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 is 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: Newly created small (<~3.4k) 136 files can be written into inode block. 137inline_dentry Enable the inline dir feature: data in newly 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 journalled quota. 207offgrpjquota Turn off group journalled quota. 208offprjjquota Turn off project journalled 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 229test_dummy_encryption=%s 230 Enable dummy encryption, which provides a fake fscrypt 231 context. The fake fscrypt context is used by xfstests. 232 The argument may be either "v1" or "v2", in order to 233 select the corresponding fscrypt policy version. 234checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" 235 to reenable checkpointing. Is enabled by default. While 236 disabled, any unmounting or unexpected shutdowns will cause 237 the filesystem contents to appear as they did when the 238 filesystem was mounted with that option. 239 While mounting with checkpoint=disabled, the filesystem must 240 run garbage collection to ensure that all available space can 241 be used. If this takes too much time, the mount may return 242 EAGAIN. You may optionally add a value to indicate how much 243 of the disk you would be willing to temporarily give up to 244 avoid additional garbage collection. This can be given as a 245 number of blocks, or as a percent. For instance, mounting 246 with checkpoint=disable:100% would always succeed, but it may 247 hide up to all remaining free space. The actual space that 248 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable 249 This space is reclaimed once checkpoint=enable. 250compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", 251 "lz4", "zstd" and "lzo-rle" algorithm. 252compress_log_size=%u Support configuring compress cluster size, the size will 253 be 4KB * (1 << %u), 16KB is minimum size, also it's 254 default size. 255compress_extension=%s Support adding specified extension, so that f2fs can enable 256 compression on those corresponding files, e.g. if all files 257 with '.ext' has high compression rate, we can set the '.ext' 258 on compression extension list and enable compression on 259 these file by default rather than to enable it via ioctl. 260 For other files, we can still enable compression via ioctl. 261 Note that, there is one reserved special extension '*', it 262 can be set to enable compression for all files. 263inlinecrypt When possible, encrypt/decrypt the contents of encrypted 264 files using the blk-crypto framework rather than 265 filesystem-layer encryption. This allows the use of 266 inline encryption hardware. The on-disk format is 267 unaffected. For more details, see 268 Documentation/block/inline-encryption.rst. 269atgc Enable age-threshold garbage collection, it provides high 270 effectiveness and efficiency on background GC. 271======================== ============================================================ 272 273Debugfs Entries 274=============== 275 276/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 277f2fs. Each file shows the whole f2fs information. 278 279/sys/kernel/debug/f2fs/status includes: 280 281 - major file system information managed by f2fs currently 282 - average SIT information about whole segments 283 - current memory footprint consumed by f2fs. 284 285Sysfs Entries 286============= 287 288Information about mounted f2fs file systems can be found in 289/sys/fs/f2fs. Each mounted filesystem will have a directory in 290/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 291The files in each per-device directory are shown in table below. 292 293Files in /sys/fs/f2fs/<devname> 294(see also Documentation/ABI/testing/sysfs-fs-f2fs) 295 296Usage 297===== 298 2991. Download userland tools and compile them. 300 3012. Skip, if f2fs was compiled statically inside kernel. 302 Otherwise, insert the f2fs.ko module:: 303 304 # insmod f2fs.ko 305 3063. Create a directory to use when mounting:: 307 308 # mkdir /mnt/f2fs 309 3104. Format the block device, and then mount as f2fs:: 311 312 # mkfs.f2fs -l label /dev/block_device 313 # mount -t f2fs /dev/block_device /mnt/f2fs 314 315mkfs.f2fs 316--------- 317The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 318which builds a basic on-disk layout. 319 320The quick options consist of: 321 322=============== =========================================================== 323``-l [label]`` Give a volume label, up to 512 unicode name. 324``-a [0 or 1]`` Split start location of each area for heap-based allocation. 325 326 1 is set by default, which performs this. 327``-o [int]`` Set overprovision ratio in percent over volume size. 328 329 5 is set by default. 330``-s [int]`` Set the number of segments per section. 331 332 1 is set by default. 333``-z [int]`` Set the number of sections per zone. 334 335 1 is set by default. 336``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 337``-t [0 or 1]`` Disable discard command or not. 338 339 1 is set by default, which conducts discard. 340=============== =========================================================== 341 342Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 343 344fsck.f2fs 345--------- 346The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 347partition, which examines whether the filesystem metadata and user-made data 348are cross-referenced correctly or not. 349Note that, initial version of the tool does not fix any inconsistency. 350 351The quick options consist of:: 352 353 -d debug level [default:0] 354 355Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 356 357dump.f2fs 358--------- 359The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 360file. Each file is dump_ssa and dump_sit. 361 362The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 363It shows on-disk inode information recognized by a given inode number, and is 364able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 365./dump_sit respectively. 