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 reporting bugs and sending patches, please use the following mailing list: 29 30- linux-f2fs-devel@lists.sourceforge.net 31 32Background and Design issues 33============================ 34 35Log-structured File System (LFS) 36-------------------------------- 37"A log-structured file system writes all modifications to disk sequentially in 38a log-like structure, thereby speeding up both file writing and crash recovery. 39The log is the only structure on disk; it contains indexing information so that 40files can be read back from the log efficiently. In order to maintain large free 41areas on disk for fast writing, we divide the log into segments and use a 42segment cleaner to compress the live information from heavily fragmented 43segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and 44implementation of a log-structured file system", ACM Trans. Computer Systems 4510, 1, 26–52. 46 47Wandering Tree Problem 48---------------------- 49In LFS, when a file data is updated and written to the end of log, its direct 50pointer block is updated due to the changed location. Then the indirect pointer 51block is also updated due to the direct pointer block update. In this manner, 52the upper index structures such as inode, inode map, and checkpoint block are 53also updated recursively. This problem is called as wandering tree problem [1], 54and in order to enhance the performance, it should eliminate or relax the update 55propagation as much as possible. 56 57[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ 58 59Cleaning Overhead 60----------------- 61Since LFS is based on out-of-place writes, it produces so many obsolete blocks 62scattered across the whole storage. In order to serve new empty log space, it 63needs to reclaim these obsolete blocks seamlessly to users. This job is called 64as a cleaning process. 65 66The process consists of three operations as follows. 67 681. A victim segment is selected through referencing segment usage table. 692. It loads parent index structures of all the data in the victim identified by 70 segment summary blocks. 713. It checks the cross-reference between the data and its parent index structure. 724. It moves valid data selectively. 73 74This cleaning job may cause unexpected long delays, so the most important goal 75is to hide the latencies to users. And also definitely, it should reduce the 76amount of valid data to be moved, and move them quickly as well. 77 78Key Features 79============ 80 81Flash Awareness 82--------------- 83- Enlarge the random write area for better performance, but provide the high 84 spatial locality 85- Align FS data structures to the operational units in FTL as best efforts 86 87Wandering Tree Problem 88---------------------- 89- Use a term, “node”, that represents inodes as well as various pointer blocks 90- Introduce Node Address Table (NAT) containing the locations of all the “node” 91 blocks; this will cut off the update propagation. 92 93Cleaning Overhead 94----------------- 95- Support a background cleaning process 96- Support greedy and cost-benefit algorithms for victim selection policies 97- Support multi-head logs for static/dynamic hot and cold data separation 98- Introduce adaptive logging for efficient block allocation 99 100Mount Options 101============= 102 103 104======================== ============================================================ 105background_gc=%s Turn on/off cleaning operations, namely garbage 106 collection, triggered in background when I/O subsystem is 107 idle. If background_gc=on, it will turn on the garbage 108 collection and if background_gc=off, garbage collection 109 will be turned off. If background_gc=sync, it will turn 110 on synchronous garbage collection running in background. 111 Default value for this option is on. So garbage 112 collection is on by default. 113gc_merge When background_gc is on, this option can be enabled to 114 let background GC thread to handle foreground GC requests, 115 it can eliminate the sluggish issue caused by slow foreground 116 GC operation when GC is triggered from a process with limited 117 I/O and CPU resources. 118nogc_merge Disable GC merge feature. 119disable_roll_forward Disable the roll-forward recovery routine 120norecovery Disable the roll-forward recovery routine, mounted read- 121 only (i.e., -o ro,disable_roll_forward) 122discard/nodiscard Enable/disable real-time discard in f2fs, if discard is 123 enabled, f2fs will issue discard/TRIM commands when a 124 segment is cleaned. 125no_heap Disable heap-style segment allocation which finds free 126 segments for data from the beginning of main area, while 127 for node from the end of main area. 128nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 129 by default if CONFIG_F2FS_FS_XATTR is selected. 