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 FAULT_INCONSISTENT_FOOTER 0x000200000 210 =========================== =========== 211mode=%s Control block allocation mode which supports "adaptive" 212 and "lfs". In "lfs" mode, there should be no random 213 writes towards main area. 214 "fragment:segment" and "fragment:block" are newly added here. 215 These are developer options for experiments to simulate filesystem 216 fragmentation/after-GC situation itself. The developers use these 217 modes to understand filesystem fragmentation/after-GC condition well, 218 and eventually get some insights to handle them better. 219 In "fragment:segment", f2fs allocates a new segment in ramdom 220 position. With this, we can simulate the after-GC condition. 221 In "fragment:block", we can scatter block allocation with 222 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. 223 We added some randomness to both chunk and hole size to make 224 it close to realistic IO pattern. So, in this mode, f2fs will allocate 225 1..<max_fragment_chunk> blocks in a chunk and make a hole in the 226 length of 1..<max_fragment_hole> by turns. With this, the newly 227 allocated blocks will be scattered throughout the whole partition. 228 Note that "fragment:block" implicitly enables "fragment:segment" 229 option for more randomness. 230 Please, use these options for your experiments and we strongly 231 recommend to re-format the filesystem after using these options. 232usrquota Enable plain user disk quota accounting. 233grpquota Enable plain group disk quota accounting. 234prjquota Enable plain project quota accounting. 235usrjquota=<file> Appoint specified file and type during mount, so that quota 236grpjquota=<file> information can be properly updated during recovery flow, 237prjjquota=<file> <quota file>: must be in root directory; 238jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 239offusrjquota Turn off user journalled quota. 240offgrpjquota Turn off group journalled quota. 241offprjjquota Turn off project journalled quota. 242quota Enable plain user disk quota accounting. 243noquota Disable all plain disk quota option. 244alloc_mode=%s Adjust block allocation policy, which supports "reuse" 245 and "default". 246fsync_mode=%s Control the policy of fsync. Currently supports "posix", 247 "strict", and "nobarrier". In "posix" mode, which is 248 default, fsync will follow POSIX semantics and does a 249 light operation to improve the filesystem performance. 250 In "strict" mode, fsync will be heavy and behaves in line 251 with xfs, ext4 and btrfs, where xfstest generic/342 will 252 pass, but the performance will regress. "nobarrier" is 253 based on "posix", but doesn't issue flush command for 254 non-atomic files likewise "nobarrier" mount option. 255test_dummy_encryption 256test_dummy_encryption=%s 257 Enable dummy encryption, which provides a fake fscrypt 258 context. The fake fscrypt context is used by xfstests. 259 The argument may be either "v1" or "v2", in order to 260 select the corresponding fscrypt policy version. 261checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 262 to reenable checkpointing. Is enabled by default. While 263 disabled, any unmounting or unexpected shutdowns will cause 264 the filesystem contents to appear as they did when the 265 filesystem was mounted with that option. 266 While mounting with checkpoint=disable, the filesystem must 267 run garbage collection to ensure that all available space can 268 be used. If this takes too much time, the mount may return 269 EAGAIN. You may optionally add a value to indicate how much 270 of the disk you would be willing to temporarily give up to 271 avoid additional garbage collection. This can be given as a 272 number of blocks, or as a percent. For instance, mounting 273 with checkpoint=disable:100% would always succeed, but it may 274 hide up to all remaining free space. The actual space that 275 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 276 This space is reclaimed once checkpoint=enable. 277checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 278 daemon and make it to merge concurrent checkpoint requests as 279 much as possible to eliminate redundant checkpoint issues. Plus, 280 we can eliminate the sluggish issue caused by slow checkpoint 281 operation when the checkpoint is done in a process context in 282 a cgroup having low i/o budget and cpu shares. To make this 283 do better, we set the default i/o priority of the kernel daemon 284 to "3", to give one higher priority than other kernel threads. 285 This is the same way to give a I/O priority to the jbd2 286 journaling thread of ext4 filesystem. 287nocheckpoint_merge Disable checkpoint merge feature. 288compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 289 "lz4", "zstd" and "lzo-rle" algorithm. 290compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 291 "lz4" and "zstd" support compress level config. 292 algorithm level range 293 lz4 3 - 16 294 zstd 1 - 22 295compress_log_size=%u Support configuring compress cluster size. The size will 296 be 4KB * (1 << %u). The default and minimum sizes are 16KB. 297compress_extension=%s Support adding specified extension, so that f2fs can enable 298 compression on those corresponding files, e.g. if all files 299 with '.ext' has high compression rate, we can set the '.ext' 300 on compression extension list and enable compression on 301 these file by default rather than to enable it via ioctl. 302 For other files, we can still enable compression via ioctl. 303 Note that, there is one reserved special extension '*', it 304 can be set to enable compression for all files. 305nocompress_extension=%s Support adding specified extension, so that f2fs can disable 306 compression on those corresponding files, just contrary to compression extension. 307 If you know exactly which files cannot be compressed, you can use this. 308 The same extension name can't appear in both compress and nocompress 309 extension at the same time. 310 If the compress extension specifies all files, the types specified by the 311 nocompress extension will be treated as special cases and will not be compressed. 312 Don't allow use '*' to specifie all file in nocompress extension. 313 After add nocompress_extension, the priority should be: 314 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 315 See more in compression sections. 316 317compress_chksum Support verifying chksum of raw data in compressed cluster. 318compress_mode=%s Control file compression mode. This supports "fs" and "user" 319 modes. In "fs" mode (default), f2fs does automatic compression 320 on the compression enabled files. In "user" mode, f2fs disables 321 the automaic compression and gives the user discretion of 322 choosing the target file and the timing. The user can do manual 323 compression/decompression on the compression enabled files using 324 ioctls. 325compress_cache Support to use address space of a filesystem managed inode to 326 cache compressed block, in order to improve cache hit ratio of 327 random read. 328inlinecrypt When possible, encrypt/decrypt the contents of encrypted 329 files using the blk-crypto framework rather than 330 filesystem-layer encryption. This allows the use of 331 inline encryption hardware. The on-disk format is 332 unaffected. For more details, see 333 Documentation/block/inline-encryption.rst. 334atgc Enable age-threshold garbage collection, it provides high 335 effectiveness and efficiency on background GC. 336discard_unit=%s Control discard unit, the argument can be "block", "segment" 337 and "section", issued discard command's offset/size will be 338 aligned to the unit, by default, "discard_unit=block" is set, 339 so that small discard functionality is enabled. 340 For blkzoned device, "discard_unit=section" will be set by 341 default, it is helpful for large sized SMR or ZNS devices to 342 reduce memory cost by getting rid of fs metadata supports small 343 discard. 344memory=%s Control memory mode. This supports "normal" and "low" modes. 345 "low" mode is introduced to support low memory devices. 346 Because of the nature of low memory devices, in this mode, f2fs 347 will try to save memory sometimes by sacrificing performance. 348 "normal" mode is the default mode and same as before. 349age_extent_cache Enable an age extent cache based on rb-tree. It records 350 data block update frequency of the extent per inode, in 351 order to provide better temperature hints for data block 352 allocation. 353errors=%s Specify f2fs behavior on critical errors. This supports modes: 354 "panic", "continue" and "remount-ro", respectively, trigger 355 panic immediately, continue without doing anything, and remount 356 the partition in read-only mode. By default it uses "continue" 357 mode. 358 ====================== =============== =============== ======== 359 mode continue remount-ro panic 360 ====================== =============== =============== ======== 361 access ops normal normal N/A 362 syscall errors -EIO -EROFS N/A 363 mount option rw ro N/A 364 pending dir write keep keep N/A 365 pending non-dir write drop keep N/A 366 pending node write drop keep N/A 367 pending meta write keep keep N/A 368 ====================== =============== =============== ======== 369nat_bits Enable nat_bits feature to enhance full/empty nat blocks access, 370 by default it's disabled. 