1================================================================================ 2WHAT IS Flash-Friendly File System (F2FS)? 3================================================================================ 4 5NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have 6been equipped on a variety systems ranging from mobile to server systems. Since 7they are known to have different characteristics from the conventional rotating 8disks, a file system, an upper layer to the storage device, should adapt to the 9changes from the sketch in the design level. 10 11F2FS is a file system exploiting NAND flash memory-based storage devices, which 12is based on Log-structured File System (LFS). The design has been focused on 13addressing the fundamental issues in LFS, which are snowball effect of wandering 14tree and high cleaning overhead. 15 16Since a NAND flash memory-based storage device shows different characteristic 17according to its internal geometry or flash memory management scheme, namely FTL, 18F2FS and its tools support various parameters not only for configuring on-disk 19layout, but also for selecting allocation and cleaning algorithms. 20 21The following git tree provides the file system formatting tool (mkfs.f2fs), 22a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs). 23>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git 24 25For reporting bugs and sending patches, please use the following mailing list: 26>> linux-f2fs-devel@lists.sourceforge.net 27 28================================================================================ 29BACKGROUND AND DESIGN ISSUES 30================================================================================ 31 32Log-structured File System (LFS) 33-------------------------------- 34"A log-structured file system writes all modifications to disk sequentially in 35a log-like structure, thereby speeding up both file writing and crash recovery. 36The log is the only structure on disk; it contains indexing information so that 37files can be read back from the log efficiently. In order to maintain large free 38areas on disk for fast writing, we divide the log into segments and use a 39segment cleaner to compress the live information from heavily fragmented 40segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and 41implementation of a log-structured file system", ACM Trans. Computer Systems 4210, 1, 26–52. 43 44Wandering Tree Problem 45---------------------- 46In LFS, when a file data is updated and written to the end of log, its direct 47pointer block is updated due to the changed location. Then the indirect pointer 48block is also updated due to the direct pointer block update. In this manner, 49the upper index structures such as inode, inode map, and checkpoint block are 50also updated recursively. This problem is called as wandering tree problem [1], 51and in order to enhance the performance, it should eliminate or relax the update 52propagation as much as possible. 53 54[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ 55 56Cleaning Overhead 57----------------- 58Since LFS is based on out-of-place writes, it produces so many obsolete blocks 59scattered across the whole storage. In order to serve new empty log space, it 60needs to reclaim these obsolete blocks seamlessly to users. This job is called 61as a cleaning process. 62 63The process consists of three operations as follows. 641. A victim segment is selected through referencing segment usage table. 652. It loads parent index structures of all the data in the victim identified by 66 segment summary blocks. 673. It checks the cross-reference between the data and its parent index structure. 684. It moves valid data selectively. 69 70This cleaning job may cause unexpected long delays, so the most important goal 71is to hide the latencies to users. And also definitely, it should reduce the 72amount of valid data to be moved, and move them quickly as well. 73 74================================================================================ 75KEY FEATURES 76================================================================================ 77 78Flash Awareness 79--------------- 80- Enlarge the random write area for better performance, but provide the high 81 spatial locality 82- Align FS data structures to the operational units in FTL as best efforts 83 84Wandering Tree Problem 85---------------------- 86- Use a term, “node”, that represents inodes as well as various pointer blocks 87- Introduce Node Address Table (NAT) containing the locations of all the “node” 88 blocks; this will cut off the update propagation. 