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. 263compress_chksum Support verifying chksum of raw data in compressed cluster. 264compress_mode=%s Control file compression mode. This supports "fs" and "user" 265 modes. In "fs" mode (default), f2fs does automatic compression 266 on the compression enabled files. In "user" mode, f2fs disables 267 the automaic compression and gives the user discretion of 268 choosing the target file and the timing. The user can do manual 269 compression/decompression on the compression enabled files using 270 ioctls. 271inlinecrypt When possible, encrypt/decrypt the contents of encrypted 272 files using the blk-crypto framework rather than 273 filesystem-layer encryption. This allows the use of 274 inline encryption hardware. The on-disk format is 275 unaffected. For more details, see 276 Documentation/block/inline-encryption.rst. 277atgc Enable age-threshold garbage collection, it provides high 278 effectiveness and efficiency on background GC. 279======================== ============================================================ 280 281Debugfs Entries 282=============== 283 284/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 285f2fs. Each file shows the whole f2fs information. 286 287/sys/kernel/debug/f2fs/status includes: 288 289 - major file system information managed by f2fs currently 290 - average SIT information about whole segments 291 - current memory footprint consumed by f2fs. 292 293Sysfs Entries 294============= 295 296Information about mounted f2fs file systems can be found in 297/sys/fs/f2fs. Each mounted filesystem will have a directory in 298/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 299The files in each per-device directory are shown in table below. 300 301Files in /sys/fs/f2fs/<devname> 302(see also Documentation/ABI/testing/sysfs-fs-f2fs) 303 304Usage 305===== 306 3071. Download userland tools and compile them. 308 3092. Skip, if f2fs was compiled statically inside kernel. 310 Otherwise, insert the f2fs.ko module:: 311 312 # insmod f2fs.ko 313 3143. Create a directory to use when mounting:: 315 316 # mkdir /mnt/f2fs 317 3184. Format the block device, and then mount as f2fs:: 319 320 # mkfs.f2fs -l label /dev/block_device 321 # mount -t f2fs /dev/block_device /mnt/f2fs 322 323mkfs.f2fs 324--------- 325The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 326which builds a basic on-disk layout. 327 328The quick options consist of: 329 330=============== =========================================================== 331``-l [label]`` Give a volume label, up to 512 unicode name. 332``-a [0 or 1]`` Split start location of each area for heap-based allocation. 333 334 1 is set by default, which performs this. 335``-o [int]`` Set overprovision ratio in percent over volume size. 336 337 5 is set by default. 338``-s [int]`` Set the number of segments per section. 339 340 1 is set by default. 341``-z [int]`` Set the number of sections per zone. 342 343 1 is set by default. 344``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" 345``-t [0 or 1]`` Disable discard command or not. 346 347 1 is set by default, which conducts discard. 348=============== =========================================================== 349 350Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. 351 352fsck.f2fs 353--------- 354The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 355partition, which examines whether the filesystem metadata and user-made data 356are cross-referenced correctly or not. 357Note that, initial version of the tool does not fix any inconsistency. 358 359The quick options consist of:: 360 361 -d debug level [default:0] 362 363Note: please refer to the manpage of fsck.f2fs(8) to get full option list. 364 365dump.f2fs 366--------- 367The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 368file. Each file is dump_ssa and dump_sit. 369 370The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 371It shows on-disk inode information recognized by a given inode number, and is 372able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 373./dump_sit respectively. 374 375The options consist of:: 376 377 -d debug level [default:0] 378 -i inode no (hex) 379 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 380 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 381 382Examples:: 383 384 # dump.f2fs -i [ino] /dev/sdx 385 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 386 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 387 388Note: please refer to the manpage of dump.f2fs(8) to get full option list. 389 390sload.f2fs 391---------- 392The sload.f2fs gives a way to insert files and directories in the exisiting disk 393image. This tool is useful when building f2fs images given compiled files. 394 395Note: please refer to the manpage of sload.f2fs(8) to get full option list. 396 397resize.f2fs 398----------- 399The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving 400all the files and directories stored in the image. 401 402Note: please refer to the manpage of resize.f2fs(8) to get full option list. 403 404defrag.f2fs 405----------- 406The defrag.f2fs can be used to defragment scattered written data as well as 407filesystem metadata across the disk. This can improve the write speed by giving 408more free consecutive space. 409 410Note: please refer to the manpage of defrag.f2fs(8) to get full option list. 411 412f2fs_io 413------- 414The f2fs_io is a simple tool to issue various filesystem APIs as well as 415f2fs-specific ones, which is very useful for QA tests. 