371======================== ============================================================ 372 373Debugfs Entries 374=============== 375 376/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 377f2fs. Each file shows the whole f2fs information. 378 379/sys/kernel/debug/f2fs/status includes: 380 381 - major file system information managed by f2fs currently 382 - average SIT information about whole segments 383 - current memory footprint consumed by f2fs. 384 385Sysfs Entries 386============= 387 388Information about mounted f2fs file systems can be found in 389/sys/fs/f2fs. Each mounted filesystem will have a directory in 390/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 391The files in each per-device directory are shown in table below. 392 393Files in /sys/fs/f2fs/<devname> 394(see also Documentation/ABI/testing/sysfs-fs-f2fs) 395 396Usage 397===== 398 3991. Download userland tools and compile them. 400 4012. Skip, if f2fs was compiled statically inside kernel. 402 Otherwise, insert the f2fs.ko module:: 403 404 # insmod f2fs.ko 405 4063. Create a directory to use when mounting:: 407 408 # mkdir /mnt/f2fs 409 4104. Format the block device, and then mount as f2fs:: 411 412 # mkfs.f2fs -l label /dev/block_device 413 # mount -t f2fs /dev/block_device /mnt/f2fs 414 415mkfs.f2fs 416--------- 417The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 418which builds a basic on-disk layout. 419 420The quick options consist of: 421 422=============== =========================================================== 423``-l [label]`` Give a volume label, up to 512 unicode name. 424``-a [0 or 1]`` Split start location of each area for heap-based allocation. 425 426 1 is set by default, which performs this. 427``-o [int]`` Set overprovision ratio in percent over volume size. 428 429 5 is set by default. 430``-s [int]`` Set the number of segments per section. 431 432 1 is set by default. 433``-z [int]`` Set the number of sections per zone. 434 435 1 is set by default. 436``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 437``-t [0 or 1]`` Disable discard command or not. 438 439 1 is set by default, which conducts discard. 440=============== =========================================================== 441 442Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 443 444fsck.f2fs 445--------- 446The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 447partition, which examines whether the filesystem metadata and user-made data 448are cross-referenced correctly or not. 449Note that, initial version of the tool does not fix any inconsistency. 450 451The quick options consist of:: 452 453 -d debug level [default:0] 454 455Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 456 457dump.f2fs 458--------- 459The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 460file. Each file is dump_ssa and dump_sit. 461 462The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 463It shows on-disk inode information recognized by a given inode number, and is 464able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 465./dump_sit respectively. 466 467The options consist of:: 468 469 -d debug level [default:0] 470 -i inode no (hex) 471 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 472 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 473 474Examples:: 475 476 # dump.f2fs -i [ino] /dev/sdx 477 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 478 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 479 480Note: please refer to the manpage of dump.f2fs(8) to get full option list. 481 482sload.f2fs 483---------- 484The sload.f2fs gives a way to insert files and directories in the existing disk 485image. This tool is useful when building f2fs images given compiled files. 486 487Note: please refer to the manpage of sload.f2fs(8) to get full option list. 488 489resize.f2fs 490----------- 491The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 492all the files and directories stored in the image. 493 494Note: please refer to the manpage of resize.f2fs(8) to get full option list. 495 496defrag.f2fs 497----------- 498The defrag.f2fs can be used to defragment scattered written data as well as 499filesystem metadata across the disk. This can improve the write speed by giving 500more free consecutive space. 501 502Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 503 504f2fs_io 505------- 506The f2fs_io is a simple tool to issue various filesystem APIs as well as 507f2fs-specific ones, which is very useful for QA tests. 