 N : MAX_DIR_HASH_DEPTH 661 ---------------------- 662 663 level #0 | A(2B) 664 | 665 level #1 | A(2B) - A(2B) 666 | 667 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 668 . | . . . . 669 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 670 . | . . . . 671 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 672 673The number of blocks and buckets are determined by:: 674 675 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 676 # of blocks in level #n = | 677 `- 4, Otherwise 678 679 ,- 2^(n + dir_level), 680 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 681 # of buckets in level #n = | 682 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 683 Otherwise 684 685When F2FS finds a file name in a directory, at first a hash value of the file 686name is calculated. Then, F2FS scans the hash table in level #0 to find the 687dentry consisting of the file name and its inode number. If not found, F2FS 688scans the next hash table in level #1. In this way, F2FS scans hash tables in 689each levels incrementally from 1 to N. In each level F2FS needs to scan only 690one bucket determined by the following equation, which shows O(log(# of files)) 691complexity:: 692 693 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 694 695In the case of file creation, F2FS finds empty consecutive slots that cover the 696file name. F2FS searches the empty slots in the hash tables of whole levels from 6971 to N in the same way as the lookup operation. 698 699The following figure shows an example of two cases holding children:: 700 701 --------------> Dir <-------------- 702 | | 703 child child 704 705 child - child [hole] - child 706 707 child - child - child [hole] - [hole] - child 708 709 Case 1: Case 2: 710 Number of children = 6, Number of children = 3, 711 File size = 7 File size = 7 712 713Default Block Allocation 714------------------------ 715 716At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 717and Hot/Warm/Cold data. 718 719- Hot node contains direct node blocks of directories. 720- Warm node contains direct node blocks except hot node blocks. 721- Cold node contains indirect node blocks 722- Hot data contains dentry blocks 723- Warm data contains data blocks except hot and cold data blocks 724- Cold data contains multimedia data or migrated data blocks 725 726LFS has two schemes for free space management: threaded log and copy-and-compac- 727tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 728for devices showing very good sequential write performance, since free segments 729are served all the time for writing new data. However, it suffers from cleaning 730overhead under high utilization. Contrarily, the threaded log scheme suffers 731from random writes, but no cleaning process is needed. F2FS adopts a hybrid 732scheme where the copy-and-compaction scheme is adopted by default, but the 733policy is dynamically changed to the threaded log scheme according to the file 734system status. 735 736In order to align F2FS with underlying flash-based storage, F2FS allocates a 737segment in a unit of section. F2FS expects that the section size would be the 738same as the unit size of garbage collection in FTL. Furthermore, with respect 739to the mapping granularity in FTL, F2FS allocates each section of the active 740logs from different zones as much as possible, since FTL can write the data in 741the active logs into one allocation unit according to its mapping granularity. 742 743Cleaning process 744---------------- 745 746F2FS does cleaning both on demand and in the background. On-demand cleaning is 747triggered when there are not enough free segments to serve VFS calls. Background 748cleaner is operated by a kernel thread, and triggers the cleaning job when the 749system is idle. 750 751F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 752In the greedy algorithm, F2FS selects a victim segment having the smallest number 753of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 754according to the segment age and the number of valid blocks in order to address 755log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 756algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 757algorithm. 758 759In order to identify whether the data in the victim segment are valid or not, 760F2FS manages a bitmap. Each bit represents the validity of a block, and the 761bitmap is composed of a bit stream covering whole blocks in main area. 762 763Fallocate(2) Policy 764------------------- 765 766The default policy follows the below POSIX rule. 767 768Allocating disk space 769 The default operation (i.e., mode is zero) of fallocate() allocates 770 the disk space within the range specified by offset and len. The 771 file size (as reported by stat(2)) will be changed if offset+len is 772 greater than the file size. Any subregion within the range specified 773 by offset and len that did not contain data before the call will be 774 initialized to zero. This default behavior closely resembles the 775 behavior of the posix_fallocate(3) library function, and is intended 776 as a method of optimally implementing that function. 