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 0x00000001 189 FAULT_KVMALLOC 0x00000002 190 FAULT_PAGE_ALLOC 0x00000004 191 FAULT_PAGE_GET 0x00000008 192 FAULT_ALLOC_BIO 0x00000010 (obsolete) 193 FAULT_ALLOC_NID 0x00000020 194 FAULT_ORPHAN 0x00000040 195 FAULT_BLOCK 0x00000080 196 FAULT_DIR_DEPTH 0x00000100 197 FAULT_EVICT_INODE 0x00000200 198 FAULT_TRUNCATE 0x00000400 199 FAULT_READ_IO 0x00000800 200 FAULT_CHECKPOINT 0x00001000 201 FAULT_DISCARD 0x00002000 202 FAULT_WRITE_IO 0x00004000 203 FAULT_SLAB_ALLOC 0x00008000 204 FAULT_DQUOT_INIT 0x00010000 205 FAULT_LOCK_OP 0x00020000 206 FAULT_BLKADDR_VALIDITY 0x00040000 207 FAULT_BLKADDR_CONSISTENCE 0x00080000 208 FAULT_NO_SEGMENT 0x00100000 209 FAULT_INCONSISTENT_FOOTER 0x00200000 210 FAULT_TIMEOUT 0x00400000 (1000ms) 211 FAULT_VMALLOC 0x00800000 212 =========================== ========== 213mode=%s Control block allocation mode which supports "adaptive" 214 and "lfs". In "lfs" mode, there should be no random 215 writes towards main area. 216 "fragment:segment" and "fragment:block" are newly added here. 217 These are developer options for experiments to simulate filesystem 218 fragmentation/after-GC situation itself. The developers use these 219 modes to understand filesystem fragmentation/after-GC condition well, 220 and eventually get some insights to handle them better. 221 In "fragment:segment", f2fs allocates a new segment in ramdom 222 position. With this, we can simulate the after-GC condition. 223 In "fragment:block", we can scatter block allocation with 224 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. 225 We added some randomness to both chunk and hole size to make 226 it close to realistic IO pattern. So, in this mode, f2fs will allocate 227 1..<max_fragment_chunk> blocks in a chunk and make a hole in the 228 length of 1..<max_fragment_hole> by turns. With this, the newly 229 allocated blocks will be scattered throughout the whole partition. 230 Note that "fragment:block" implicitly enables "fragment:segment" 231 option for more randomness. 232 Please, use these options for your experiments and we strongly 233 recommend to re-format the filesystem after using these options. 234usrquota Enable plain user disk quota accounting. 235grpquota Enable plain group disk quota accounting. 236prjquota Enable plain project quota accounting. 237usrjquota=<file> Appoint specified file and type during mount, so that quota 238grpjquota=<file> information can be properly updated during recovery flow, 239prjjquota=<file> <quota file>: must be in root directory; 240jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 241offusrjquota Turn off user journalled quota. 242offgrpjquota Turn off group journalled quota. 243offprjjquota Turn off project journalled quota. 244quota Enable plain user disk quota accounting. 245noquota Disable all plain disk quota option. 246alloc_mode=%s Adjust block allocation policy, which supports "reuse" 247 and "default". 248fsync_mode=%s Control the policy of fsync. Currently supports "posix", 249 "strict", and "nobarrier". In "posix" mode, which is 250 default, fsync will follow POSIX semantics and does a 251 light operation to improve the filesystem performance. 252 In "strict" mode, fsync will be heavy and behaves in line 253 with xfs, ext4 and btrfs, where xfstest generic/342 will 254 pass, but the performance will regress. "nobarrier" is 255 based on "posix", but doesn't issue flush command for 256 non-atomic files likewise "nobarrier" mount option. 257test_dummy_encryption 258test_dummy_encryption=%s 259 Enable dummy encryption, which provides a fake fscrypt 260 context. The fake fscrypt context is used by xfstests. 261 The argument may be either "v1" or "v2", in order to 262 select the corresponding fscrypt policy version. 263checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 264 to reenable checkpointing. Is enabled by default. While 265 disabled, any unmounting or unexpected shutdowns will cause 266 the filesystem contents to appear as they did when the 267 filesystem was mounted with that option. 268 While mounting with checkpoint=disable, the filesystem must 269 run garbage collection to ensure that all available space can 270 be used. If this takes too much time, the mount may return 271 EAGAIN. You may optionally add a value to indicate how much 272 of the disk you would be willing to temporarily give up to 273 avoid additional garbage collection. This can be given as a 274 number of blocks, or as a percent. For instance, mounting 275 with checkpoint=disable:100% would always succeed, but it may 276 hide up to all remaining free space. The actual space that 277 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 278 This space is reclaimed once checkpoint=enable. 279checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 280 daemon and make it to merge concurrent checkpoint requests as 281 much as possible to eliminate redundant checkpoint issues. Plus, 282 we can eliminate the sluggish issue caused by slow checkpoint 283 operation when the checkpoint is done in a process context in 284 a cgroup having low i/o budget and cpu shares. To make this 285 do better, we set the default i/o priority of the kernel daemon 286 to "3", to give one higher priority than other kernel threads. 287 This is the same way to give a I/O priority to the jbd2 288 journaling thread of ext4 filesystem. 289nocheckpoint_merge Disable checkpoint merge feature. 290compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 291 "lz4", "zstd" and "lzo-rle" algorithm. 292compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 293 "lz4" and "zstd" support compress level config. 294 algorithm level range 295 lz4 3 - 16 296 zstd 1 - 22 297compress_log_size=%u Support configuring compress cluster size. The size will 298 be 4KB * (1 << %u). The default and minimum sizes are 16KB. 299compress_extension=%s Support adding specified extension, so that f2fs can enable 300 compression on those corresponding files, e.g. if all files 301 with '.ext' has high compression rate, we can set the '.ext' 302 on compression extension list and enable compression on 303 these file by default rather than to enable it via ioctl. 304 For other files, we can still enable compression via ioctl. 305 Note that, there is one reserved special extension '*', it 306 can be set to enable compression for all files. 307nocompress_extension=%s Support adding specified extension, so that f2fs can disable 308 compression on those corresponding files, just contrary to compression extension. 309 If you know exactly which files cannot be compressed, you can use this. 310 The same extension name can't appear in both compress and nocompress 311 extension at the same time. 312 If the compress extension specifies all files, the types specified by the 313 nocompress extension will be treated as special cases and will not be compressed. 314 Don't allow use '*' to specifie all file in nocompress extension. 315 After add nocompress_extension, the priority should be: 316 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 317 See more in compression sections. 318 319compress_chksum Support verifying chksum of raw data in compressed cluster. 320compress_mode=%s Control file compression mode. This supports "fs" and "user" 321 modes. In "fs" mode (default), f2fs does automatic compression 322 on the compression enabled files. In "user" mode, f2fs disables 323 the automaic compression and gives the user discretion of 324 choosing the target file and the timing. The user can do manual 325 compression/decompression on the compression enabled files using 326 ioctls. 327compress_cache Support to use address space of a filesystem managed inode to 328 cache compressed block, in order to improve cache hit ratio of 329 random read. 330inlinecrypt When possible, encrypt/decrypt the contents of encrypted 331 files using the blk-crypto framework rather than 332 filesystem-layer encryption. This allows the use of 333 inline encryption hardware. The on-disk format is 334 unaffected. For more details, see 335 Documentation/block/inline-encryption.rst. 336atgc Enable age-threshold garbage collection, it provides high 337 effectiveness and efficiency on background GC. 338discard_unit=%s Control discard unit, the argument can be "block", "segment" 339 and "section", issued discard command's offset/size will be 340 aligned to the unit, by default, "discard_unit=block" is set, 341 so that small discard functionality is enabled. 342 For blkzoned device, "discard_unit=section" will be set by 343 default, it is helpful for large sized SMR or ZNS devices to 344 reduce memory cost by getting rid of fs metadata supports small 345 discard. 346memory=%s Control memory mode. This supports "normal" and "low" modes. 347 "low" mode is introduced to support low memory devices. 348 Because of the nature of low memory devices, in this mode, f2fs 349 will try to save memory sometimes by sacrificing performance. 350 "normal" mode is the default mode and same as before. 351age_extent_cache Enable an age extent cache based on rb-tree. It records 352 data block update frequency of the extent per inode, in 353 order to provide better temperature hints for data block 354 allocation. 355errors=%s Specify f2fs behavior on critical errors. This supports modes: 356 "panic", "continue" and "remount-ro", respectively, trigger 357 panic immediately, continue without doing anything, and remount 358 the partition in read-only mode. By default it uses "continue" 359 mode. 360 ====================== =============== =============== ======== 361 mode continue remount-ro panic 362 ====================== =============== =============== ======== 363 access ops normal normal N/A 364 syscall errors -EIO -EROFS N/A 365 mount option rw ro N/A 366 pending dir write keep keep N/A 367 pending non-dir write drop keep N/A 368 pending node write drop keep N/A 369 pending meta write keep keep N/A 370 ====================== =============== =============== ======== 371nat_bits Enable nat_bits feature to enhance full/empty nat blocks access, 372 by default it's disabled. 