89 90Cleaning Overhead 91----------------- 92- Support a background cleaning process 93- Support greedy and cost-benefit algorithms for victim selection policies 94- Support multi-head logs for static/dynamic hot and cold data separation 95- Introduce adaptive logging for efficient block allocation 96 97================================================================================ 98MOUNT OPTIONS 99================================================================================ 100 101background_gc=%s Turn on/off cleaning operations, namely garbage 102 collection, triggered in background when I/O subsystem is 103 idle. If background_gc=on, it will turn on the garbage 104 collection and if background_gc=off, garbage collection 105 will be turned off. If background_gc=sync, it will turn 106 on synchronous garbage collection running in background. 107 Default value for this option is on. So garbage 108 collection is on by default. 109disable_roll_forward Disable the roll-forward recovery routine 110norecovery Disable the roll-forward recovery routine, mounted read- 111 only (i.e., -o ro,disable_roll_forward) 112discard/nodiscard Enable/disable real-time discard in f2fs, if discard is 113 enabled, f2fs will issue discard/TRIM commands when a 114 segment is cleaned. 115no_heap Disable heap-style segment allocation which finds free 116 segments for data from the beginning of main area, while 117 for node from the end of main area. 118nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 119 by default if CONFIG_F2FS_FS_XATTR is selected. 120noacl Disable POSIX Access Control List. Note: acl is enabled 121 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 122active_logs=%u Support configuring the number of active logs. In the 123 current design, f2fs supports only 2, 4, and 6 logs. 124 Default number is 6. 125disable_ext_identify Disable the extension list configured by mkfs, so f2fs 126 does not aware of cold files such as media files. 127inline_xattr Enable the inline xattrs feature. 128noinline_xattr Disable the inline xattrs feature. 129inline_data Enable the inline data feature: New created small(<~3.4k) 130 files can be written into inode block. 131inline_dentry Enable the inline dir feature: data in new created 132 directory entries can be written into inode block. The 133 space of inode block which is used to store inline 134 dentries is limited to ~3.4k. 135noinline_dentry Disable the inline dentry feature. 136flush_merge Merge concurrent cache_flush commands as much as possible 137 to eliminate redundant command issues. If the underlying 138 device handles the cache_flush command relatively slowly, 139 recommend to enable this option. 140nobarrier This option can be used if underlying storage guarantees 141 its cached data should be written to the novolatile area. 142 If this option is set, no cache_flush commands are issued 143 but f2fs still guarantees the write ordering of all the 144 data writes. 145fastboot This option is used when a system wants to reduce mount 146 time as much as possible, even though normal performance 147 can be sacrificed. 148extent_cache Enable an extent cache based on rb-tree, it can cache 149 as many as extent which map between contiguous logical 150 address and physical address per inode, resulting in 151 increasing the cache hit ratio. Set by default. 152noextent_cache Disable an extent cache based on rb-tree explicitly, see 153 the above extent_cache mount option. 154noinline_data Disable the inline data feature, inline data feature is 155 enabled by default. 156data_flush Enable data flushing before checkpoint in order to 157 persist data of regular and symlink. 158fault_injection=%d Enable fault injection in all supported types with 159 specified injection rate. 160fault_type=%d Support configuring fault injection type, should be 161 enabled with fault_injection option, fault type value 162 is shown below, it supports single or combined type. 163 Type_Name Type_Value 164 FAULT_KMALLOC 0x000000001 165 FAULT_KVMALLOC 0x000000002 166 FAULT_PAGE_ALLOC 0x000000004 167 FAULT_PAGE_GET 0x000000008 168 FAULT_ALLOC_BIO 0x000000010 169 FAULT_ALLOC_NID 0x000000020 170 FAULT_ORPHAN 0x000000040 171 FAULT_BLOCK 0x000000080 172 FAULT_DIR_DEPTH 0x000000100 173 FAULT_EVICT_INODE 0x000000200 174 FAULT_TRUNCATE 0x000000400 175 FAULT_IO 0x000000800 176 FAULT_CHECKPOINT 0x000001000 177 FAULT_DISCARD 0x000002000 178mode=%s Control block allocation mode which supports "adaptive" 179 and "lfs". In "lfs" mode, there should be no random 180 writes towards main area. 181io_bits=%u Set the bit size of write IO requests. It should be set 182 with "mode=lfs". 183usrquota Enable plain user disk quota accounting. 184grpquota Enable plain group disk quota accounting. 185prjquota Enable plain project quota accounting. 186usrjquota=<file> Appoint specified file and type during mount, so that quota 187grpjquota=<file> information can be properly updated during recovery flow, 188prjjquota=<file> <quota file>: must be in root directory; 189jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 190offusrjquota Turn off user journelled quota. 191offgrpjquota Turn off group journelled quota. 