508 509Note: please refer to the manpage of f2fs_io(8) to get full option list. 510 511Design 512====== 513 514On-disk Layout 515-------------- 516 517F2FS divides the whole volume into a number of segments, each of which is fixed 518to 2MB in size. A section is composed of consecutive segments, and a zone 519consists of a set of sections. By default, section and zone sizes are set to one 520segment size identically, but users can easily modify the sizes by mkfs. 521 522F2FS splits the entire volume into six areas, and all the areas except superblock 523consist of multiple segments as described below:: 524 525 align with the zone size <-| 526 |-> align with the segment size 527 _________________________________________________________________________ 528 | | | Segment | Node | Segment | | 529 | Superblock | Checkpoint | Info. | Address | Summary | Main | 530 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 531 |____________|_____2______|______N______|______N______|______N_____|__N___| 532 . . 533 . . 534 . . 535 ._________________________________________. 536 |_Segment_|_..._|_Segment_|_..._|_Segment_| 537 . . 538 ._________._________ 539 |_section_|__...__|_ 540 . . 541 .________. 542 |__zone__| 543 544- Superblock (SB) 545 It is located at the beginning of the partition, and there exist two copies 546 to avoid file system crash. It contains basic partition information and some 547 default parameters of f2fs. 548 549- Checkpoint (CP) 550 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 551 inode lists, and summary entries of current active segments. 552 553- Segment Information Table (SIT) 554 It contains segment information such as valid block count and bitmap for the 555 validity of all the blocks. 556 557- Node Address Table (NAT) 558 It is composed of a block address table for all the node blocks stored in 559 Main area. 560 561- Segment Summary Area (SSA) 562 It contains summary entries which contains the owner information of all the 563 data and node blocks stored in Main area. 564 565- Main Area 566 It contains file and directory data including their indices. 567 568In order to avoid misalignment between file system and flash-based storage, F2FS 569aligns the start block address of CP with the segment size. Also, it aligns the 570start block address of Main area with the zone size by reserving some segments 571in SSA area. 572 573Reference the following survey for additional technical details. 574https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 575 576File System Metadata Structure 577------------------------------ 578 579F2FS adopts the checkpointing scheme to maintain file system consistency. At 580mount time, F2FS first tries to find the last valid checkpoint data by scanning 581CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 582One of them always indicates the last valid data, which is called as shadow copy 583mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 584 585For file system consistency, each CP points to which NAT and SIT copies are 586valid, as shown as below:: 587 588 +--------+----------+---------+ 589 | CP | SIT | NAT | 590 +--------+----------+---------+ 591 . . . . 592 . . . . 593 . . . . 594 +-------+-------+--------+--------+--------+--------+ 595 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 596 +-------+-------+--------+--------+--------+--------+ 597 | ^ ^ 598 | | | 599 `----------------------------------------' 600 601Index Structure 602--------------- 603 604The key data structure to manage the data locations is a "node". Similar to 605traditional file structures, F2FS has three types of node: inode, direct node, 606indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 607indices, two direct node pointers, two indirect node pointers, and one double 608indirect node pointer as described below. One direct node block contains 1018 609data blocks, and one indirect node block contains also 1018 node blocks. Thus, 610one inode block (i.e., a file) covers:: 611 612 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 613 614 Inode block (4KB) 615 |- data (923) 616 |- direct node (2) 617 | `- data (1018) 618 |- indirect node (2) 619 | `- direct node (1018) 620 | `- data (1018) 621 `- double indirect node (1) 622 `- indirect node (1018) 623 `- direct node (1018) 624 `- data (1018) 625 626Note that all the node blocks are mapped by NAT which means the location of 627each node is translated by the NAT table. In the consideration of the wandering 628tree problem, F2FS is able to cut off the propagation of node updates caused by 629leaf data writes. 630 631Directory Structure 632------------------- 633 634A directory entry occupies 11 bytes, which consists of the following attributes. 635 636- hash hash value of the file name 637- ino inode number 638- len the length of file name 639- type file type such as directory, symlink, etc 640 641A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 642used to represent whether each dentry is valid or not. A dentry block occupies 6434KB with the following composition. 