416 417Note: please refer to the manpage of f2fs_io(8) to get full option list. 418 419Design 420====== 421 422On-disk Layout 423-------------- 424 425F2FS divides the whole volume into a number of segments, each of which is fixed 426to 2MB in size. A section is composed of consecutive segments, and a zone 427consists of a set of sections. By default, section and zone sizes are set to one 428segment size identically, but users can easily modify the sizes by mkfs. 429 430F2FS splits the entire volume into six areas, and all the areas except superblock 431consist of multiple segments as described below:: 432 433 align with the zone size <-| 434 |-> align with the segment size 435 _________________________________________________________________________ 436 | | | Segment | Node | Segment | | 437 | Superblock | Checkpoint | Info. | Address | Summary | Main | 438 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 439 |____________|_____2______|______N______|______N______|______N_____|__N___| 440 . . 441 . . 442 . . 443 ._________________________________________. 444 |_Segment_|_..._|_Segment_|_..._|_Segment_| 445 . . 446 ._________._________ 447 |_section_|__...__|_ 448 . . 449 .________. 450 |__zone__| 451 452- Superblock (SB) 453 It is located at the beginning of the partition, and there exist two copies 454 to avoid file system crash. It contains basic partition information and some 455 default parameters of f2fs. 456 457- Checkpoint (CP) 458 It contains file system information, bitmaps for valid NAT/SIT sets, orphan 459 inode lists, and summary entries of current active segments. 460 461- Segment Information Table (SIT) 462 It contains segment information such as valid block count and bitmap for the 463 validity of all the blocks. 464 465- Node Address Table (NAT) 466 It is composed of a block address table for all the node blocks stored in 467 Main area. 468 469- Segment Summary Area (SSA) 470 It contains summary entries which contains the owner information of all the 471 data and node blocks stored in Main area. 472 473- Main Area 474 It contains file and directory data including their indices. 475 476In order to avoid misalignment between file system and flash-based storage, F2FS 477aligns the start block address of CP with the segment size. Also, it aligns the 478start block address of Main area with the zone size by reserving some segments 479in SSA area. 480 481Reference the following survey for additional technical details. 482https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 483 484File System Metadata Structure 485------------------------------ 486 487F2FS adopts the checkpointing scheme to maintain file system consistency. At 488mount time, F2FS first tries to find the last valid checkpoint data by scanning 489CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 490One of them always indicates the last valid data, which is called as shadow copy 491mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 492 493For file system consistency, each CP points to which NAT and SIT copies are 494valid, as shown as below:: 495 496 +--------+----------+---------+ 497 | CP | SIT | NAT | 498 +--------+----------+---------+ 499 . . . . 500 . . . . 501 . . . . 502 +-------+-------+--------+--------+--------+--------+ 503 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 504 +-------+-------+--------+--------+--------+--------+ 505 | ^ ^ 506 | | | 507 `----------------------------------------' 508 509Index Structure 510--------------- 511 512The key data structure to manage the data locations is a "node". Similar to 513traditional file structures, F2FS has three types of node: inode, direct node, 514indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 515indices, two direct node pointers, two indirect node pointers, and one double 516indirect node pointer as described below. One direct node block contains 1018 517data blocks, and one indirect node block contains also 1018 node blocks. Thus, 518one inode block (i.e., a file) covers:: 519 520 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 521 522 Inode block (4KB) 523 |- data (923) 524 |- direct node (2) 525 | `- data (1018) 526 |- indirect node (2) 527 | `- direct node (1018) 528 | `- data (1018) 529 `- double indirect node (1) 530 `- indirect node (1018) 531 `- direct node (1018) 532 `- data (1018) 533 534Note that all the node blocks are mapped by NAT which means the location of 535each node is translated by the NAT table. In the consideration of the wandering 536tree problem, F2FS is able to cut off the propagation of node updates caused by 537leaf data writes. 538 539Directory Structure 540------------------- 541 542A directory entry occupies 11 bytes, which consists of the following attributes. 543 544- hash hash value of the file name 545- ino inode number 546- len the length of file name 547- type file type such as directory, symlink, etc 548 549A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 550used to represent whether each dentry is valid or not. A dentry block occupies 5514KB with the following composition. 552 553:: 554 555 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 556 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 557 558 [Bucket] 559 +--------------------------------+ 560 |dentry block 1 | dentry block 2 | 561 +--------------------------------+ 562 . . 563 . . 564 . [Dentry Block Structure: 4KB] . 565 +--------+----------+----------+------------+ 566 | bitmap | reserved | dentries | file names | 567 +--------+----------+----------+------------+ 568 [Dentry Block: 4KB] . . 569 . . 570 . . 571 +------+------+-----+------+ 572 | hash | ino | len | type | 573 +------+------+-----+------+ 574 [Dentry Structure: 11 bytes] 575 576F2FS implements multi-level hash tables for directory structure. Each level has 577a hash table with dedicated number of hash buckets as shown below. Note that 578"A(2B)" means a bucket includes 2 data blocks. 