130noacl Disable POSIX Access Control List. Note: acl is enabled 131 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 132active_logs=%u Support configuring the number of active logs. In the 133 current design, f2fs supports only 2, 4, and 6 logs. 134 Default number is 6. 135disable_ext_identify Disable the extension list configured by mkfs, so f2fs 136 is not aware of cold files such as media files. 137inline_xattr Enable the inline xattrs feature. 138noinline_xattr Disable the inline xattrs feature. 139inline_xattr_size=%u Support configuring inline xattr size, it depends on 140 flexible inline xattr feature. 141inline_data Enable the inline data feature: Newly created small (<~3.4k) 142 files can be written into inode block. 143inline_dentry Enable the inline dir feature: data in newly created 144 directory entries can be written into inode block. The 145 space of inode block which is used to store inline 146 dentries is limited to ~3.4k. 147noinline_dentry Disable the inline dentry feature. 148flush_merge Merge concurrent cache_flush commands as much as possible 149 to eliminate redundant command issues. If the underlying 150 device handles the cache_flush command relatively slowly, 151 recommend to enable this option. 152nobarrier This option can be used if underlying storage guarantees 153 its cached data should be written to the novolatile area. 154 If this option is set, no cache_flush commands are issued 155 but f2fs still guarantees the write ordering of all the 156 data writes. 157fastboot This option is used when a system wants to reduce mount 158 time as much as possible, even though normal performance 159 can be sacrificed. 160extent_cache Enable an extent cache based on rb-tree, it can cache 161 as many as extent which map between contiguous logical 162 address and physical address per inode, resulting in 163 increasing the cache hit ratio. Set by default. 164noextent_cache Disable an extent cache based on rb-tree explicitly, see 165 the above extent_cache mount option. 166noinline_data Disable the inline data feature, inline data feature is 167 enabled by default. 168data_flush Enable data flushing before checkpoint in order to 169 persist data of regular and symlink. 170reserve_root=%d Support configuring reserved space which is used for 171 allocation from a privileged user with specified uid or 172 gid, unit: 4KB, the default limit is 0.2% of user blocks. 173resuid=%d The user ID which may use the reserved blocks. 174resgid=%d The group ID which may use the reserved blocks. 175fault_injection=%d Enable fault injection in all supported types with 176 specified injection rate. 177fault_type=%d Support configuring fault injection type, should be 178 enabled with fault_injection option, fault type value 179 is shown below, it supports single or combined type. 180 181 =================== =========== 182 Type_Name Type_Value 183 =================== =========== 184 FAULT_KMALLOC 0x000000001 185 FAULT_KVMALLOC 0x000000002 186 FAULT_PAGE_ALLOC 0x000000004 187 FAULT_PAGE_GET 0x000000008 188 FAULT_ALLOC_NID 0x000000020 189 FAULT_ORPHAN 0x000000040 190 FAULT_BLOCK 0x000000080 191 FAULT_DIR_DEPTH 0x000000100 192 FAULT_EVICT_INODE 0x000000200 193 FAULT_TRUNCATE 0x000000400 194 FAULT_READ_IO 0x000000800 195 FAULT_CHECKPOINT 0x000001000 196 FAULT_DISCARD 0x000002000 197 FAULT_WRITE_IO 0x000004000 198 =================== =========== 199mode=%s Control block allocation mode which supports "adaptive" 200 and "lfs". In "lfs" mode, there should be no random 201 writes towards main area. 202io_bits=%u Set the bit size of write IO requests. It should be set 203 with "mode=lfs". 204usrquota Enable plain user disk quota accounting. 205grpquota Enable plain group disk quota accounting. 206prjquota Enable plain project quota accounting. 207usrjquota=<file> Appoint specified file and type during mount, so that quota 208grpjquota=<file> information can be properly updated during recovery flow, 209prjjquota=<file> <quota file>: must be in root directory; 210jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 211offusrjquota Turn off user journalled quota. 212offgrpjquota Turn off group journalled quota. 213offprjjquota Turn off project journalled quota. 214quota Enable plain user disk quota accounting. 215noquota Disable all plain disk quota option. 216whint_mode=%s Control which write hints are passed down to block 217 layer. This supports "off", "user-based", and 218 "fs-based". In "off" mode (default), f2fs does not pass 219 down hints. In "user-based" mode, f2fs tries to pass 220 down hints given by users. And in "fs-based" mode, f2fs 221 passes down hints with its policy. 