644 645:: 646 647 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 648 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 649 650 [Bucket] 651 +--------------------------------+ 652 |dentry block 1 | dentry block 2 | 653 +--------------------------------+ 654 . . 655 . . 656 . [Dentry Block Structure: 4KB] . 657 +--------+----------+----------+------------+ 658 | bitmap | reserved | dentries | file names | 659 +--------+----------+----------+------------+ 660 [Dentry Block: 4KB] . . 661 . . 662 . . 663 +------+------+-----+------+ 664 | hash | ino | len | type | 665 +------+------+-----+------+ 666 [Dentry Structure: 11 bytes] 667 668F2FS implements multi-level hash tables for directory structure. Each level has 669a hash table with dedicated number of hash buckets as shown below. Note that 670"A(2B)" means a bucket includes 2 data blocks. 671 672:: 673 674 ---------------------- 675 A : bucket 676 B : block 677 N : MAX_DIR_HASH_DEPTH 678 ---------------------- 679 680 level #0 | A(2B) 681 | 682 level #1 | A(2B) - A(2B) 683 | 684 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 685 . | . . . . 686 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 687 . | . . . . 688 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 689 690The number of blocks and buckets are determined by:: 691 692 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 693 # of blocks in level #n = | 694 `- 4, Otherwise 695 696 ,- 2^(n + dir_level), 697 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 698 # of buckets in level #n = | 699 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 700 Otherwise 701 702When F2FS finds a file name in a directory, at first a hash value of the file 703name is calculated. Then, F2FS scans the hash table in level #0 to find the 704dentry consisting of the file name and its inode number. If not found, F2FS 705scans the next hash table in level #1. In this way, F2FS scans hash tables in 706each levels incrementally from 1 to N. In each level F2FS needs to scan only 707one bucket determined by the following equation, which shows O(log(# of files)) 708complexity:: 709 710 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 711 712In the case of file creation, F2FS finds empty consecutive slots that cover the 713file name. F2FS searches the empty slots in the hash tables of whole levels from 7141 to N in the same way as the lookup operation. 715 716The following figure shows an example of two cases holding children:: 717 718 --------------> Dir <-------------- 719 | | 720 child child 721 722 child - child [hole] - child 723 724 child - child - child [hole] - [hole] - child 725 726 Case 1: Case 2: 727 Number of children = 6, Number of children = 3, 728 File size = 7 File size = 7 729 730Default Block Allocation 731------------------------ 732 733At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 734and Hot/Warm/Cold data. 735 736- Hot node contains direct node blocks of directories. 737- Warm node contains direct node blocks except hot node blocks. 738- Cold node contains indirect node blocks 739- Hot data contains dentry blocks 740- Warm data contains data blocks except hot and cold data blocks 741- Cold data contains multimedia data or migrated data blocks 742 743LFS has two schemes for free space management: threaded log and copy-and-compac- 744tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 745for devices showing very good sequential write performance, since free segments 746are served all the time for writing new data. However, it suffers from cleaning 747overhead under high utilization. Contrarily, the threaded log scheme suffers 748from random writes, but no cleaning process is needed. F2FS adopts a hybrid 749scheme where the copy-and-compaction scheme is adopted by default, but the 750policy is dynamically changed to the threaded log scheme according to the file 751system status. 752 753In order to align F2FS with underlying flash-based storage, F2FS allocates a 754segment in a unit of section. F2FS expects that the section size would be the 755same as the unit size of garbage collection in FTL. Furthermore, with respect 756to the mapping granularity in FTL, F2FS allocates each section of the active 757logs from different zones as much as possible, since FTL can write the data in 758the active logs into one allocation unit according to its mapping granularity. 