366 367The options consist of:: 368 369 -d debug level [default:0] 370 -i inode no (hex) 371 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 372 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 373 374Examples:: 375 376 # dump.f2fs -i [ino] /dev/sdx 377 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 378 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 379 380Note: please refer to the manpage of dump.f2fs(8) to get full option list. 381 382sload.f2fs 383---------- 384The sload.f2fs gives a way to insert files and directories in the exisiting disk 385image. This tool is useful when building f2fs images given compiled files. 386 387Note: please refer to the manpage of sload.f2fs(8) to get full option list. 388 389resize.f2fs 390----------- 391The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 392all the files and directories stored in the image. 393 394Note: please refer to the manpage of resize.f2fs(8) to get full option list. 395 396defrag.f2fs 397----------- 398The defrag.f2fs can be used to defragment scattered written data as well as 399filesystem metadata across the disk. This can improve the write speed by giving 400more free consecutive space. 401 402Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 403 404f2fs_io 405------- 406The f2fs_io is a simple tool to issue various filesystem APIs as well as 407f2fs-specific ones, which is very useful for QA tests. 408 409Note: please refer to the manpage of f2fs_io(8) to get full option list. 410 411Design 412====== 413 414On-disk Layout 415-------------- 416 417F2FS divides the whole volume into a number of segments, each of which is fixed 418to 2MB in size. A section is composed of consecutive segments, and a zone 419consists of a set of sections. By default, section and zone sizes are set to one 420segment size identically, but users can easily modify the sizes by mkfs. 421 422F2FS splits the entire volume into six areas, and all the areas except superblock 423consist of multiple segments as described below:: 424 425 align with the zone size <-| 426 |-> align with the segment size 427 _________________________________________________________________________ 428 | | | Segment | Node | Segment | | 429 | Superblock | Checkpoint | Info. | Address | Summary | Main | 430 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 431 |____________|_____2______|______N______|______N______|______N_____|__N___| 432 . . 433 . . 434 . . 435 ._________________________________________. 436 |_Segment_|_..._|_Segment_|_..._|_Segment_| 437 . . 438 ._________._________ 439 |_section_|__...__|_ 440 . . 441 .________. 442 |__zone__| 443 444- Superblock (SB) 445 It is located at the beginning of the partition, and there exist two copies 446 to avoid file system crash. It contains basic partition information and some 447 default parameters of f2fs. 448 449- Checkpoint (CP) 450 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 451 inode lists, and summary entries of current active segments. 452 453- Segment Information Table (SIT) 454 It contains segment information such as valid block count and bitmap for the 455 validity of all the blocks. 456 457- Node Address Table (NAT) 458 It is composed of a block address table for all the node blocks stored in 459 Main area. 460 461- Segment Summary Area (SSA) 462 It contains summary entries which contains the owner information of all the 463 data and node blocks stored in Main area. 464 465- Main Area 466 It contains file and directory data including their indices. 467 468In order to avoid misalignment between file system and flash-based storage, F2FS 469aligns the start block address of CP with the segment size. Also, it aligns the 470start block address of Main area with the zone size by reserving some segments 471in SSA area. 472 473Reference the following survey for additional technical details. 474https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 475 476File System Metadata Structure 477------------------------------ 478 479F2FS adopts the checkpointing scheme to maintain file system consistency. At 480mount time, F2FS first tries to find the last valid checkpoint data by scanning 481CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 482One of them always indicates the last valid data, which is called as shadow copy 483mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 484 485For file system consistency, each CP points to which NAT and SIT copies are 486valid, as shown as below:: 487 488 +--------+----------+---------+ 489 | CP | SIT | NAT | 490 +--------+----------+---------+ 491 . . . . 492 . . . . 493 . . . . 494 +-------+-------+--------+--------+--------+--------+ 495 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 496 +-------+-------+--------+--------+--------+--------+ 497 | ^ ^ 498 | | | 499 `----------------------------------------' 500 501Index Structure 502--------------- 503 504The key data structure to manage the data locations is a "node". Similar to 505traditional file structures, F2FS has three types of node: inode, direct node, 506indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 507indices, two direct node pointers, two indirect node pointers, and one double 508indirect node pointer as described below. One direct node block contains 1018 509data blocks, and one indirect node block contains also 1018 node blocks. Thus, 510one inode block (i.e., a file) covers:: 511 512 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 513 514 Inode block (4KB) 515 |- data (923) 516 |- direct node (2) 517 | `- data (1018) 518 |- indirect node (2) 519 | `- direct node (1018) 520 | `- data (1018) 521 `- double indirect node (1) 522 `- indirect node (1018) 523 `- direct node (1018) 524 `- data (1018) 525 526Note that all the node blocks are mapped by NAT which means the location of 527each node is translated by the NAT table. In the consideration of the wandering 528tree problem, F2FS is able to cut off the propagation of node updates caused by 529leaf data writes. 