222alloc_mode=%s Adjust block allocation policy, which supports "reuse" 223 and "default". 224fsync_mode=%s Control the policy of fsync. Currently supports "posix", 225 "strict", and "nobarrier". In "posix" mode, which is 226 default, fsync will follow POSIX semantics and does a 227 light operation to improve the filesystem performance. 228 In "strict" mode, fsync will be heavy and behaves in line 229 with xfs, ext4 and btrfs, where xfstest generic/342 will 230 pass, but the performance will regress. "nobarrier" is 231 based on "posix", but doesn't issue flush command for 232 non-atomic files likewise "nobarrier" mount option. 233test_dummy_encryption 234test_dummy_encryption=%s 235 Enable dummy encryption, which provides a fake fscrypt 236 context. The fake fscrypt context is used by xfstests. 237 The argument may be either "v1" or "v2", in order to 238 select the corresponding fscrypt policy version. 239checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 240 to reenable checkpointing. Is enabled by default. While 241 disabled, any unmounting or unexpected shutdowns will cause 242 the filesystem contents to appear as they did when the 243 filesystem was mounted with that option. 244 While mounting with checkpoint=disabled, the filesystem must 245 run garbage collection to ensure that all available space can 246 be used. If this takes too much time, the mount may return 247 EAGAIN. You may optionally add a value to indicate how much 248 of the disk you would be willing to temporarily give up to 249 avoid additional garbage collection. This can be given as a 250 number of blocks, or as a percent. For instance, mounting 251 with checkpoint=disable:100% would always succeed, but it may 252 hide up to all remaining free space. The actual space that 253 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 254 This space is reclaimed once checkpoint=enable. 255checkpoint_merge When checkpoint is enabled, this can be used to create a kernel 256 daemon and make it to merge concurrent checkpoint requests as 257 much as possible to eliminate redundant checkpoint issues. Plus, 258 we can eliminate the sluggish issue caused by slow checkpoint 259 operation when the checkpoint is done in a process context in 260 a cgroup having low i/o budget and cpu shares. To make this 261 do better, we set the default i/o priority of the kernel daemon 262 to "3", to give one higher priority than other kernel threads. 263 This is the same way to give a I/O priority to the jbd2 264 journaling thread of ext4 filesystem. 265nocheckpoint_merge Disable checkpoint merge feature. 266compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 267 "lz4", "zstd" and "lzo-rle" algorithm. 268compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only 269 "lz4" and "zstd" support compress level config. 270 algorithm level range 271 lz4 3 - 16 272 zstd 1 - 22 273compress_log_size=%u Support configuring compress cluster size, the size will 274 be 4KB * (1 << %u), 16KB is minimum size, also it's 275 default size. 276compress_extension=%s Support adding specified extension, so that f2fs can enable 277 compression on those corresponding files, e.g. if all files 278 with '.ext' has high compression rate, we can set the '.ext' 279 on compression extension list and enable compression on 280 these file by default rather than to enable it via ioctl. 281 For other files, we can still enable compression via ioctl. 282 Note that, there is one reserved special extension '*', it 283 can be set to enable compression for all files. 284nocompress_extension=%s Support adding specified extension, so that f2fs can disable 285 compression on those corresponding files, just contrary to compression extension. 286 If you know exactly which files cannot be compressed, you can use this. 287 The same extension name can't appear in both compress and nocompress 288 extension at the same time. 289 If the compress extension specifies all files, the types specified by the 290 nocompress extension will be treated as special cases and will not be compressed. 291 Don't allow use '*' to specifie all file in nocompress extension. 292 After add nocompress_extension, the priority should be: 293 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 294 See more in compression sections. 295 296compress_chksum Support verifying chksum of raw data in compressed cluster. 297compress_mode=%s Control file compression mode. This supports "fs" and "user" 298 modes. In "fs" mode (default), f2fs does automatic compression 299 on the compression enabled files. In "user" mode, f2fs disables 300 the automaic compression and gives the user discretion of 301 choosing the target file and the timing. The user can do manual 302 compression/decompression on the compression enabled files using 303 ioctls. 