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_BIO 0x000000010 (obsolete) 189 FAULT_ALLOC_NID 0x000000020 190 FAULT_ORPHAN 0x000000040 191 FAULT_BLOCK 0x000000080 192 FAULT_DIR_DEPTH 0x000000100 193 FAULT_EVICT_INODE 0x000000200 194 FAULT_TRUNCATE 0x000000400 195 FAULT_READ_IO 0x000000800 196 FAULT_CHECKPOINT 0x000001000 197 FAULT_DISCARD 0x000002000 198 FAULT_WRITE_IO 0x000004000 199 FAULT_SLAB_ALLOC 0x000008000 200 FAULT_DQUOT_INIT 0x000010000 201 FAULT_LOCK_OP 0x000020000 202 =================== =========== 203mode=%s Control block allocation mode which supports "adaptive" 204 and "lfs". In "lfs" mode, there should be no random 205 writes towards main area. 206 "fragment:segment" and "fragment:block" are newly added here. 207 These are developer options for experiments to simulate filesystem 208 fragmentation/after-GC situation itself. The developers use these 209 modes to understand filesystem fragmentation/after-GC condition well, 210 and eventually get some insights to handle them better. 211 In "fragment:segment", f2fs allocates a new segment in ramdom 212 position. With this, we can simulate the after-GC condition. 213 In "fragment:block", we can scatter block allocation with 214 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. 215 We added some randomness to both chunk and hole size to make 216 it close to realistic IO pattern. So, in this mode, f2fs will allocate 217 1..<max_fragment_chunk> blocks in a chunk and make a hole in the 218 length of 1..<max_fragment_hole> by turns. With this, the newly 219 allocated blocks will be scattered throughout the whole partition. 220 Note that "fragment:block" implicitly enables "fragment:segment" 221 option for more randomness. 222 Please, use these options for your experiments and we strongly 223 recommend to re-format the filesystem after using these options. 224io_bits=%u Set the bit size of write IO requests. It should be set 225 with "mode=lfs". 226usrquota Enable plain user disk quota accounting. 227grpquota Enable plain group disk quota accounting. 228prjquota Enable plain project quota accounting. 229usrjquota=<file> Appoint specified file and type during mount, so that quota 230grpjquota=<file> information can be properly updated during recovery flow, 231prjjquota=<file> <quota file>: must be in root directory; 232jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 233offusrjquota Turn off user journalled quota. 234offgrpjquota Turn off group journalled quota. 235offprjjquota Turn off project journalled quota. 236quota Enable plain user disk quota accounting. 237noquota Disable all plain disk quota option. 238whint_mode=%s Control which write hints are passed down to block 239 layer. This supports "off", "user-based", and 240 "fs-based". In "off" mode (default), f2fs does not pass 241 down hints. In "user-based" mode, f2fs tries to pass 242 down hints given by users. And in "fs-based" mode, f2fs 243 passes down hints with its policy. 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=disabled, 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), 16KB is minimum size, also it's 297 default size. 298compress_extension=%s Support adding specified extension, so that f2fs can enable 299 compression on those corresponding files, e.g. if all files 300 with '.ext' has high compression rate, we can set the '.ext' 301 on compression extension list and enable compression on 302 these file by default rather than to enable it via ioctl. 303 For other files, we can still enable compression via ioctl. 304 Note that, there is one reserved special extension '*', it 305 can be set to enable compression for all files. 306nocompress_extension=%s Support adding specified extension, so that f2fs can disable 307 compression on those corresponding files, just contrary to compression extension. 308 If you know exactly which files cannot be compressed, you can use this. 309 The same extension name can't appear in both compress and nocompress 310 extension at the same time. 311 If the compress extension specifies all files, the types specified by the 312 nocompress extension will be treated as special cases and will not be compressed. 313 Don't allow use '*' to specifie all file in nocompress extension. 314 After add nocompress_extension, the priority should be: 315 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. 316 See more in compression sections. 317 318compress_chksum Support verifying chksum of raw data in compressed cluster. 319compress_mode=%s Control file compression mode. This supports "fs" and "user" 320 modes. In "fs" mode (default), f2fs does automatic compression 321 on the compression enabled files. In "user" mode, f2fs disables 322 the automaic compression and gives the user discretion of 323 choosing the target file and the timing. The user can do manual 324 compression/decompression on the compression enabled files using 325 ioctls. 