759 760Cleaning process 761---------------- 762 763F2FS does cleaning both on demand and in the background. On-demand cleaning is 764triggered when there are not enough free segments to serve VFS calls. Background 765cleaner is operated by a kernel thread, and triggers the cleaning job when the 766system is idle. 767 768F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 769In the greedy algorithm, F2FS selects a victim segment having the smallest number 770of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 771according to the segment age and the number of valid blocks in order to address 772log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 773algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 774algorithm. 775 776In order to identify whether the data in the victim segment are valid or not, 777F2FS manages a bitmap. Each bit represents the validity of a block, and the 778bitmap is composed of a bit stream covering whole blocks in main area. 779 780Write-hint Policy 781----------------- 782 783F2FS sets the whint all the time with the below policy. 784 785===================== ======================== =================== 786User F2FS Block 787===================== ======================== =================== 788N/A META WRITE_LIFE_NONE|REQ_META 789N/A HOT_NODE WRITE_LIFE_NONE 790N/A WARM_NODE WRITE_LIFE_MEDIUM 791N/A COLD_NODE WRITE_LIFE_LONG 792ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 793extension list " " 794 795-- buffered io 796N/A COLD_DATA WRITE_LIFE_EXTREME 797N/A HOT_DATA WRITE_LIFE_SHORT 798N/A WARM_DATA WRITE_LIFE_NOT_SET 799 800-- direct io 801WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 802WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 803WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 804WRITE_LIFE_NONE " WRITE_LIFE_NONE 805WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 806WRITE_LIFE_LONG " WRITE_LIFE_LONG 807===================== ======================== =================== 808 809Fallocate(2) Policy 810------------------- 811 812The default policy follows the below POSIX rule. 813 814Allocating disk space 815 The default operation (i.e., mode is zero) of fallocate() allocates 816 the disk space within the range specified by offset and len. The 817 file size (as reported by stat(2)) will be changed if offset+len is 818 greater than the file size. Any subregion within the range specified 819 by offset and len that did not contain data before the call will be 820 initialized to zero. This default behavior closely resembles the 821 behavior of the posix_fallocate(3) library function, and is intended 822 as a method of optimally implementing that function. 823 824However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 825fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having 826zero or random data, which is useful to the below scenario where: 827 828 1. create(fd) 829 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 830 3. fallocate(fd, 0, 0, size) 831 4. address = fibmap(fd, offset) 832 5. open(blkdev) 833 6. write(blkdev, address) 834 835Compression implementation 836-------------------------- 837 838- New term named cluster is defined as basic unit of compression, file can 839 be divided into multiple clusters logically. One cluster includes 4 << n 840 (n >= 0) logical pages, compression size is also cluster size, each of 841 cluster can be compressed or not. 842 843- In cluster metadata layout, one special block address is used to indicate 844 a cluster is a compressed one or normal one; for compressed cluster, following 845 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 846 stores data including compress header and compressed data. 847 848- In order to eliminate write amplification during overwrite, F2FS only 849 support compression on write-once file, data can be compressed only when 850 all logical blocks in cluster contain valid data and compress ratio of 851 cluster data is lower than specified threshold. 852 853- To enable compression on regular inode, there are four ways: 854 855 * chattr +c file 856 * chattr +c dir; touch dir/file 857 * mount w/ -o compress_extension=ext; touch file.ext 858 * mount w/ -o compress_extension=*; touch any_file 859 860- To disable compression on regular inode, there are two ways: 861 862 * chattr -c file 863 * mount w/ -o nocompress_extension=ext; touch file.ext 864 865- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 866 867 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 868 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 869 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 870 can enable compress on bar.zip. 871 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 872 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 873 compresse, bar.zip and baz.txt should be non-compressed. 874 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 875 and baz.txt. 876 877- At this point, compression feature doesn't expose compressed space to user 878 directly in order to guarantee potential data updates later to the space. 