304compress_cache Support to use address space of a filesystem managed inode to 305 cache compressed block, in order to improve cache hit ratio of 306 random read. 307inlinecrypt When possible, encrypt/decrypt the contents of encrypted 308 files using the blk-crypto framework rather than 309 filesystem-layer encryption. This allows the use of 310 inline encryption hardware. The on-disk format is 311 unaffected. For more details, see 312 Documentation/block/inline-encryption.rst. 313atgc Enable age-threshold garbage collection, it provides high 314 effectiveness and efficiency on background GC. 315======================== ============================================================ 316 317Debugfs Entries 318=============== 319 320/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 321f2fs. Each file shows the whole f2fs information. 322 323/sys/kernel/debug/f2fs/status includes: 324 325 - major file system information managed by f2fs currently 326 - average SIT information about whole segments 327 - current memory footprint consumed by f2fs. 328 329Sysfs Entries 330============= 331 332Information about mounted f2fs file systems can be found in 333/sys/fs/f2fs. Each mounted filesystem will have a directory in 334/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 335The files in each per-device directory are shown in table below. 336 337Files in /sys/fs/f2fs/<devname> 338(see also Documentation/ABI/testing/sysfs-fs-f2fs) 339 340Usage 341===== 342 3431. Download userland tools and compile them. 344 3452. Skip, if f2fs was compiled statically inside kernel. 346 Otherwise, insert the f2fs.ko module:: 347 348 # insmod f2fs.ko 349 3503. Create a directory to use when mounting:: 351 352 # mkdir /mnt/f2fs 353 3544. Format the block device, and then mount as f2fs:: 355 356 # mkfs.f2fs -l label /dev/block_device 357 # mount -t f2fs /dev/block_device /mnt/f2fs 358 359mkfs.f2fs 360--------- 361The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 362which builds a basic on-disk layout. 363 364The quick options consist of: 365 366=============== =========================================================== 367``-l [label]`` Give a volume label, up to 512 unicode name. 368``-a [0 or 1]`` Split start location of each area for heap-based allocation. 369 370 1 is set by default, which performs this. 371``-o [int]`` Set overprovision ratio in percent over volume size. 372 373 5 is set by default. 374``-s [int]`` Set the number of segments per section. 375 376 1 is set by default. 377``-z [int]`` Set the number of sections per zone. 378 379 1 is set by default. 380``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 381``-t [0 or 1]`` Disable discard command or not. 382 383 1 is set by default, which conducts discard. 384=============== =========================================================== 385 386Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 387 388fsck.f2fs 389--------- 390The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 391partition, which examines whether the filesystem metadata and user-made data 392are cross-referenced correctly or not. 393Note that, initial version of the tool does not fix any inconsistency. 394 395The quick options consist of:: 396 397 -d debug level [default:0] 398 399Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 400 401dump.f2fs 402--------- 403The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 404file. Each file is dump_ssa and dump_sit. 405 406The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 407It shows on-disk inode information recognized by a given inode number, and is 408able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 409./dump_sit respectively. 410 411The options consist of:: 412 413 -d debug level [default:0] 414 -i inode no (hex) 415 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 416 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 417 418Examples:: 419 420 # dump.f2fs -i [ino] /dev/sdx 421 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 422 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 423 424Note: please refer to the manpage of dump.f2fs(8) to get full option list. 425 426sload.f2fs 427---------- 428The sload.f2fs gives a way to insert files and directories in the exisiting disk 429image. This tool is useful when building f2fs images given compiled files. 430 431Note: please refer to the manpage of sload.f2fs(8) to get full option list. 432 433resize.f2fs 434----------- 435The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 436all the files and directories stored in the image. 437 438Note: please refer to the manpage of resize.f2fs(8) to get full option list. 439 440defrag.f2fs 441----------- 442The defrag.f2fs can be used to defragment scattered written data as well as 443filesystem metadata across the disk. This can improve the write speed by giving 444more free consecutive space. 