326compress_cache Support to use address space of a filesystem managed inode to 327 cache compressed block, in order to improve cache hit ratio of 328 random read. 329inlinecrypt When possible, encrypt/decrypt the contents of encrypted 330 files using the blk-crypto framework rather than 331 filesystem-layer encryption. This allows the use of 332 inline encryption hardware. The on-disk format is 333 unaffected. For more details, see 334 Documentation/block/inline-encryption.rst. 335atgc Enable age-threshold garbage collection, it provides high 336 effectiveness and efficiency on background GC. 337discard_unit=%s Control discard unit, the argument can be "block", "segment" 338 and "section", issued discard command's offset/size will be 339 aligned to the unit, by default, "discard_unit=block" is set, 340 so that small discard functionality is enabled. 341 For blkzoned device, "discard_unit=section" will be set by 342 default, it is helpful for large sized SMR or ZNS devices to 343 reduce memory cost by getting rid of fs metadata supports small 344 discard. 345======================== ============================================================ 346 347Debugfs Entries 348=============== 349 350/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 351f2fs. Each file shows the whole f2fs information. 352 353/sys/kernel/debug/f2fs/status includes: 354 355 - major file system information managed by f2fs currently 356 - average SIT information about whole segments 357 - current memory footprint consumed by f2fs. 358 359Sysfs Entries 360============= 361 362Information about mounted f2fs file systems can be found in 363/sys/fs/f2fs. Each mounted filesystem will have a directory in 364/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 365The files in each per-device directory are shown in table below. 366 367Files in /sys/fs/f2fs/<devname> 368(see also Documentation/ABI/testing/sysfs-fs-f2fs) 369 370Usage 371===== 372 3731. Download userland tools and compile them. 374 3752. Skip, if f2fs was compiled statically inside kernel. 376 Otherwise, insert the f2fs.ko module:: 377 378 # insmod f2fs.ko 379 3803. Create a directory to use when mounting:: 381 382 # mkdir /mnt/f2fs 383 3844. Format the block device, and then mount as f2fs:: 385 386 # mkfs.f2fs -l label /dev/block_device 387 # mount -t f2fs /dev/block_device /mnt/f2fs 388 389mkfs.f2fs 390--------- 391The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 392which builds a basic on-disk layout. 393 394The quick options consist of: 395 396=============== =========================================================== 397``-l [label]`` Give a volume label, up to 512 unicode name. 398``-a [0 or 1]`` Split start location of each area for heap-based allocation. 399 400 1 is set by default, which performs this. 401``-o [int]`` Set overprovision ratio in percent over volume size. 402 403 5 is set by default. 404``-s [int]`` Set the number of segments per section. 405 406 1 is set by default. 407``-z [int]`` Set the number of sections per zone. 408 409 1 is set by default. 410``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 411``-t [0 or 1]`` Disable discard command or not. 412 413 1 is set by default, which conducts discard. 414=============== =========================================================== 415 416Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 417 418fsck.f2fs 419--------- 420The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 421partition, which examines whether the filesystem metadata and user-made data 422are cross-referenced correctly or not. 423Note that, initial version of the tool does not fix any inconsistency. 424 425The quick options consist of:: 426 427 -d debug level [default:0] 428 429Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 430 431dump.f2fs 432--------- 433The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 434file. Each file is dump_ssa and dump_sit. 435 436The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 437It shows on-disk inode information recognized by a given inode number, and is 438able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 439./dump_sit respectively. 440 441The options consist of:: 442 443 -d debug level [default:0] 444 -i inode no (hex) 445 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 446 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 447 448Examples:: 449 450 # dump.f2fs -i [ino] /dev/sdx 451 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 452 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 453 454Note: please refer to the manpage of dump.f2fs(8) to get full option list. 455 456sload.f2fs 457---------- 458The sload.f2fs gives a way to insert files and directories in the exisiting disk 459image. This tool is useful when building f2fs images given compiled files. 