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. 113disable_roll_forward Disable the roll-forward recovery routine 114norecovery Disable the roll-forward recovery routine, mounted read- 115 only (i.e., -o ro,disable_roll_forward) 116discard/nodiscard Enable/disable real-time discard in f2fs, if discard is 117 enabled, f2fs will issue discard/TRIM commands when a 118 segment is cleaned. 119no_heap Disable heap-style segment allocation which finds free 120 segments for data from the beginning of main area, while 121 for node from the end of main area. 122nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 123 by default if CONFIG_F2FS_FS_XATTR is selected. 124noacl Disable POSIX Access Control List. Note: acl is enabled 125 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 126active_logs=%u Support configuring the number of active logs. In the 127 current design, f2fs supports only 2, 4, and 6 logs. 128 Default number is 6. 129disable_ext_identify Disable the extension list configured by mkfs, so f2fs 130 does not aware of cold files such as media files. 131inline_xattr Enable the inline xattrs feature. 132noinline_xattr Disable the inline xattrs feature. 133inline_xattr_size=%u Support configuring inline xattr size, it depends on 134 flexible inline xattr feature. 135inline_data Enable the inline data feature: New created small(<~3.4k) 136 files can be written into inode block. 137inline_dentry Enable the inline dir feature: data in new created 138 directory entries can be written into inode block. The 139 space of inode block which is used to store inline 140 dentries is limited to ~3.4k. 141noinline_dentry Disable the inline dentry feature. 142flush_merge Merge concurrent cache_flush commands as much as possible 143 to eliminate redundant command issues. If the underlying 144 device handles the cache_flush command relatively slowly, 145 recommend to enable this option. 146nobarrier This option can be used if underlying storage guarantees 147 its cached data should be written to the novolatile area. 148 If this option is set, no cache_flush commands are issued 149 but f2fs still guarantees the write ordering of all the 150 data writes. 151fastboot This option is used when a system wants to reduce mount 152 time as much as possible, even though normal performance 153 can be sacrificed. 154extent_cache Enable an extent cache based on rb-tree, it can cache 155 as many as extent which map between contiguous logical 156 address and physical address per inode, resulting in 157 increasing the cache hit ratio. Set by default. 158noextent_cache Disable an extent cache based on rb-tree explicitly, see 159 the above extent_cache mount option. 160noinline_data Disable the inline data feature, inline data feature is 161 enabled by default. 162data_flush Enable data flushing before checkpoint in order to 163 persist data of regular and symlink. 164reserve_root=%d Support configuring reserved space which is used for 165 allocation from a privileged user with specified uid or 166 gid, unit: 4KB, the default limit is 0.2% of user blocks. 167resuid=%d The user ID which may use the reserved blocks. 168resgid=%d The group ID which may use the reserved blocks. 169fault_injection=%d Enable fault injection in all supported types with 170 specified injection rate. 171fault_type=%d Support configuring fault injection type, should be 172 enabled with fault_injection option, fault type value 173 is shown below, it supports single or combined type. 174 175 =================== =========== 176 Type_Name Type_Value 177 =================== =========== 178 FAULT_KMALLOC 0x000000001 179 FAULT_KVMALLOC 0x000000002 180 FAULT_PAGE_ALLOC 0x000000004 181 FAULT_PAGE_GET 0x000000008 182 FAULT_ALLOC_BIO 0x000000010 183 FAULT_ALLOC_NID 0x000000020 184 FAULT_ORPHAN 0x000000040 185 FAULT_BLOCK 0x000000080 186 FAULT_DIR_DEPTH 0x000000100 187 FAULT_EVICT_INODE 0x000000200 188 FAULT_TRUNCATE 0x000000400 189 FAULT_READ_IO 0x000000800 190 FAULT_CHECKPOINT 0x000001000 191 FAULT_DISCARD 0x000002000 192 FAULT_WRITE_IO 0x000004000 193 =================== =========== 194mode=%s Control block allocation mode which supports "adaptive" 195 and "lfs". In "lfs" mode, there should be no random 196 writes towards main area. 197io_bits=%u Set the bit size of write IO requests. It should be set 198 with "mode=lfs". 199usrquota Enable plain user disk quota accounting. 200grpquota Enable plain group disk quota accounting. 201prjquota Enable plain project quota accounting. 202usrjquota=<file> Appoint specified file and type during mount, so that quota 203grpjquota=<file> information can be properly updated during recovery flow, 204prjjquota=<file> <quota file>: must be in root directory; 205jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 206offusrjquota Turn off user journelled quota. 207offgrpjquota Turn off group journelled quota. 208offprjjquota Turn off project journelled quota. 209quota Enable plain user disk quota accounting. 210noquota Disable all plain disk quota option. 211whint_mode=%s Control which write hints are passed down to block 212 layer. This supports "off", "user-based", and 213 "fs-based". In "off" mode (default), f2fs does not pass 214 down hints. In "user-based" mode, f2fs tries to pass 215 down hints given by users. And in "fs-based" mode, f2fs 216 passes down hints with its policy. 