373======================== ============================================================ 374 375Debugfs Entries 376=============== 377 378/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 379f2fs. Each file shows the whole f2fs information. 380 381/sys/kernel/debug/f2fs/status includes: 382 383 - major file system information managed by f2fs currently 384 - average SIT information about whole segments 385 - current memory footprint consumed by f2fs. 386 387Sysfs Entries 388============= 389 390Information about mounted f2fs file systems can be found in 391/sys/fs/f2fs. Each mounted filesystem will have a directory in 392/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 393The files in each per-device directory are shown in table below. 394 395Files in /sys/fs/f2fs/<devname> 396(see also Documentation/ABI/testing/sysfs-fs-f2fs) 397 398Usage 399===== 400 4011. Download userland tools and compile them. 402 4032. Skip, if f2fs was compiled statically inside kernel. 404 Otherwise, insert the f2fs.ko module:: 405 406 # insmod f2fs.ko 407 4083. Create a directory to use when mounting:: 409 410 # mkdir /mnt/f2fs 411 4124. Format the block device, and then mount as f2fs:: 413 414 # mkfs.f2fs -l label /dev/block_device 415 # mount -t f2fs /dev/block_device /mnt/f2fs 416 417mkfs.f2fs 418--------- 419The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 420which builds a basic on-disk layout. 421 422The quick options consist of: 423 424=============== =========================================================== 425``-l [label]`` Give a volume label, up to 512 unicode name. 426``-a [0 or 1]`` Split start location of each area for heap-based allocation. 427 428 1 is set by default, which performs this. 429``-o [int]`` Set overprovision ratio in percent over volume size. 430 431 5 is set by default. 432``-s [int]`` Set the number of segments per section. 433 434 1 is set by default. 435``-z [int]`` Set the number of sections per zone. 436 437 1 is set by default. 438``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 439``-t [0 or 1]`` Disable discard command or not. 440 441 1 is set by default, which conducts discard. 442=============== =========================================================== 443 444Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 445 446fsck.f2fs 447--------- 448The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 449partition, which examines whether the filesystem metadata and user-made data 450are cross-referenced correctly or not. 451Note that, initial version of the tool does not fix any inconsistency. 452 453The quick options consist of:: 454 455 -d debug level [default:0] 456 457Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 458 459dump.f2fs 460--------- 461The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 462file. Each file is dump_ssa and dump_sit. 463 464The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 465It shows on-disk inode information recognized by a given inode number, and is 466able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 467./dump_sit respectively. 468 469The options consist of:: 470 471 -d debug level [default:0] 472 -i inode no (hex) 473 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 474 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 475 476Examples:: 477 478 # dump.f2fs -i [ino] /dev/sdx 479 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 480 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 481 482Note: please refer to the manpage of dump.f2fs(8) to get full option list. 483 484sload.f2fs 485---------- 486The sload.f2fs gives a way to insert files and directories in the existing disk 487image. This tool is useful when building f2fs images given compiled files. 488 489Note: please refer to the manpage of sload.f2fs(8) to get full option list. 490 491resize.f2fs 492----------- 493The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 494all the files and directories stored in the image. 495 496Note: please refer to the manpage of resize.f2fs(8) to get full option list. 497 498defrag.f2fs 499----------- 500The defrag.f2fs can be used to defragment scattered written data as well as 501filesystem metadata across the disk. This can improve the write speed by giving 502more free consecutive space. 503 504Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 505 506f2fs_io 507------- 508The f2fs_io is a simple tool to issue various filesystem APIs as well as 509f2fs-specific ones, which is very useful for QA tests. 