445 446Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 447 448f2fs_io 449------- 450The f2fs_io is a simple tool to issue various filesystem APIs as well as 451f2fs-specific ones, which is very useful for QA tests. 452 453Note: please refer to the manpage of f2fs_io(8) to get full option list. 454 455Design 456====== 457 458On-disk Layout 459-------------- 460 461F2FS divides the whole volume into a number of segments, each of which is fixed 462to 2MB in size. A section is composed of consecutive segments, and a zone 463consists of a set of sections. By default, section and zone sizes are set to one 464segment size identically, but users can easily modify the sizes by mkfs. 465 466F2FS splits the entire volume into six areas, and all the areas except superblock 467consist of multiple segments as described below:: 468 469 align with the zone size <-| 470 |-> align with the segment size 471 _________________________________________________________________________ 472 | | | Segment | Node | Segment | | 473 | Superblock | Checkpoint | Info. | Address | Summary | Main | 474 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 475 |____________|_____2______|______N______|______N______|______N_____|__N___| 476 . . 477 . . 478 . . 479 ._________________________________________. 480 |_Segment_|_..._|_Segment_|_..._|_Segment_| 481 . . 482 ._________._________ 483 |_section_|__...__|_ 484 . . 485 .________. 486 |__zone__| 487 488- Superblock (SB) 489 It is located at the beginning of the partition, and there exist two copies 490 to avoid file system crash. It contains basic partition information and some 491 default parameters of f2fs. 492 493- Checkpoint (CP) 494 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 495 inode lists, and summary entries of current active segments. 496 497- Segment Information Table (SIT) 498 It contains segment information such as valid block count and bitmap for the 499 validity of all the blocks. 500 501- Node Address Table (NAT) 502 It is composed of a block address table for all the node blocks stored in 503 Main area. 504 505- Segment Summary Area (SSA) 506 It contains summary entries which contains the owner information of all the 507 data and node blocks stored in Main area. 508 509- Main Area 510 It contains file and directory data including their indices. 511 512In order to avoid misalignment between file system and flash-based storage, F2FS 513aligns the start block address of CP with the segment size. Also, it aligns the 514start block address of Main area with the zone size by reserving some segments 515in SSA area. 516 517Reference the following survey for additional technical details. 518https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 519 520File System Metadata Structure 521------------------------------ 522 523F2FS adopts the checkpointing scheme to maintain file system consistency. At 524mount time, F2FS first tries to find the last valid checkpoint data by scanning 525CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 526One of them always indicates the last valid data, which is called as shadow copy 527mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 528 529For file system consistency, each CP points to which NAT and SIT copies are 530valid, as shown as below:: 531 532 +--------+----------+---------+ 533 | CP | SIT | NAT | 534 +--------+----------+---------+ 535 . . . . 536 . . . . 537 . . . . 538 +-------+-------+--------+--------+--------+--------+ 539 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 540 +-------+-------+--------+--------+--------+--------+ 541 | ^ ^ 542 | | | 543 `----------------------------------------' 544 545Index Structure 546--------------- 547 548The key data structure to manage the data locations is a "node". Similar to 549traditional file structures, F2FS has three types of node: inode, direct node, 550indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 551indices, two direct node pointers, two indirect node pointers, and one double 552indirect node pointer as described below. One direct node block contains 1018 553data blocks, and one indirect node block contains also 1018 node blocks. Thus, 554one inode block (i.e., a file) covers:: 555 556 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 557 558 Inode block (4KB) 559 |- data (923) 560 |- direct node (2) 561 | `- data (1018) 562 |- indirect node (2) 563 | `- direct node (1018) 564 | `- data (1018) 565 `- double indirect node (1) 566 `- indirect node (1018) 567 `- direct node (1018) 568 `- data (1018) 569 570Note that all the node blocks are mapped by NAT which means the location of 571each node is translated by the NAT table. In the consideration of the wandering 572tree problem, F2FS is able to cut off the propagation of node updates caused by 573leaf data writes. 