530 531Directory Structure 532------------------- 533 534A directory entry occupies 11 bytes, which consists of the following attributes. 535 536- hash hash value of the file name 537- ino inode number 538- len the length of file name 539- type file type such as directory, symlink, etc 540 541A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 542used to represent whether each dentry is valid or not. A dentry block occupies 5434KB with the following composition. 544 545:: 546 547 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 548 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 549 550 [Bucket] 551 +--------------------------------+ 552 |dentry block 1 | dentry block 2 | 553 +--------------------------------+ 554 . . 555 . . 556 . [Dentry Block Structure: 4KB] . 557 +--------+----------+----------+------------+ 558 | bitmap | reserved | dentries | file names | 559 +--------+----------+----------+------------+ 560 [Dentry Block: 4KB] . . 561 . . 562 . . 563 +------+------+-----+------+ 564 | hash | ino | len | type | 565 +------+------+-----+------+ 566 [Dentry Structure: 11 bytes] 567 568F2FS implements multi-level hash tables for directory structure. Each level has 569a hash table with dedicated number of hash buckets as shown below. Note that 570"A(2B)" means a bucket includes 2 data blocks. 571 572:: 573 574 ---------------------- 575 A : bucket 576 B : block 577 N : MAX_DIR_HASH_DEPTH 578 ---------------------- 579 580 level #0 | A(2B) 581 | 582 level #1 | A(2B) - A(2B) 583 | 584 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 585 . | . . . . 586 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 587 . | . . . . 588 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 589 590The number of blocks and buckets are determined by:: 591 592 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 593 # of blocks in level #n = | 594 `- 4, Otherwise 595 596 ,- 2^(n + dir_level), 597 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 598 # of buckets in level #n = | 599 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 600 Otherwise 601 602When F2FS finds a file name in a directory, at first a hash value of the file 603name is calculated. Then, F2FS scans the hash table in level #0 to find the 604dentry consisting of the file name and its inode number. If not found, F2FS 605scans the next hash table in level #1. In this way, F2FS scans hash tables in 606each levels incrementally from 1 to N. In each level F2FS needs to scan only 607one bucket determined by the following equation, which shows O(log(# of files)) 608complexity:: 609 610 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 611 612In the case of file creation, F2FS finds empty consecutive slots that cover the 613file name. F2FS searches the empty slots in the hash tables of whole levels from 6141 to N in the same way as the lookup operation. 615 616The following figure shows an example of two cases holding children:: 617 618 --------------> Dir <-------------- 619 | | 620 child child 621 622 child - child [hole] - child 623 624 child - child - child [hole] - [hole] - child 625 626 Case 1: Case 2: 627 Number of children = 6, Number of children = 3, 628 File size = 7 File size = 7 629 630Default Block Allocation 631------------------------ 632 633At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 634and Hot/Warm/Cold data. 635 636- Hot node contains direct node blocks of directories. 637- Warm node contains direct node blocks except hot node blocks. 638- Cold node contains indirect node blocks 639- Hot data contains dentry blocks 640- Warm data contains data blocks except hot and cold data blocks 641- Cold data contains multimedia data or migrated data blocks 642 643LFS has two schemes for free space management: threaded log and copy-and-compac- 644tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 645for devices showing very good sequential write performance, since free segments 646are served all the time for writing new data. However, it suffers from cleaning 647overhead under high utilization. Contrarily, the threaded log scheme suffers 648from random writes, but no cleaning process is needed. F2FS adopts a hybrid 649scheme where the copy-and-compaction scheme is adopted by default, but the 650policy is dynamically changed to the threaded log scheme according to the file 651system status. 652 653In order to align F2FS with underlying flash-based storage, F2FS allocates a 654segment in a unit of section. F2FS expects that the section size would be the 655same as the unit size of garbage collection in FTL. Furthermore, with respect 656to the mapping granularity in FTL, F2FS allocates each section of the active 657logs from different zones as much as possible, since FTL can write the data in 658the active logs into one allocation unit according to its mapping granularity. 659 660Cleaning process 661---------------- 662 663F2FS does cleaning both on demand and in the background. On-demand cleaning is 664triggered when there are not enough free segments to serve VFS calls. Background 665cleaner is operated by a kernel thread, and triggers the cleaning job when the 666system is idle. 667 668F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 669In the greedy algorithm, F2FS selects a victim segment having the smallest number 670of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 671according to the segment age and the number of valid blocks in order to address 672log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 673algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 674algorithm. 675 676In order to identify whether the data in the victim segment are valid or not, 677F2FS manages a bitmap. Each bit represents the validity of a block, and the 678bitmap is composed of a bit stream covering whole blocks in main area. 679 680Write-hint Policy 681----------------- 682 6831) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 684 6852) whint_mode=user-based. F2FS tries to pass down hints given by 686users. 