217alloc_mode=%s Adjust block allocation policy, which supports "reuse" 218 and "default". 219fsync_mode=%s Control the policy of fsync. Currently supports "posix", 220 "strict", and "nobarrier". In "posix" mode, which is 221 default, fsync will follow POSIX semantics and does a 222 light operation to improve the filesystem performance. 223 In "strict" mode, fsync will be heavy and behaves in line 224 with xfs, ext4 and btrfs, where xfstest generic/342 will 225 pass, but the performance will regress. "nobarrier" is 226 based on "posix", but doesn't issue flush command for 227 non-atomic files likewise "nobarrier" mount option. 228test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt 229 context. The fake fscrypt context is used by xfstests. 230checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 231 to reenable checkpointing. Is enabled by default. While 232 disabled, any unmounting or unexpected shutdowns will cause 233 the filesystem contents to appear as they did when the 234 filesystem was mounted with that option. 235 While mounting with checkpoint=disabled, the filesystem must 236 run garbage collection to ensure that all available space can 237 be used. If this takes too much time, the mount may return 238 EAGAIN. You may optionally add a value to indicate how much 239 of the disk you would be willing to temporarily give up to 240 avoid additional garbage collection. This can be given as a 241 number of blocks, or as a percent. For instance, mounting 242 with checkpoint=disable:100% would always succeed, but it may 243 hide up to all remaining free space. The actual space that 244 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 245 This space is reclaimed once checkpoint=enable. 246compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 247 "lz4" and "zstd" algorithm. 248compress_log_size=%u Support configuring compress cluster size, the size will 249 be 4KB * (1 << %u), 16KB is minimum size, also it's 250 default size. 251compress_extension=%s Support adding specified extension, so that f2fs can enable 252 compression on those corresponding files, e.g. if all files 253 with '.ext' has high compression rate, we can set the '.ext' 254 on compression extension list and enable compression on 255 these file by default rather than to enable it via ioctl. 256 For other files, we can still enable compression via ioctl. 257====================== ============================================================ 258 259Debugfs Entries 260=============== 261 262/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 263f2fs. Each file shows the whole f2fs information. 264 265/sys/kernel/debug/f2fs/status includes: 266 267 - major file system information managed by f2fs currently 268 - average SIT information about whole segments 269 - current memory footprint consumed by f2fs. 270 271Sysfs Entries 272============= 273 274Information about mounted f2fs file systems can be found in 275/sys/fs/f2fs. Each mounted filesystem will have a directory in 276/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 277The files in each per-device directory are shown in table below. 278 279Files in /sys/fs/f2fs/<devname> 280(see also Documentation/ABI/testing/sysfs-fs-f2fs) 281 282Usage 283===== 284 2851. Download userland tools and compile them. 286 2872. Skip, if f2fs was compiled statically inside kernel. 288 Otherwise, insert the f2fs.ko module:: 289 290 # insmod f2fs.ko 291 2923. Create a directory trying to mount:: 293 294 # mkdir /mnt/f2fs 295 2964. Format the block device, and then mount as f2fs:: 297 298 # mkfs.f2fs -l label /dev/block_device 299 # mount -t f2fs /dev/block_device /mnt/f2fs 300 301mkfs.f2fs 302--------- 303The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 304which builds a basic on-disk layout. 305 306The options consist of: 307 308=============== =========================================================== 309``-l [label]`` Give a volume label, up to 512 unicode name. 310``-a [0 or 1]`` Split start location of each area for heap-based allocation. 311 312 1 is set by default, which performs this. 313``-o [int]`` Set overprovision ratio in percent over volume size. 314 315 5 is set by default. 316``-s [int]`` Set the number of segments per section. 317 318 1 is set by default. 319``-z [int]`` Set the number of sections per zone. 320 321 1 is set by default. 322``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 323``-t [0 or 1]`` Disable discard command or not. 324 325 1 is set by default, which conducts discard. 326=============== =========================================================== 327 328fsck.f2fs 329--------- 330The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 331partition, which examines whether the filesystem metadata and user-made data 332are cross-referenced correctly or not. 333Note that, initial version of the tool does not fix any inconsistency. 334 335The options consist of:: 336 337 -d debug level [default:0] 338 339dump.f2fs 340--------- 341The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 342file. Each file is dump_ssa and dump_sit. 343 344The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 345It shows on-disk inode information recognized by a given inode number, and is 346able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 347./dump_sit respectively. 