579 580:: 581 582 ---------------------- 583 A : bucket 584 B : block 585 N : MAX_DIR_HASH_DEPTH 586 ---------------------- 587 588 level #0 | A(2B) 589 | 590 level #1 | A(2B) - A(2B) 591 | 592 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 593 . | . . . . 594 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 595 . | . . . . 596 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 597 598The number of blocks and buckets are determined by:: 599 600 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 601 # of blocks in level #n = | 602 `- 4, Otherwise 603 604 ,- 2^(n + dir_level), 605 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 606 # of buckets in level #n = | 607 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 608 Otherwise 609 610When F2FS finds a file name in a directory, at first a hash value of the file 611name is calculated. Then, F2FS scans the hash table in level #0 to find the 612dentry consisting of the file name and its inode number. If not found, F2FS 613scans the next hash table in level #1. In this way, F2FS scans hash tables in 614each levels incrementally from 1 to N. In each level F2FS needs to scan only 615one bucket determined by the following equation, which shows O(log(# of files)) 616complexity:: 617 618 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 619 620In the case of file creation, F2FS finds empty consecutive slots that cover the 621file name. F2FS searches the empty slots in the hash tables of whole levels from 6221 to N in the same way as the lookup operation. 623 624The following figure shows an example of two cases holding children:: 625 626 --------------> Dir <-------------- 627 | | 628 child child 629 630 child - child [hole] - child 631 632 child - child - child [hole] - [hole] - child 633 634 Case 1: Case 2: 635 Number of children = 6, Number of children = 3, 636 File size = 7 File size = 7 637 638Default Block Allocation 639------------------------ 640 641At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 642and Hot/Warm/Cold data. 643 644- Hot node contains direct node blocks of directories. 645- Warm node contains direct node blocks except hot node blocks. 646- Cold node contains indirect node blocks 647- Hot data contains dentry blocks 648- Warm data contains data blocks except hot and cold data blocks 649- Cold data contains multimedia data or migrated data blocks 650 651LFS has two schemes for free space management: threaded log and copy-and-compac- 652tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 653for devices showing very good sequential write performance, since free segments 654are served all the time for writing new data. However, it suffers from cleaning 655overhead under high utilization. Contrarily, the threaded log scheme suffers 656from random writes, but no cleaning process is needed. F2FS adopts a hybrid 657scheme where the copy-and-compaction scheme is adopted by default, but the 658policy is dynamically changed to the threaded log scheme according to the file 659system status. 660 661In order to align F2FS with underlying flash-based storage, F2FS allocates a 662segment in a unit of section. F2FS expects that the section size would be the 663same as the unit size of garbage collection in FTL. Furthermore, with respect 664to the mapping granularity in FTL, F2FS allocates each section of the active 665logs from different zones as much as possible, since FTL can write the data in 666the active logs into one allocation unit according to its mapping granularity. 667 668Cleaning process 669---------------- 670 671F2FS does cleaning both on demand and in the background. On-demand cleaning is 672triggered when there are not enough free segments to serve VFS calls. Background 673cleaner is operated by a kernel thread, and triggers the cleaning job when the 674system is idle. 675 676F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 677In the greedy algorithm, F2FS selects a victim segment having the smallest number 678of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 679according to the segment age and the number of valid blocks in order to address 680log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 681algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 682algorithm. 683 684In order to identify whether the data in the victim segment are valid or not, 685F2FS manages a bitmap. Each bit represents the validity of a block, and the 686bitmap is composed of a bit stream covering whole blocks in main area. 687 688Write-hint Policy 689----------------- 690 6911) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 692 6932) whint_mode=user-based. F2FS tries to pass down hints given by 694users. 695 696===================== ======================== =================== 697User F2FS Block 698===================== ======================== =================== 699 META WRITE_LIFE_NOT_SET 700 HOT_NODE " 701 WARM_NODE " 702 COLD_NODE " 703ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 704extension list " " 705 706-- buffered 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 " " 711WRITE_LIFE_MEDIUM " " 712WRITE_LIFE_LONG " " 713 714-- direct io 715WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 716WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 717WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 718WRITE_LIFE_NONE " WRITE_LIFE_NONE 719WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 720WRITE_LIFE_LONG " WRITE_LIFE_LONG 721===================== ======================== =================== 722 7233) whint_mode=fs-based. F2FS passes down hints with its policy. 