879 Instead, the main goal is to reduce data writes to flash disk as much as 880 possible, resulting in extending disk life time as well as relaxing IO 881 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 882 interface to reclaim compressed space and show it to user after setting a 883 special flag to the inode. Once the compressed space is released, the flag 884 will block writing data to the file until either the compressed space is 885 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is 886 truncated to zero. 887 888Compress metadata layout:: 889 890 [Dnode Structure] 891 +-----------------------------------------------+ 892 | cluster 1 | cluster 2 | ......... | cluster N | 893 +-----------------------------------------------+ 894 . . . . 895 . . . . 896 . Compressed Cluster . . Normal Cluster . 897 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 898 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 899 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 900 . . 901 . . 902 . . 903 +-------------+-------------+----------+----------------------------+ 904 | data length | data chksum | reserved | compressed data | 905 +-------------+-------------+----------+----------------------------+ 906 907Compression mode 908-------------------------- 909 910f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 911With this option, f2fs provides a choice to select the way how to compress the 912compression enabled files (refer to "Compression implementation" section for how to 913enable compression on a regular inode). 914 9151) compress_mode=fs 916This is the default option. f2fs does automatic compression in the writeback of the 917compression enabled files. 918 9192) compress_mode=user 920This disables the automatic compression and gives the user discretion of choosing the 921target file and the timing. The user can do manual compression/decompression on the 922compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 923ioctls like the below. 924 925To decompress a file, 926 927fd = open(filename, O_WRONLY, 0); 928ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 929 930To compress a file, 931 932fd = open(filename, O_WRONLY, 0); 933ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 934 935NVMe Zoned Namespace devices 936---------------------------- 937 938- ZNS defines a per-zone capacity which can be equal or less than the 939 zone-size. Zone-capacity is the number of usable blocks in the zone. 940 F2FS checks if zone-capacity is less than zone-size, if it is, then any 941 segment which starts after the zone-capacity is marked as not-free in 942 the free segment bitmap at initial mount time. These segments are marked 943 as permanently used so they are not allocated for writes and 944 consequently are not needed to be garbage collected. In case the 945 zone-capacity is not aligned to default segment size(2MB), then a segment 946 can start before the zone-capacity and span across zone-capacity boundary. 947 Such spanning segments are also considered as usable segments. All blocks 948 past the zone-capacity are considered unusable in these segments. 949 950Device aliasing feature 951----------------------- 952 953f2fs can utilize a special file called a "device aliasing file." This file allows 954the entire storage device to be mapped with a single, large extent, not using 955the usual f2fs node structures. This mapped area is pinned and primarily intended 956for holding the space. 957 958Essentially, this mechanism allows a portion of the f2fs area to be temporarily 959reserved and used by another filesystem or for different purposes. Once that 960external usage is complete, the device aliasing file can be deleted, releasing 961the reserved space back to F2FS for its own use. 962 963<use-case> 964 965# ls /dev/vd* 966/dev/vdb (32GB) /dev/vdc (32GB) 967# mkfs.ext4 /dev/vdc 968# mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb 969# mount /dev/vdb /mnt/f2fs 970# ls -l /mnt/f2fs 971vdc.file 972# df -h 973/dev/vdb 64G 33G 32G 52% /mnt/f2fs 974 975# mount -o loop /dev/vdc /mnt/ext4 976# df -h 977/dev/vdb 64G 33G 32G 52% /mnt/f2fs 978/dev/loop7 32G 24K 30G 1% /mnt/ext4 979# umount /mnt/ext4 980 981# f2fs_io getflags /mnt/f2fs/vdc.file 982get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable 983# f2fs_io setflags noimmutable /mnt/f2fs/vdc.file 984get a flag on noimmutable ret=0, flags=800010 985set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable 986# rm /mnt/f2fs/vdc.file 987# df -h 988/dev/vdb 64G 753M 64G 2% /mnt/f2fs 989 990So, the key idea is, user can do any file operations on /dev/vdc, and 991reclaim the space after the use, while the space is counted as /data. 992That doesn't require modifying partition size and filesystem format. 993