460 461Note: please refer to the manpage of sload.f2fs(8) to get full option list. 462 463resize.f2fs 464----------- 465The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 466all the files and directories stored in the image. 467 468Note: please refer to the manpage of resize.f2fs(8) to get full option list. 469 470defrag.f2fs 471----------- 472The defrag.f2fs can be used to defragment scattered written data as well as 473filesystem metadata across the disk. This can improve the write speed by giving 474more free consecutive space. 475 476Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 477 478f2fs_io 479------- 480The f2fs_io is a simple tool to issue various filesystem APIs as well as 481f2fs-specific ones, which is very useful for QA tests. 482 483Note: please refer to the manpage of f2fs_io(8) to get full option list. 484 485Design 486====== 487 488On-disk Layout 489-------------- 490 491F2FS divides the whole volume into a number of segments, each of which is fixed 492to 2MB in size. A section is composed of consecutive segments, and a zone 493consists of a set of sections. By default, section and zone sizes are set to one 494segment size identically, but users can easily modify the sizes by mkfs. 495 496F2FS splits the entire volume into six areas, and all the areas except superblock 497consist of multiple segments as described below:: 498 499 align with the zone size <-| 500 |-> align with the segment size 501 _________________________________________________________________________ 502 | | | Segment | Node | Segment | | 503 | Superblock | Checkpoint | Info. | Address | Summary | Main | 504 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 505 |____________|_____2______|______N______|______N______|______N_____|__N___| 506 . . 507 . . 508 . . 509 ._________________________________________. 510 |_Segment_|_..._|_Segment_|_..._|_Segment_| 511 . . 512 ._________._________ 513 |_section_|__...__|_ 514 . . 515 .________. 516 |__zone__| 517 518- Superblock (SB) 519 It is located at the beginning of the partition, and there exist two copies 520 to avoid file system crash. It contains basic partition information and some 521 default parameters of f2fs. 522 523- Checkpoint (CP) 524 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 525 inode lists, and summary entries of current active segments. 526 527- Segment Information Table (SIT) 528 It contains segment information such as valid block count and bitmap for the 529 validity of all the blocks. 530 531- Node Address Table (NAT) 532 It is composed of a block address table for all the node blocks stored in 533 Main area. 534 535- Segment Summary Area (SSA) 536 It contains summary entries which contains the owner information of all the 537 data and node blocks stored in Main area. 538 539- Main Area 540 It contains file and directory data including their indices. 541 542In order to avoid misalignment between file system and flash-based storage, F2FS 543aligns the start block address of CP with the segment size. Also, it aligns the 544start block address of Main area with the zone size by reserving some segments 545in SSA area. 546 547Reference the following survey for additional technical details. 548https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 549 550File System Metadata Structure 551------------------------------ 552 553F2FS adopts the checkpointing scheme to maintain file system consistency. At 554mount time, F2FS first tries to find the last valid checkpoint data by scanning 555CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 556One of them always indicates the last valid data, which is called as shadow copy 557mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 558 559For file system consistency, each CP points to which NAT and SIT copies are 560valid, as shown as below:: 561 562 +--------+----------+---------+ 563 | CP | SIT | NAT | 564 +--------+----------+---------+ 565 . . . . 566 . . . . 567 . . . . 568 +-------+-------+--------+--------+--------+--------+ 569 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 570 +-------+-------+--------+--------+--------+--------+ 571 | ^ ^ 572 | | | 573 `----------------------------------------' 574 575Index Structure 576--------------- 577 578The key data structure to manage the data locations is a "node". Similar to 579traditional file structures, F2FS has three types of node: inode, direct node, 580indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 581indices, two direct node pointers, two indirect node pointers, and one double 582indirect node pointer as described below. One direct node block contains 1018 583data blocks, and one indirect node block contains also 1018 node blocks. Thus, 584one inode block (i.e., a file) covers:: 585 586 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 587 588 Inode block (4KB) 589 |- data (923) 590 |- direct node (2) 591 | `- data (1018) 592 |- indirect node (2) 593 | `- direct node (1018) 594 | `- data (1018) 595 `- double indirect node (1) 596 `- indirect node (1018) 597 `- direct node (1018) 598 `- data (1018) 599 600Note that all the node blocks are mapped by NAT which means the location of 601each node is translated by the NAT table. In the consideration of the wandering 602tree problem, F2FS is able to cut off the propagation of node updates caused by 603leaf data writes. 