510 511Note: please refer to the manpage of f2fs_io(8) to get full option list. 512 513Design 514====== 515 516On-disk Layout 517-------------- 518 519F2FS divides the whole volume into a number of segments, each of which is fixed 520to 2MB in size. A section is composed of consecutive segments, and a zone 521consists of a set of sections. By default, section and zone sizes are set to one 522segment size identically, but users can easily modify the sizes by mkfs. 523 524F2FS splits the entire volume into six areas, and all the areas except superblock 525consist of multiple segments as described below:: 526 527 align with the zone size <-| 528 |-> align with the segment size 529 _________________________________________________________________________ 530 | | | Segment | Node | Segment | | 531 | Superblock | Checkpoint | Info. | Address | Summary | Main | 532 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 533 |____________|_____2______|______N______|______N______|______N_____|__N___| 534 . . 535 . . 536 . . 537 ._________________________________________. 538 |_Segment_|_..._|_Segment_|_..._|_Segment_| 539 . . 540 ._________._________ 541 |_section_|__...__|_ 542 . . 543 .________. 544 |__zone__| 545 546- Superblock (SB) 547 It is located at the beginning of the partition, and there exist two copies 548 to avoid file system crash. It contains basic partition information and some 549 default parameters of f2fs. 550 551- Checkpoint (CP) 552 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 553 inode lists, and summary entries of current active segments. 554 555- Segment Information Table (SIT) 556 It contains segment information such as valid block count and bitmap for the 557 validity of all the blocks. 558 559- Node Address Table (NAT) 560 It is composed of a block address table for all the node blocks stored in 561 Main area. 562 563- Segment Summary Area (SSA) 564 It contains summary entries which contains the owner information of all the 565 data and node blocks stored in Main area. 566 567- Main Area 568 It contains file and directory data including their indices. 569 570In order to avoid misalignment between file system and flash-based storage, F2FS 571aligns the start block address of CP with the segment size. Also, it aligns the 572start block address of Main area with the zone size by reserving some segments 573in SSA area. 574 575Reference the following survey for additional technical details. 576https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 577 578File System Metadata Structure 579------------------------------ 580 581F2FS adopts the checkpointing scheme to maintain file system consistency. At 582mount time, F2FS first tries to find the last valid checkpoint data by scanning 583CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 584One of them always indicates the last valid data, which is called as shadow copy 585mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 586 587For file system consistency, each CP points to which NAT and SIT copies are 588valid, as shown as below:: 589 590 +--------+----------+---------+ 591 | CP | SIT | NAT | 592 +--------+----------+---------+ 593 . . . . 594 . . . . 595 . . . . 596 +-------+-------+--------+--------+--------+--------+ 597 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 598 +-------+-------+--------+--------+--------+--------+ 599 | ^ ^ 600 | | | 601 `----------------------------------------' 602 603Index Structure 604--------------- 605 606The key data structure to manage the data locations is a "node". Similar to 607traditional file structures, F2FS has three types of node: inode, direct node, 608indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 609indices, two direct node pointers, two indirect node pointers, and one double 610indirect node pointer as described below. One direct node block contains 1018 611data blocks, and one indirect node block contains also 1018 node blocks. Thus, 612one inode block (i.e., a file) covers:: 613 614 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 615 616 Inode block (4KB) 617 |- data (923) 618 |- direct node (2) 619 | `- data (1018) 620 |- indirect node (2) 621 | `- direct node (1018) 622 | `- data (1018) 623 `- double indirect node (1) 624 `- indirect node (1018) 625 `- direct node (1018) 626 `- data (1018) 627 628Note that all the node blocks are mapped by NAT which means the location of 629each node is translated by the NAT table. In the consideration of the wandering 630tree problem, F2FS is able to cut off the propagation of node updates caused by 631leaf data writes. 632 633Directory Structure 634------------------- 635 636A directory entry occupies 11 bytes, which consists of the following attributes. 