348 349The options consist of:: 350 351 -d debug level [default:0] 352 -i inode no (hex) 353 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 354 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 355 356Examples:: 357 358 # dump.f2fs -i [ino] /dev/sdx 359 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 360 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 361 362Design 363====== 364 365On-disk Layout 366-------------- 367 368F2FS divides the whole volume into a number of segments, each of which is fixed 369to 2MB in size. A section is composed of consecutive segments, and a zone 370consists of a set of sections. By default, section and zone sizes are set to one 371segment size identically, but users can easily modify the sizes by mkfs. 372 373F2FS splits the entire volume into six areas, and all the areas except superblock 374consists of multiple segments as described below:: 375 376 align with the zone size <-| 377 |-> align with the segment size 378 _________________________________________________________________________ 379 | | | Segment | Node | Segment | | 380 | Superblock | Checkpoint | Info. | Address | Summary | Main | 381 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 382 |____________|_____2______|______N______|______N______|______N_____|__N___| 383 . . 384 . . 385 . . 386 ._________________________________________. 387 |_Segment_|_..._|_Segment_|_..._|_Segment_| 388 . . 389 ._________._________ 390 |_section_|__...__|_ 391 . . 392 .________. 393 |__zone__| 394 395- Superblock (SB) 396 It is located at the beginning of the partition, and there exist two copies 397 to avoid file system crash. It contains basic partition information and some 398 default parameters of f2fs. 399 400- Checkpoint (CP) 401 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 402 inode lists, and summary entries of current active segments. 403 404- Segment Information Table (SIT) 405 It contains segment information such as valid block count and bitmap for the 406 validity of all the blocks. 407 408- Node Address Table (NAT) 409 It is composed of a block address table for all the node blocks stored in 410 Main area. 411 412- Segment Summary Area (SSA) 413 It contains summary entries which contains the owner information of all the 414 data and node blocks stored in Main area. 415 416- Main Area 417 It contains file and directory data including their indices. 418 419In order to avoid misalignment between file system and flash-based storage, F2FS 420aligns the start block address of CP with the segment size. Also, it aligns the 421start block address of Main area with the zone size by reserving some segments 422in SSA area. 423 424Reference the following survey for additional technical details. 425https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 426 427File System Metadata Structure 428------------------------------ 429 430F2FS adopts the checkpointing scheme to maintain file system consistency. At 431mount time, F2FS first tries to find the last valid checkpoint data by scanning 432CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 433One of them always indicates the last valid data, which is called as shadow copy 434mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 435 436For file system consistency, each CP points to which NAT and SIT copies are 437valid, as shown as below:: 438 439 +--------+----------+---------+ 440 | CP | SIT | NAT | 441 +--------+----------+---------+ 442 . . . . 443 . . . . 444 . . . . 445 +-------+-------+--------+--------+--------+--------+ 446 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 447 +-------+-------+--------+--------+--------+--------+ 448 | ^ ^ 449 | | | 450 `----------------------------------------' 451 452Index Structure 453--------------- 454 455The key data structure to manage the data locations is a "node". Similar to 456traditional file structures, F2FS has three types of node: inode, direct node, 457indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 458indices, two direct node pointers, two indirect node pointers, and one double 459indirect node pointer as described below. One direct node block contains 1018 460data blocks, and one indirect node block contains also 1018 node blocks. Thus, 461one inode block (i.e., a file) covers:: 462 463 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 464 465 Inode block (4KB) 466 |- data (923) 467 |- direct node (2) 468 | `- data (1018) 469 |- indirect node (2) 470 | `- direct node (1018) 471 | `- data (1018) 472 `- double indirect node (1) 473 `- indirect node (1018) 474 `- direct node (1018) 475 `- data (1018) 476 477Note that, all the node blocks are mapped by NAT which means the location of 478each node is translated by the NAT table. In the consideration of the wandering 479tree problem, F2FS is able to cut off the propagation of node updates caused by 480leaf data writes. 481 482Directory Structure 483------------------- 484 485A directory entry occupies 11 bytes, which consists of the following attributes. 486 487- hash hash value of the file name 488- ino inode number 489- len the length of file name 490- type file type such as directory, symlink, etc 491 492A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 493used to represent whether each dentry is valid or not. A dentry block occupies 4944KB with the following composition. 