687 688===================== ======================== =================== 689User F2FS Block 690===================== ======================== =================== 691 META WRITE_LIFE_NOT_SET 692 HOT_NODE " 693 WARM_NODE " 694 COLD_NODE " 695ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 696extension list " " 697 698-- buffered io 699WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 700WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 701WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 702WRITE_LIFE_NONE " " 703WRITE_LIFE_MEDIUM " " 704WRITE_LIFE_LONG " " 705 706-- direct io 707WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 708WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 709WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 710WRITE_LIFE_NONE " WRITE_LIFE_NONE 711WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 712WRITE_LIFE_LONG " WRITE_LIFE_LONG 713===================== ======================== =================== 714 7153) whint_mode=fs-based. F2FS passes down hints with its policy. 716 717===================== ======================== =================== 718User F2FS Block 719===================== ======================== =================== 720 META WRITE_LIFE_MEDIUM; 721 HOT_NODE WRITE_LIFE_NOT_SET 722 WARM_NODE " 723 COLD_NODE WRITE_LIFE_NONE 724ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 725extension list " " 726 727-- buffered io 728WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 729WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 730WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG 731WRITE_LIFE_NONE " " 732WRITE_LIFE_MEDIUM " " 733WRITE_LIFE_LONG " " 734 735-- direct io 736WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 737WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 738WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 739WRITE_LIFE_NONE " WRITE_LIFE_NONE 740WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 741WRITE_LIFE_LONG " WRITE_LIFE_LONG 742===================== ======================== =================== 743 744Fallocate(2) Policy 745------------------- 746 747The default policy follows the below POSIX rule. 748 749Allocating disk space 750 The default operation (i.e., mode is zero) of fallocate() allocates 751 the disk space within the range specified by offset and len. The 752 file size (as reported by stat(2)) will be changed if offset+len is 753 greater than the file size. Any subregion within the range specified 754 by offset and len that did not contain data before the call will be 755 initialized to zero. This default behavior closely resembles the 756 behavior of the posix_fallocate(3) library function, and is intended 757 as a method of optimally implementing that function. 758 759However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 760fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having 761zero or random data, which is useful to the below scenario where: 762 763 1. create(fd) 764 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 765 3. fallocate(fd, 0, 0, size) 766 4. address = fibmap(fd, offset) 767 5. open(blkdev) 768 6. write(blkdev, address) 769 770Compression implementation 771-------------------------- 772 773- New term named cluster is defined as basic unit of compression, file can 774 be divided into multiple clusters logically. One cluster includes 4 << n 775 (n >= 0) logical pages, compression size is also cluster size, each of 776 cluster can be compressed or not. 777 778- In cluster metadata layout, one special block address is used to indicate 779 a cluster is a compressed one or normal one; for compressed cluster, following 780 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 781 stores data including compress header and compressed data. 782 783- In order to eliminate write amplification during overwrite, F2FS only 784 support compression on write-once file, data can be compressed only when 785 all logical blocks in cluster contain valid data and compress ratio of 786 cluster data is lower than specified threshold. 787 788- To enable compression on regular inode, there are three ways: 789 790 * chattr +c file 791 * chattr +c dir; touch dir/file 792 * mount w/ -o compress_extension=ext; touch file.ext 793 794Compress metadata layout:: 795 796 [Dnode Structure] 797 +-----------------------------------------------+ 798 | cluster 1 | cluster 2 | ......... | cluster N | 799 +-----------------------------------------------+ 800 . . . . 801 . . . . 802 . Compressed Cluster . . Normal Cluster . 803 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 804 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 805 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 806 . . 807 . . 808 . . 809 +-------------+-------------+----------+----------------------------+ 810 | data length | data chksum | reserved | compressed data | 811 +-------------+-------------+----------+----------------------------+ 812 813NVMe Zoned Namespace devices 814---------------------------- 815 816- ZNS defines a per-zone capacity which can be equal or less than the 817 zone-size. Zone-capacity is the number of usable blocks in the zone. 818 F2FS checks if zone-capacity is less than zone-size, if it is, then any 819 segment which starts after the zone-capacity is marked as not-free in 820 the free segment bitmap at initial mount time. These segments are marked 821 as permanently used so they are not allocated for writes and 822 consequently are not needed to be garbage collected. In case the 823 zone-capacity is not aligned to default segment size(2MB), then a segment 824 can start before the zone-capacity and span across zone-capacity boundary. 825 Such spanning segments are also considered as usable segments. All blocks 826 past the zone-capacity are considered unusable in these segments. 827