637 638- hash hash value of the file name 639- ino inode number 640- len the length of file name 641- type file type such as directory, symlink, etc 642 643A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 644used to represent whether each dentry is valid or not. A dentry block occupies 6454KB with the following composition. 646 647:: 648 649 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 650 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 651 652 [Bucket] 653 +--------------------------------+ 654 |dentry block 1 | dentry block 2 | 655 +--------------------------------+ 656 . . 657 . . 658 . [Dentry Block Structure: 4KB] . 659 +--------+----------+----------+------------+ 660 | bitmap | reserved | dentries | file names | 661 +--------+----------+----------+------------+ 662 [Dentry Block: 4KB] . . 663 . . 664 . . 665 +------+------+-----+------+ 666 | hash | ino | len | type | 667 +------+------+-----+------+ 668 [Dentry Structure: 11 bytes] 669 670F2FS implements multi-level hash tables for directory structure. Each level has 671a hash table with dedicated number of hash buckets as shown below. Note that 672"A(2B)" means a bucket includes 2 data blocks. 673 674:: 675 676 ---------------------- 677 A : bucket 678 B : block 679 N : MAX_DIR_HASH_DEPTH 680 ---------------------- 681 682 level #0 | A(2B) 683 | 684 level #1 | A(2B) - A(2B) 685 | 686 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 687 . | . . . . 688 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 689 . | . . . . 690 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 691 692The number of blocks and buckets are determined by:: 693 694 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 695 # of blocks in level #n = | 696 `- 4, Otherwise 697 698 ,- 2^(n + dir_level), 699 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 700 # of buckets in level #n = | 701 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 702 Otherwise 703 704When F2FS finds a file name in a directory, at first a hash value of the file 705name is calculated. Then, F2FS scans the hash table in level #0 to find the 706dentry consisting of the file name and its inode number. If not found, F2FS 707scans the next hash table in level #1. In this way, F2FS scans hash tables in 708each levels incrementally from 1 to N. In each level F2FS needs to scan only 709one bucket determined by the following equation, which shows O(log(# of files)) 710complexity:: 711 712 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 713 714In the case of file creation, F2FS finds empty consecutive slots that cover the 715file name. F2FS searches the empty slots in the hash tables of whole levels from 7161 to N in the same way as the lookup operation. 717 718The following figure shows an example of two cases holding children:: 719 720 --------------> Dir <-------------- 721 | | 722 child child 723 724 child - child [hole] - child 725 726 child - child - child [hole] - [hole] - child 727 728 Case 1: Case 2: 729 Number of children = 6, Number of children = 3, 730 File size = 7 File size = 7 731 732Default Block Allocation 733------------------------ 734 735At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 736and Hot/Warm/Cold data. 737 738- Hot node contains direct node blocks of directories. 739- Warm node contains direct node blocks except hot node blocks. 740- Cold node contains indirect node blocks 741- Hot data contains dentry blocks 742- Warm data contains data blocks except hot and cold data blocks 743- Cold data contains multimedia data or migrated data blocks 744 745LFS has two schemes for free space management: threaded log and copy-and-compac- 746tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 747for devices showing very good sequential write performance, since free segments 748are served all the time for writing new data. However, it suffers from cleaning 749overhead under high utilization. Contrarily, the threaded log scheme suffers 750from random writes, but no cleaning process is needed. F2FS adopts a hybrid 751scheme where the copy-and-compaction scheme is adopted by default, but the 752policy is dynamically changed to the threaded log scheme according to the file 753system status. 754 755In order to align F2FS with underlying flash-based storage, F2FS allocates a 756segment in a unit of section. F2FS expects that the section size would be the 757same as the unit size of garbage collection in FTL. Furthermore, with respect 758to the mapping granularity in FTL, F2FS allocates each section of the active 759logs from different zones as much as possible, since FTL can write the data in 760the active logs into one allocation unit according to its mapping granularity. 