777 778However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 779fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having 780zero or random data, which is useful to the below scenario where: 781 782 1. create(fd) 783 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 784 3. fallocate(fd, 0, 0, size) 785 4. address = fibmap(fd, offset) 786 5. open(blkdev) 787 6. write(blkdev, address) 788 789Compression implementation 790-------------------------- 791 792- New term named cluster is defined as basic unit of compression, file can 793 be divided into multiple clusters logically. One cluster includes 4 << n 794 (n >= 0) logical pages, compression size is also cluster size, each of 795 cluster can be compressed or not. 796 797- In cluster metadata layout, one special block address is used to indicate 798 a cluster is a compressed one or normal one; for compressed cluster, following 799 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 800 stores data including compress header and compressed data. 801 802- In order to eliminate write amplification during overwrite, F2FS only 803 support compression on write-once file, data can be compressed only when 804 all logical blocks in cluster contain valid data and compress ratio of 805 cluster data is lower than specified threshold. 806 807- To enable compression on regular inode, there are four ways: 808 809 * chattr +c file 810 * chattr +c dir; touch dir/file 811 * mount w/ -o compress_extension=ext; touch file.ext 812 * mount w/ -o compress_extension=*; touch any_file 813 814- To disable compression on regular inode, there are two ways: 815 816 * chattr -c file 817 * mount w/ -o nocompress_extension=ext; touch file.ext 818 819- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 820 821 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 822 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 823 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 824 can enable compress on bar.zip. 825 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 826 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 827 compresse, bar.zip and baz.txt should be non-compressed. 828 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 829 and baz.txt. 830 831- At this point, compression feature doesn't expose compressed space to user 832 directly in order to guarantee potential data updates later to the space. 833 Instead, the main goal is to reduce data writes to flash disk as much as 834 possible, resulting in extending disk life time as well as relaxing IO 835 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 836 interface to reclaim compressed space and show it to user after setting a 837 special flag to the inode. Once the compressed space is released, the flag 838 will block writing data to the file until either the compressed space is 839 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 840 truncated to zero. 841 842Compress metadata layout:: 843 844 [Dnode Structure] 845 +-----------------------------------------------+ 846 | cluster 1 | cluster 2 | ......... | cluster N | 847 +-----------------------------------------------+ 848 . . . . 849 . . . . 850 . Compressed Cluster . . Normal Cluster . 851 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 852 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 853 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 854 . . 855 . . 856 . . 857 +-------------+-------------+----------+----------------------------+ 858 | data length | data chksum | reserved | compressed data | 859 +-------------+-------------+----------+----------------------------+ 860 861Compression mode 862-------------------------- 863 864f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 865With this option, f2fs provides a choice to select the way how to compress the 866compression enabled files (refer to "Compression implementation" section for how to 867enable compression on a regular inode). 868 8691) compress_mode=fs 870This is the default option. f2fs does automatic compression in the writeback of the 871compression enabled files. 872 8732) compress_mode=user 874This disables the automatic compression and gives the user discretion of choosing the 875target file and the timing. The user can do manual compression/decompression on the 876compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 877ioctls like the below. 878 879To decompress a file, 880 881fd = open(filename, O_WRONLY, 0); 882ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 883 884To compress a file, 885 886fd = open(filename, O_WRONLY, 0); 887ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 888 889NVMe Zoned Namespace devices 890---------------------------- 891 892- ZNS defines a per-zone capacity which can be equal or less than the 893 zone-size. Zone-capacity is the number of usable blocks in the zone. 894 F2FS checks if zone-capacity is less than zone-size, if it is, then any 895 segment which starts after the zone-capacity is marked as not-free in 896 the free segment bitmap at initial mount time. These segments are marked 897 as permanently used so they are not allocated for writes and 898 consequently are not needed to be garbage collected. In case the 899 zone-capacity is not aligned to default segment size(2MB), then a segment 900 can start before the zone-capacity and span across zone-capacity boundary. 901 Such spanning segments are also considered as usable segments. All blocks 902 past the zone-capacity are considered unusable in these segments. 903