574 575Directory Structure 576------------------- 577 578A directory entry occupies 11 bytes, which consists of the following attributes. 579 580- hash hash value of the file name 581- ino inode number 582- len the length of file name 583- type file type such as directory, symlink, etc 584 585A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 586used to represent whether each dentry is valid or not. A dentry block occupies 5874KB with the following composition. 588 589:: 590 591 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 592 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 593 594 [Bucket] 595 +--------------------------------+ 596 |dentry block 1 | dentry block 2 | 597 +--------------------------------+ 598 . . 599 . . 600 . [Dentry Block Structure: 4KB] . 601 +--------+----------+----------+------------+ 602 | bitmap | reserved | dentries | file names | 603 +--------+----------+----------+------------+ 604 [Dentry Block: 4KB] . . 605 . . 606 . . 607 +------+------+-----+------+ 608 | hash | ino | len | type | 609 +------+------+-----+------+ 610 [Dentry Structure: 11 bytes] 611 612F2FS implements multi-level hash tables for directory structure. Each level has 613a hash table with dedicated number of hash buckets as shown below. Note that 614"A(2B)" means a bucket includes 2 data blocks. 615 616:: 617 618 ---------------------- 619 A : bucket 620 B : block 621 N : MAX_DIR_HASH_DEPTH 622 ---------------------- 623 624 level #0 | A(2B) 625 | 626 level #1 | A(2B) - A(2B) 627 | 628 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 629 . | . . . . 630 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 631 . | . . . . 632 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 633 634The number of blocks and buckets are determined by:: 635 636 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 637 # of blocks in level #n = | 638 `- 4, Otherwise 639 640 ,- 2^(n + dir_level), 641 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 642 # of buckets in level #n = | 643 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 644 Otherwise 645 646When F2FS finds a file name in a directory, at first a hash value of the file 647name is calculated. Then, F2FS scans the hash table in level #0 to find the 648dentry consisting of the file name and its inode number. If not found, F2FS 649scans the next hash table in level #1. In this way, F2FS scans hash tables in 650each levels incrementally from 1 to N. In each level F2FS needs to scan only 651one bucket determined by the following equation, which shows O(log(# of files)) 652complexity:: 653 654 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 655 656In the case of file creation, F2FS finds empty consecutive slots that cover the 657file name. F2FS searches the empty slots in the hash tables of whole levels from 6581 to N in the same way as the lookup operation. 659 660The following figure shows an example of two cases holding children:: 661 662 --------------> Dir <-------------- 663 | | 664 child child 665 666 child - child [hole] - child 667 668 child - child - child [hole] - [hole] - child 669 670 Case 1: Case 2: 671 Number of children = 6, Number of children = 3, 672 File size = 7 File size = 7 673 674Default Block Allocation 675------------------------ 676 677At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 678and Hot/Warm/Cold data. 679 680- Hot node contains direct node blocks of directories. 681- Warm node contains direct node blocks except hot node blocks. 682- Cold node contains indirect node blocks 683- Hot data contains dentry blocks 684- Warm data contains data blocks except hot and cold data blocks 685- Cold data contains multimedia data or migrated data blocks 686 687LFS has two schemes for free space management: threaded log and copy-and-compac- 688tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 689for devices showing very good sequential write performance, since free segments 690are served all the time for writing new data. However, it suffers from cleaning 691overhead under high utilization. Contrarily, the threaded log scheme suffers 692from random writes, but no cleaning process is needed. F2FS adopts a hybrid 693scheme where the copy-and-compaction scheme is adopted by default, but the 694policy is dynamically changed to the threaded log scheme according to the file 695system status. 