761 762Cleaning process 763---------------- 764 765F2FS does cleaning both on demand and in the background. On-demand cleaning is 766triggered when there are not enough free segments to serve VFS calls. Background 767cleaner is operated by a kernel thread, and triggers the cleaning job when the 768system is idle. 769 770F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 771In the greedy algorithm, F2FS selects a victim segment having the smallest number 772of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 773according to the segment age and the number of valid blocks in order to address 774log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 775algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 776algorithm. 777 778In order to identify whether the data in the victim segment are valid or not, 779F2FS manages a bitmap. Each bit represents the validity of a block, and the 780bitmap is composed of a bit stream covering whole blocks in main area. 781 782Write-hint Policy 783----------------- 784 785F2FS sets the whint all the time with the below policy. 786 787===================== ======================== =================== 788User F2FS Block 789===================== ======================== =================== 790N/A META WRITE_LIFE_NONE|REQ_META 791N/A HOT_NODE WRITE_LIFE_NONE 792N/A WARM_NODE WRITE_LIFE_MEDIUM 793N/A COLD_NODE WRITE_LIFE_LONG 794ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 795extension list " " 796 797-- buffered io 798N/A COLD_DATA WRITE_LIFE_EXTREME 799N/A HOT_DATA WRITE_LIFE_SHORT 800N/A WARM_DATA WRITE_LIFE_NOT_SET 801 802-- direct io 803WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 804WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 805WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 806WRITE_LIFE_NONE " WRITE_LIFE_NONE 807WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 808WRITE_LIFE_LONG " WRITE_LIFE_LONG 809===================== ======================== =================== 810 811Fallocate(2) Policy 812------------------- 813 814The default policy follows the below POSIX rule. 815 816Allocating disk space 817 The default operation (i.e., mode is zero) of fallocate() allocates 818 the disk space within the range specified by offset and len. The 819 file size (as reported by stat(2)) will be changed if offset+len is 820 greater than the file size. Any subregion within the range specified 821 by offset and len that did not contain data before the call will be 822 initialized to zero. This default behavior closely resembles the 823 behavior of the posix_fallocate(3) library function, and is intended 824 as a method of optimally implementing that function. 825 826However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 827fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having 828zero or random data, which is useful to the below scenario where: 829 830 1. create(fd) 831 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 832 3. fallocate(fd, 0, 0, size) 833 4. address = fibmap(fd, offset) 834 5. open(blkdev) 835 6. write(blkdev, address) 836 837Compression implementation 838-------------------------- 839 840- New term named cluster is defined as basic unit of compression, file can 841 be divided into multiple clusters logically. One cluster includes 4 << n 842 (n >= 0) logical pages, compression size is also cluster size, each of 843 cluster can be compressed or not. 844 845- In cluster metadata layout, one special block address is used to indicate 846 a cluster is a compressed one or normal one; for compressed cluster, following 847 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 848 stores data including compress header and compressed data. 849 850- In order to eliminate write amplification during overwrite, F2FS only 851 support compression on write-once file, data can be compressed only when 852 all logical blocks in cluster contain valid data and compress ratio of 853 cluster data is lower than specified threshold. 854 855- To enable compression on regular inode, there are four ways: 856 857 * chattr +c file 858 * chattr +c dir; touch dir/file 859 * mount w/ -o compress_extension=ext; touch file.ext 860 * mount w/ -o compress_extension=*; touch any_file 861 862- To disable compression on regular inode, there are two ways: 863 864 * chattr -c file 865 * mount w/ -o nocompress_extension=ext; touch file.ext 866 867- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 868 869 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 870 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 871 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 872 can enable compress on bar.zip. 873 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 874 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 875 compresse, bar.zip and baz.txt should be non-compressed. 876 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 877 and baz.txt. 878 879- At this point, compression feature doesn't expose compressed space to user 880 directly in order to guarantee potential data updates later to the space. 