604 605Directory Structure 606------------------- 607 608A directory entry occupies 11 bytes, which consists of the following attributes. 609 610- hash hash value of the file name 611- ino inode number 612- len the length of file name 613- type file type such as directory, symlink, etc 614 615A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 616used to represent whether each dentry is valid or not. A dentry block occupies 6174KB with the following composition. 618 619:: 620 621 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 622 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 623 624 [Bucket] 625 +--------------------------------+ 626 |dentry block 1 | dentry block 2 | 627 +--------------------------------+ 628 . . 629 . . 630 . [Dentry Block Structure: 4KB] . 631 +--------+----------+----------+------------+ 632 | bitmap | reserved | dentries | file names | 633 +--------+----------+----------+------------+ 634 [Dentry Block: 4KB] . . 635 . . 636 . . 637 +------+------+-----+------+ 638 | hash | ino | len | type | 639 +------+------+-----+------+ 640 [Dentry Structure: 11 bytes] 641 642F2FS implements multi-level hash tables for directory structure. Each level has 643a hash table with dedicated number of hash buckets as shown below. Note that 644"A(2B)" means a bucket includes 2 data blocks. 645 646:: 647 648 ---------------------- 649 A : bucket 650 B : block 651 N : MAX_DIR_HASH_DEPTH 652 ---------------------- 653 654 level #0 | A(2B) 655 | 656 level #1 | A(2B) - A(2B) 657 | 658 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 659 . | . . . . 660 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 661 . | . . . . 662 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 663 664The number of blocks and buckets are determined by:: 665 666 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 667 # of blocks in level #n = | 668 `- 4, Otherwise 669 670 ,- 2^(n + dir_level), 671 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 672 # of buckets in level #n = | 673 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 674 Otherwise 675 676When F2FS finds a file name in a directory, at first a hash value of the file 677name is calculated. Then, F2FS scans the hash table in level #0 to find the 678dentry consisting of the file name and its inode number. If not found, F2FS 679scans the next hash table in level #1. In this way, F2FS scans hash tables in 680each levels incrementally from 1 to N. In each level F2FS needs to scan only 681one bucket determined by the following equation, which shows O(log(# of files)) 682complexity:: 683 684 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 685 686In the case of file creation, F2FS finds empty consecutive slots that cover the 687file name. F2FS searches the empty slots in the hash tables of whole levels from 6881 to N in the same way as the lookup operation. 689 690The following figure shows an example of two cases holding children:: 691 692 --------------> Dir <-------------- 693 | | 694 child child 695 696 child - child [hole] - child 697 698 child - child - child [hole] - [hole] - child 699 700 Case 1: Case 2: 701 Number of children = 6, Number of children = 3, 702 File size = 7 File size = 7 703 704Default Block Allocation 705------------------------ 706 707At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 708and Hot/Warm/Cold data. 709 710- Hot node contains direct node blocks of directories. 711- Warm node contains direct node blocks except hot node blocks. 712- Cold node contains indirect node blocks 713- Hot data contains dentry blocks 714- Warm data contains data blocks except hot and cold data blocks 715- Cold data contains multimedia data or migrated data blocks 716 717LFS has two schemes for free space management: threaded log and copy-and-compac- 718tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 719for devices showing very good sequential write performance, since free segments 720are served all the time for writing new data. However, it suffers from cleaning 721overhead under high utilization. Contrarily, the threaded log scheme suffers 722from random writes, but no cleaning process is needed. F2FS adopts a hybrid 723scheme where the copy-and-compaction scheme is adopted by default, but the 724policy is dynamically changed to the threaded log scheme according to the file 725system status. 