696 697In order to align F2FS with underlying flash-based storage, F2FS allocates a 698segment in a unit of section. F2FS expects that the section size would be the 699same as the unit size of garbage collection in FTL. Furthermore, with respect 700to the mapping granularity in FTL, F2FS allocates each section of the active 701logs from different zones as much as possible, since FTL can write the data in 702the active logs into one allocation unit according to its mapping granularity. 703 704Cleaning process 705---------------- 706 707F2FS does cleaning both on demand and in the background. On-demand cleaning is 708triggered when there are not enough free segments to serve VFS calls. Background 709cleaner is operated by a kernel thread, and triggers the cleaning job when the 710system is idle. 711 712F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 713In the greedy algorithm, F2FS selects a victim segment having the smallest number 714of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 715according to the segment age and the number of valid blocks in order to address 716log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 717algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 718algorithm. 719 720In order to identify whether the data in the victim segment are valid or not, 721F2FS manages a bitmap. Each bit represents the validity of a block, and the 722bitmap is composed of a bit stream covering whole blocks in main area. 723 724Write-hint Policy 725----------------- 726 7271) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 728 7292) whint_mode=user-based. F2FS tries to pass down hints given by 730users. 731 732===================== ======================== =================== 733User F2FS Block 734===================== ======================== =================== 735N/A META WRITE_LIFE_NOT_SET 736N/A HOT_NODE " 737N/A WARM_NODE " 738N/A COLD_NODE " 739ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 740extension list " " 741 742-- buffered io 743WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 744WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 745WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 746WRITE_LIFE_NONE " " 747WRITE_LIFE_MEDIUM " " 748WRITE_LIFE_LONG " " 749 750-- direct io 751WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 752WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 753WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 754WRITE_LIFE_NONE " WRITE_LIFE_NONE 755WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 756WRITE_LIFE_LONG " WRITE_LIFE_LONG 757===================== ======================== =================== 758 7593) whint_mode=fs-based. F2FS passes down hints with its policy. 760 761===================== ======================== =================== 762User F2FS Block 763===================== ======================== =================== 764N/A META WRITE_LIFE_MEDIUM; 765N/A HOT_NODE WRITE_LIFE_NOT_SET 766N/A WARM_NODE " 767N/A COLD_NODE WRITE_LIFE_NONE 768ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 769extension list " " 770 771-- buffered io 772WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 773WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 774WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG 775WRITE_LIFE_NONE " " 776WRITE_LIFE_MEDIUM " " 777WRITE_LIFE_LONG " " 778 779-- direct io 780WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 781WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 782WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 783WRITE_LIFE_NONE " WRITE_LIFE_NONE 784WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 785WRITE_LIFE_LONG " WRITE_LIFE_LONG 786===================== ======================== =================== 787 788Fallocate(2) Policy 789------------------- 790 791The default policy follows the below POSIX rule. 792 793Allocating disk space 794 The default operation (i.e., mode is zero) of fallocate() allocates 795 the disk space within the range specified by offset and len. The 796 file size (as reported by stat(2)) will be changed if offset+len is 797 greater than the file size. Any subregion within the range specified 798 by offset and len that did not contain data before the call will be 799 initialized to zero. This default behavior closely resembles the 800 behavior of the posix_fallocate(3) library function, and is intended 801 as a method of optimally implementing that function. 