881 Instead, the main goal is to reduce data writes to flash disk as much as 882 possible, resulting in extending disk life time as well as relaxing IO 883 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 884 interface to reclaim compressed space and show it to user after setting a 885 special flag to the inode. Once the compressed space is released, the flag 886 will block writing data to the file until either the compressed space is 887 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 888 truncated to zero. 889 890Compress metadata layout:: 891 892 [Dnode Structure] 893 +-----------------------------------------------+ 894 | cluster 1 | cluster 2 | ......... | cluster N | 895 +-----------------------------------------------+ 896 . . . . 897 . . . . 898 . Compressed Cluster . . Normal Cluster . 899 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 900 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 901 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 902 . . 903 . . 904 . . 905 +-------------+-------------+----------+----------------------------+ 906 | data length | data chksum | reserved | compressed data | 907 +-------------+-------------+----------+----------------------------+ 908 909Compression mode 910-------------------------- 911 912f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 913With this option, f2fs provides a choice to select the way how to compress the 914compression enabled files (refer to "Compression implementation" section for how to 915enable compression on a regular inode). 916 9171) compress_mode=fs 918This is the default option. f2fs does automatic compression in the writeback of the 919compression enabled files. 920 9212) compress_mode=user 922This disables the automatic compression and gives the user discretion of choosing the 923target file and the timing. The user can do manual compression/decompression on the 924compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 925ioctls like the below. 926 927To decompress a file, 928 929fd = open(filename, O_WRONLY, 0); 930ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 931 932To compress a file, 933 934fd = open(filename, O_WRONLY, 0); 935ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 936 937NVMe Zoned Namespace devices 938---------------------------- 939 940- ZNS defines a per-zone capacity which can be equal or less than the 941 zone-size. Zone-capacity is the number of usable blocks in the zone. 942 F2FS checks if zone-capacity is less than zone-size, if it is, then any 943 segment which starts after the zone-capacity is marked as not-free in 944 the free segment bitmap at initial mount time. These segments are marked 945 as permanently used so they are not allocated for writes and 946 consequently are not needed to be garbage collected. In case the 947 zone-capacity is not aligned to default segment size(2MB), then a segment 948 can start before the zone-capacity and span across zone-capacity boundary. 949 Such spanning segments are also considered as usable segments. All blocks 950 past the zone-capacity are considered unusable in these segments. 951 952Device aliasing feature 953----------------------- 954 955f2fs can utilize a special file called a "device aliasing file." This file allows 956the entire storage device to be mapped with a single, large extent, not using 957the usual f2fs node structures. This mapped area is pinned and primarily intended 958for holding the space. 959 960Essentially, this mechanism allows a portion of the f2fs area to be temporarily 961reserved and used by another filesystem or for different purposes. Once that 962external usage is complete, the device aliasing file can be deleted, releasing 963the reserved space back to F2FS for its own use. 964 965<use-case> 966 967# ls /dev/vd* 968/dev/vdb (32GB) /dev/vdc (32GB) 969# mkfs.ext4 /dev/vdc 970# mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb 971# mount /dev/vdb /mnt/f2fs 972# ls -l /mnt/f2fs 973vdc.file 974# df -h 975/dev/vdb 64G 33G 32G 52% /mnt/f2fs 976 977# mount -o loop /dev/vdc /mnt/ext4 978# df -h 979/dev/vdb 64G 33G 32G 52% /mnt/f2fs 980/dev/loop7 32G 24K 30G 1% /mnt/ext4 981# umount /mnt/ext4 982 983# f2fs_io getflags /mnt/f2fs/vdc.file 984get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable 985# f2fs_io setflags noimmutable /mnt/f2fs/vdc.file 986get a flag on noimmutable ret=0, flags=800010 987set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable 988# rm /mnt/f2fs/vdc.file 989# df -h 990/dev/vdb 64G 753M 64G 2% /mnt/f2fs 991 992So, the key idea is, user can do any file operations on /dev/vdc, and 993reclaim the space after the use, while the space is counted as /data. 994That doesn't require modifying partition size and filesystem format. 995