726 727In order to align F2FS with underlying flash-based storage, F2FS allocates a 728segment in a unit of section. F2FS expects that the section size would be the 729same as the unit size of garbage collection in FTL. Furthermore, with respect 730to the mapping granularity in FTL, F2FS allocates each section of the active 731logs from different zones as much as possible, since FTL can write the data in 732the active logs into one allocation unit according to its mapping granularity. 733 734Cleaning process 735---------------- 736 737F2FS does cleaning both on demand and in the background. On-demand cleaning is 738triggered when there are not enough free segments to serve VFS calls. Background 739cleaner is operated by a kernel thread, and triggers the cleaning job when the 740system is idle. 741 742F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 743In the greedy algorithm, F2FS selects a victim segment having the smallest number 744of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 745according to the segment age and the number of valid blocks in order to address 746log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 747algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 748algorithm. 749 750In order to identify whether the data in the victim segment are valid or not, 751F2FS manages a bitmap. Each bit represents the validity of a block, and the 752bitmap is composed of a bit stream covering whole blocks in main area. 753 754Write-hint Policy 755----------------- 756 7571) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 758 7592) whint_mode=user-based. F2FS tries to pass down hints given by 760users. 761 762===================== ======================== =================== 763User F2FS Block 764===================== ======================== =================== 765N/A META WRITE_LIFE_NOT_SET 766N/A HOT_NODE " 767N/A WARM_NODE " 768N/A COLD_NODE " 769ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 770extension list " " 771 772-- buffered io 773WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 774WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 775WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 776WRITE_LIFE_NONE " " 777WRITE_LIFE_MEDIUM " " 778WRITE_LIFE_LONG " " 779 780-- direct io 781WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 782WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 783WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 784WRITE_LIFE_NONE " WRITE_LIFE_NONE 785WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 786WRITE_LIFE_LONG " WRITE_LIFE_LONG 787===================== ======================== =================== 788 7893) whint_mode=fs-based. F2FS passes down hints with its policy. 790 791===================== ======================== =================== 792User F2FS Block 793===================== ======================== =================== 794N/A META WRITE_LIFE_MEDIUM; 795N/A HOT_NODE WRITE_LIFE_NOT_SET 796N/A WARM_NODE " 797N/A COLD_NODE WRITE_LIFE_NONE 798ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 799extension list " " 800 801-- buffered io 802WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 803WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 804WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG 805WRITE_LIFE_NONE " " 806WRITE_LIFE_MEDIUM " " 807WRITE_LIFE_LONG " " 808 809-- direct io 810WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 811WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 812WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 813WRITE_LIFE_NONE " WRITE_LIFE_NONE 814WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 815WRITE_LIFE_LONG " WRITE_LIFE_LONG 816===================== ======================== =================== 817 818Fallocate(2) Policy 819------------------- 820 821The default policy follows the below POSIX rule. 822 823Allocating disk space 824 The default operation (i.e., mode is zero) of fallocate() allocates 825 the disk space within the range specified by offset and len. The 826 file size (as reported by stat(2)) will be changed if offset+len is 827 greater than the file size. Any subregion within the range specified 828 by offset and len that did not contain data before the call will be 829 initialized to zero. This default behavior closely resembles the 830 behavior of the posix_fallocate(3) library function, and is intended 831 as a method of optimally implementing that function. 