724 725===================== ======================== =================== 726User F2FS Block 727===================== ======================== =================== 728 META WRITE_LIFE_MEDIUM; 729 HOT_NODE WRITE_LIFE_NOT_SET 730 WARM_NODE " 731 COLD_NODE WRITE_LIFE_NONE 732ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 733extension list " " 734 735-- buffered 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_LONG 739WRITE_LIFE_NONE " " 740WRITE_LIFE_MEDIUM " " 741WRITE_LIFE_LONG " " 742 743-- direct io 744WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 745WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 746WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 747WRITE_LIFE_NONE " WRITE_LIFE_NONE 748WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 749WRITE_LIFE_LONG " WRITE_LIFE_LONG 750===================== ======================== =================== 751 752Fallocate(2) Policy 753------------------- 754 755The default policy follows the below POSIX rule. 756 757Allocating disk space 758 The default operation (i.e., mode is zero) of fallocate() allocates 759 the disk space within the range specified by offset and len. The 760 file size (as reported by stat(2)) will be changed if offset+len is 761 greater than the file size. Any subregion within the range specified 762 by offset and len that did not contain data before the call will be 763 initialized to zero. This default behavior closely resembles the 764 behavior of the posix_fallocate(3) library function, and is intended 765 as a method of optimally implementing that function. 766 767However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to 768fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having 769zero or random data, which is useful to the below scenario where: 770 771 1. create(fd) 772 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 773 3. fallocate(fd, 0, 0, size) 774 4. address = fibmap(fd, offset) 775 5. open(blkdev) 776 6. write(blkdev, address) 777 778Compression implementation 779-------------------------- 780 781- New term named cluster is defined as basic unit of compression, file can 782 be divided into multiple clusters logically. One cluster includes 4 << n 783 (n >= 0) logical pages, compression size is also cluster size, each of 784 cluster can be compressed or not. 785 786- In cluster metadata layout, one special block address is used to indicate 787 a cluster is a compressed one or normal one; for compressed cluster, following 788 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs 789 stores data including compress header and compressed data. 790 791- In order to eliminate write amplification during overwrite, F2FS only 792 support compression on write-once file, data can be compressed only when 793 all logical blocks in cluster contain valid data and compress ratio of 794 cluster data is lower than specified threshold. 795 796- To enable compression on regular inode, there are three ways: 797 798 * chattr +c file 799 * chattr +c dir; touch dir/file 800 * mount w/ -o compress_extension=ext; touch file.ext 801 802Compress metadata layout:: 803 804 [Dnode Structure] 805 +-----------------------------------------------+ 806 | cluster 1 | cluster 2 | ......... | cluster N | 807 +-----------------------------------------------+ 808 . . . . 809 . . . . 810 . Compressed Cluster . . Normal Cluster . 811 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 812 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | 813 +----------+---------+---------+---------+ +---------+---------+---------+---------+ 814 . . 815 . . 816 . . 817 +-------------+-------------+----------+----------------------------+ 818 | data length | data chksum | reserved | compressed data | 819 +-------------+-------------+----------+----------------------------+ 820 821Compression mode 822-------------------------- 823 824f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. 825With this option, f2fs provides a choice to select the way how to compress the 826compression enabled files (refer to "Compression implementation" section for how to 827enable compression on a regular inode). 828 8291) compress_mode=fs 830This is the default option. f2fs does automatic compression in the writeback of the 831compression enabled files. 832 8332) compress_mode=user 834This disables the automaic compression and gives the user discretion of choosing the 835target file and the timing. The user can do manual compression/decompression on the 836compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE 837ioctls like the below. 838 839To decompress a file, 840 841fd = open(filename, O_WRONLY, 0); 842ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 843 844To compress a file, 845 846fd = open(filename, O_WRONLY, 0); 847ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 848 849NVMe Zoned Namespace devices 850---------------------------- 851 852- ZNS defines a per-zone capacity which can be equal or less than the 853 zone-size. Zone-capacity is the number of usable blocks in the zone. 854 F2FS checks if zone-capacity is less than zone-size, if it is, then any 855 segment which starts after the zone-capacity is marked as not-free in 856 the free segment bitmap at initial mount time. These segments are marked 857 as permanently used so they are not allocated for writes and 858 consequently are not needed to be garbage collected. In case the 859 zone-capacity is not aligned to default segment size(2MB), then a segment 860 can start before the zone-capacity and span across zone-capacity boundary. 861 Such spanning segments are also considered as usable segments. All blocks 862 past the zone-capacity are considered unusable in these segments. 863