495 496:: 497 498 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 499 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 500 501 [Bucket] 502 +--------------------------------+ 503 |dentry block 1 | dentry block 2 | 504 +--------------------------------+ 505 . . 506 . . 507 . [Dentry Block Structure: 4KB] . 508 +--------+----------+----------+------------+ 509 | bitmap | reserved | dentries | file names | 510 +--------+----------+----------+------------+ 511 [Dentry Block: 4KB] . . 512 . . 513 . . 514 +------+------+-----+------+ 515 | hash | ino | len | type | 516 +------+------+-----+------+ 517 [Dentry Structure: 11 bytes] 518 519F2FS implements multi-level hash tables for directory structure. Each level has 520a hash table with dedicated number of hash buckets as shown below. Note that 521"A(2B)" means a bucket includes 2 data blocks. 522 523:: 524 525 ---------------------- 526 A : bucket 527 B : block 528 N : MAX_DIR_HASH_DEPTH 529 ---------------------- 530 531 level #0 | A(2B) 532 | 533 level #1 | A(2B) - A(2B) 534 | 535 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 536 . | . . . . 537 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 538 . | . . . . 539 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 540 541The number of blocks and buckets are determined by:: 542 543 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 544 # of blocks in level #n = | 545 `- 4, Otherwise 546 547 ,- 2^(n + dir_level), 548 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 549 # of buckets in level #n = | 550 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 551 Otherwise 552 553When F2FS finds a file name in a directory, at first a hash value of the file 554name is calculated. Then, F2FS scans the hash table in level #0 to find the 555dentry consisting of the file name and its inode number. If not found, F2FS 556scans the next hash table in level #1. In this way, F2FS scans hash tables in 557each levels incrementally from 1 to N. In each levels F2FS needs to scan only 558one bucket determined by the following equation, which shows O(log(# of files)) 559complexity:: 560 561 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 562 563In the case of file creation, F2FS finds empty consecutive slots that cover the 564file name. F2FS searches the empty slots in the hash tables of whole levels from 5651 to N in the same way as the lookup operation. 566 567The following figure shows an example of two cases holding children:: 568 569 --------------> Dir <-------------- 570 | | 571 child child 572 573 child - child [hole] - child 574 575 child - child - child [hole] - [hole] - child 576 577 Case 1: Case 2: 578 Number of children = 6, Number of children = 3, 579 File size = 7 File size = 7 580 581Default Block Allocation 582------------------------ 583 584At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 585and Hot/Warm/Cold data. 586 587- Hot node contains direct node blocks of directories. 588- Warm node contains direct node blocks except hot node blocks. 589- Cold node contains indirect node blocks 590- Hot data contains dentry blocks 591- Warm data contains data blocks except hot and cold data blocks 592- Cold data contains multimedia data or migrated data blocks 593 594LFS has two schemes for free space management: threaded log and copy-and-compac- 595tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 596for devices showing very good sequential write performance, since free segments 597are served all the time for writing new data. However, it suffers from cleaning 598overhead under high utilization. Contrarily, the threaded log scheme suffers 599from random writes, but no cleaning process is needed. F2FS adopts a hybrid 600scheme where the copy-and-compaction scheme is adopted by default, but the 601policy is dynamically changed to the threaded log scheme according to the file 602system status. 603 604In order to align F2FS with underlying flash-based storage, F2FS allocates a 605segment in a unit of section. F2FS expects that the section size would be the 606same as the unit size of garbage collection in FTL. Furthermore, with respect 607to the mapping granularity in FTL, F2FS allocates each section of the active 608logs from different zones as much as possible, since FTL can write the data in 609the active logs into one allocation unit according to its mapping granularity. 610 611Cleaning process 612---------------- 613 614F2FS does cleaning both on demand and in the background. On-demand cleaning is 615triggered when there are not enough free segments to serve VFS calls. Background 616cleaner is operated by a kernel thread, and triggers the cleaning job when the 617system is idle. 618 619F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 620In the greedy algorithm, F2FS selects a victim segment having the smallest number 621of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 622according to the segment age and the number of valid blocks in order to address 623log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 624algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 625algorithm. 