832 833However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 834fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having 835zero or random data, which is useful to the below scenario where: 836 837 1. create(fd) 838 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 839 3. fallocate(fd, 0, 0, size) 840 4. address = fibmap(fd, offset) 841 5. open(blkdev) 842 6. write(blkdev, address) 843 844Compression implementation 845-------------------------- 846 847- New term named cluster is defined as basic unit of compression, file can 848 be divided into multiple clusters logically. One cluster includes 4 << n 849 (n >= 0) logical pages, compression size is also cluster size, each of 850 cluster can be compressed or not. 851 852- In cluster metadata layout, one special block address is used to indicate 853 a cluster is a compressed one or normal one; for compressed cluster, following 854 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 855 stores data including compress header and compressed data. 856 857- In order to eliminate write amplification during overwrite, F2FS only 858 support compression on write-once file, data can be compressed only when 859 all logical blocks in cluster contain valid data and compress ratio of 860 cluster data is lower than specified threshold. 861 862- To enable compression on regular inode, there are four ways: 863 864 * chattr +c file 865 * chattr +c dir; touch dir/file 866 * mount w/ -o compress_extension=ext; touch file.ext 867 * mount w/ -o compress_extension=*; touch any_file 868 869- To disable compression on regular inode, there are two ways: 870 871 * chattr -c file 872 * mount w/ -o nocompress_extension=ext; touch file.ext 873 874- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: 875 876 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch 877 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt 878 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip 879 can enable compress on bar.zip. 880 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch 881 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be 882 compresse, bar.zip and baz.txt should be non-compressed. 883 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip 884 and baz.txt. 885 886- At this point, compression feature doesn't expose compressed space to user 887 directly in order to guarantee potential data updates later to the space. 888 Instead, the main goal is to reduce data writes to flash disk as much as 889 possible, resulting in extending disk life time as well as relaxing IO 890 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) 891 interface to reclaim compressed space and show it to user after putting the 892 immutable bit. Immutable bit, after release, it doesn't allow writing/mmaping 893 on the file, until reserving compressed space via 894 ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or truncating filesize to zero. 895 896Compress metadata layout:: 897 898 [Dnode Structure] 899 +-----------------------------------------------+ 900 | cluster 1 | cluster 2 | ......... | cluster N | 901 +-----------------------------------------------+ 902 . . . . 903 . . . . 904 . Compressed Cluster . . Normal Cluster . 905 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 906 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 907 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 908 . . 909 . . 910 . . 911 +-------------+-------------+----------+----------------------------+ 912 | data length | data chksum | reserved | compressed data | 913 +-------------+-------------+----------+----------------------------+ 914 915Compression mode 916-------------------------- 917 918f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 919With this option, f2fs provides a choice to select the way how to compress the 920compression enabled files (refer to "Compression implementation" section for how to 921enable compression on a regular inode). 922 9231) compress_mode=fs 924This is the default option. f2fs does automatic compression in the writeback of the 925compression enabled files. 926 9272) compress_mode=user 928This disables the automatic compression and gives the user discretion of choosing the 929target file and the timing. The user can do manual compression/decompression on the 930compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 931ioctls like the below. 932 933To decompress a file, 934 935fd = open(filename, O_WRONLY, 0); 936ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 937 938To compress a file, 939 940fd = open(filename, O_WRONLY, 0); 941ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 942 943NVMe Zoned Namespace devices 944---------------------------- 945 946- ZNS defines a per-zone capacity which can be equal or less than the 947 zone-size. Zone-capacity is the number of usable blocks in the zone. 948 F2FS checks if zone-capacity is less than zone-size, if it is, then any 949 segment which starts after the zone-capacity is marked as not-free in 950 the free segment bitmap at initial mount time. These segments are marked 951 as permanently used so they are not allocated for writes and 952 consequently are not needed to be garbage collected. In case the 953 zone-capacity is not aligned to default segment size(2MB), then a segment 954 can start before the zone-capacity and span across zone-capacity boundary. 955 Such spanning segments are also considered as usable segments. All blocks 956 past the zone-capacity are considered unusable in these segments. 957