802 803However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 804fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having 805zero or random data, which is useful to the below scenario where: 806 807 1. create(fd) 808 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 809 3. fallocate(fd, 0, 0, size) 810 4. address = fibmap(fd, offset) 811 5. open(blkdev) 812 6. write(blkdev, address) 813 814Compression implementation 815-------------------------- 816 817- New term named cluster is defined as basic unit of compression, file can 818 be divided into multiple clusters logically. One cluster includes 4 << n 819 (n >= 0) logical pages, compression size is also cluster size, each of 820 cluster can be compressed or not. 821 822- In cluster metadata layout, one special block address is used to indicate 823 a cluster is a compressed one or normal one; for compressed cluster, following 824 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 825 stores data including compress header and compressed data. 826 827- In order to eliminate write amplification during overwrite, F2FS only 828 support compression on write-once file, data can be compressed only when 829 all logical blocks in cluster contain valid data and compress ratio of 830 cluster data is lower than specified threshold. 831 832- To enable compression on regular inode, there are four ways: 833 834 * chattr +c file 835 * chattr +c dir; touch dir/file 836 * mount w/ -o compress_extension=ext; touch file.ext 837 * mount w/ -o compress_extension=*; touch any_file 838 839- To disable compression on regular inode, there are two ways: 840 841 * chattr -c file 842 * mount w/ -o nocompress_extension=ext; touch file.ext 843 844- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 845 846 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 847 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 848 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 849 can enable compress on bar.zip. 850 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 851 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 852 compresse, bar.zip and baz.txt should be non-compressed. 853 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 854 and baz.txt. 855 856- At this point, compression feature doesn't expose compressed space to user 857 directly in order to guarantee potential data updates later to the space. 858 Instead, the main goal is to reduce data writes to flash disk as much as 859 possible, resulting in extending disk life time as well as relaxing IO 860 congestion. Alternatively, we've added ioctl interface to reclaim compressed 861 space and show it to user after putting the immutable bit. 862 863Compress metadata layout:: 864 865 [Dnode Structure] 866 +-----------------------------------------------+ 867 | cluster 1 | cluster 2 | ......... | cluster N | 868 +-----------------------------------------------+ 869 . . . . 870 . . . . 871 . Compressed Cluster . . Normal Cluster . 872 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 873 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 874 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 875 . . 876 . . 877 . . 878 +-------------+-------------+----------+----------------------------+ 879 | data length | data chksum | reserved | compressed data | 880 +-------------+-------------+----------+----------------------------+ 881 882Compression mode 883-------------------------- 884 885f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 886With this option, f2fs provides a choice to select the way how to compress the 887compression enabled files (refer to "Compression implementation" section for how to 888enable compression on a regular inode). 889 8901) compress_mode=fs 891This is the default option. f2fs does automatic compression in the writeback of the 892compression enabled files. 893 8942) compress_mode=user 895This disables the automatic compression and gives the user discretion of choosing the 896target file and the timing. The user can do manual compression/decompression on the 897compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 898ioctls like the below. 899 900To decompress a file, 901 902fd = open(filename, O_WRONLY, 0); 903ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 904 905To compress a file, 906 907fd = open(filename, O_WRONLY, 0); 908ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 909 910NVMe Zoned Namespace devices 911---------------------------- 912 913- ZNS defines a per-zone capacity which can be equal or less than the 914 zone-size. Zone-capacity is the number of usable blocks in the zone. 915 F2FS checks if zone-capacity is less than zone-size, if it is, then any 916 segment which starts after the zone-capacity is marked as not-free in 917 the free segment bitmap at initial mount time. These segments are marked 918 as permanently used so they are not allocated for writes and 919 consequently are not needed to be garbage collected. In case the 920 zone-capacity is not aligned to default segment size(2MB), then a segment 921 can start before the zone-capacity and span across zone-capacity boundary. 922 Such spanning segments are also considered as usable segments. All blocks 923 past the zone-capacity are considered unusable in these segments. 924