626 627In order to identify whether the data in the victim segment are valid or not, 628F2FS manages a bitmap. Each bit represents the validity of a block, and the 629bitmap is composed of a bit stream covering whole blocks in main area. 630 631Write-hint Policy 632----------------- 633 6341) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 635 6362) whint_mode=user-based. F2FS tries to pass down hints given by 637users. 638 639===================== ======================== =================== 640User F2FS Block 641===================== ======================== =================== 642 META WRITE_LIFE_NOT_SET 643 HOT_NODE " 644 WARM_NODE " 645 COLD_NODE " 646ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 647extension list " " 648 649-- buffered io 650WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 651WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 652WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 653WRITE_LIFE_NONE " " 654WRITE_LIFE_MEDIUM " " 655WRITE_LIFE_LONG " " 656 657-- direct io 658WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 659WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 660WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 661WRITE_LIFE_NONE " WRITE_LIFE_NONE 662WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 663WRITE_LIFE_LONG " WRITE_LIFE_LONG 664===================== ======================== =================== 665 6663) whint_mode=fs-based. F2FS passes down hints with its policy. 667 668===================== ======================== =================== 669User F2FS Block 670===================== ======================== =================== 671 META WRITE_LIFE_MEDIUM; 672 HOT_NODE WRITE_LIFE_NOT_SET 673 WARM_NODE " 674 COLD_NODE WRITE_LIFE_NONE 675ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 676extension list " " 677 678-- buffered io 679WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 680WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 681WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG 682WRITE_LIFE_NONE " " 683WRITE_LIFE_MEDIUM " " 684WRITE_LIFE_LONG " " 685 686-- direct io 687WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 688WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 689WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 690WRITE_LIFE_NONE " WRITE_LIFE_NONE 691WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 692WRITE_LIFE_LONG " WRITE_LIFE_LONG 693===================== ======================== =================== 694 695Fallocate(2) Policy 696------------------- 697 698The default policy follows the below posix rule. 699 700Allocating disk space 701 The default operation (i.e., mode is zero) of fallocate() allocates 702 the disk space within the range specified by offset and len. The 703 file size (as reported by stat(2)) will be changed if offset+len is 704 greater than the file size. Any subregion within the range specified 705 by offset and len that did not contain data before the call will be 706 initialized to zero. This default behavior closely resembles the 707 behavior of the posix_fallocate(3) library function, and is intended 708 as a method of optimally implementing that function. 709 710However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 711fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having 712zero or random data, which is useful to the below scenario where: 713 714 1. create(fd) 715 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 716 3. fallocate(fd, 0, 0, size) 717 4. address = fibmap(fd, offset) 718 5. open(blkdev) 719 6. write(blkdev, address) 720 721Compression implementation 722-------------------------- 723 724- New term named cluster is defined as basic unit of compression, file can 725 be divided into multiple clusters logically. One cluster includes 4 << n 726 (n >= 0) logical pages, compression size is also cluster size, each of 727 cluster can be compressed or not. 728 729- In cluster metadata layout, one special block address is used to indicate 730 cluster is compressed one or normal one, for compressed cluster, following 731 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 732 stores data including compress header and compressed data. 733 734- In order to eliminate write amplification during overwrite, F2FS only 735 support compression on write-once file, data can be compressed only when 736 all logical blocks in file are valid and cluster compress ratio is lower 737 than specified threshold. 738 739- To enable compression on regular inode, there are three ways: 740 741 * chattr +c file 742 * chattr +c dir; touch dir/file 743 * mount w/ -o compress_extension=ext; touch file.ext 744 745Compress metadata layout:: 746 747 [Dnode Structure] 748 +-----------------------------------------------+ 749 | cluster 1 | cluster 2 | ......... | cluster N | 750 +-----------------------------------------------+ 751 . . . . 752 . . . . 753 . Compressed Cluster . . Normal Cluster . 754 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 755 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 756 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 757 . . 758 . . 759 . . 760 +-------------+-------------+----------+----------------------------+ 761 | data length | data chksum | reserved | compressed data | 762 +-------------+-------------+----------+----------------------------+ 763