192offprjjquota Turn off project journelled quota. 193quota Enable plain user disk quota accounting. 194noquota Disable all plain disk quota option. 195whint_mode=%s Control which write hints are passed down to block 196 layer. This supports "off", "user-based", and 197 "fs-based". In "off" mode (default), f2fs does not pass 198 down hints. In "user-based" mode, f2fs tries to pass 199 down hints given by users. And in "fs-based" mode, f2fs 200 passes down hints with its policy. 201alloc_mode=%s Adjust block allocation policy, which supports "reuse" 202 and "default". 203fsync_mode=%s Control the policy of fsync. Currently supports "posix", 204 "strict", and "nobarrier". In "posix" mode, which is 205 default, fsync will follow POSIX semantics and does a 206 light operation to improve the filesystem performance. 207 In "strict" mode, fsync will be heavy and behaves in line 208 with xfs, ext4 and btrfs, where xfstest generic/342 will 209 pass, but the performance will regress. "nobarrier" is 210 based on "posix", but doesn't issue flush command for 211 non-atomic files likewise "nobarrier" mount option. 212test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt 213 context. The fake fscrypt context is used by xfstests. 214 215================================================================================ 216DEBUGFS ENTRIES 217================================================================================ 218 219/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 220f2fs. Each file shows the whole f2fs information. 221 222/sys/kernel/debug/f2fs/status includes: 223 - major file system information managed by f2fs currently 224 - average SIT information about whole segments 225 - current memory footprint consumed by f2fs. 226 227================================================================================ 228SYSFS ENTRIES 229================================================================================ 230 231Information about mounted f2fs file systems can be found in 232/sys/fs/f2fs. Each mounted filesystem will have a directory in 233/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 234The files in each per-device directory are shown in table below. 235 236Files in /sys/fs/f2fs/<devname> 237(see also Documentation/ABI/testing/sysfs-fs-f2fs) 238.............................................................................. 239 File Content 240 241 gc_max_sleep_time This tuning parameter controls the maximum sleep 242 time for the garbage collection thread. Time is 243 in milliseconds. 244 245 gc_min_sleep_time This tuning parameter controls the minimum sleep 246 time for the garbage collection thread. Time is 247 in milliseconds. 248 249 gc_no_gc_sleep_time This tuning parameter controls the default sleep 250 time for the garbage collection thread. Time is 251 in milliseconds. 252 253 gc_idle This parameter controls the selection of victim 254 policy for garbage collection. Setting gc_idle = 0 255 (default) will disable this option. Setting 256 gc_idle = 1 will select the Cost Benefit approach 257 & setting gc_idle = 2 will select the greedy approach. 258 259 gc_urgent This parameter controls triggering background GCs 260 urgently or not. Setting gc_urgent = 0 [default] 261 makes back to default behavior, while if it is set 262 to 1, background thread starts to do GC by given 263 gc_urgent_sleep_time interval. 264 265 gc_urgent_sleep_time This parameter controls sleep time for gc_urgent. 266 500 ms is set by default. See above gc_urgent. 267 268 reclaim_segments This parameter controls the number of prefree 269 segments to be reclaimed. If the number of prefree 270 segments is larger than the number of segments 271 in the proportion to the percentage over total 272 volume size, f2fs tries to conduct checkpoint to 273 reclaim the prefree segments to free segments. 274 By default, 5% over total # of segments. 275 276 max_small_discards This parameter controls the number of discard 277 commands that consist small blocks less than 2MB. 278 The candidates to be discarded are cached until 279 checkpoint is triggered, and issued during the 280 checkpoint. By default, it is disabled with 0. 281 282 trim_sections This parameter controls the number of sections 283 to be trimmed out in batch mode when FITRIM 284 conducts. 32 sections is set by default. 285 286 ipu_policy This parameter controls the policy of in-place 287 updates in f2fs. There are five policies: 288 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, 289 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, 290 0x10: F2FS_IPU_FSYNC. 291 292 min_ipu_util This parameter controls the threshold to trigger 293 in-place-updates. The number indicates percentage 294 of the filesystem utilization, and used by 295 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies. 296 297 min_fsync_blocks This parameter controls the threshold to trigger 298 in-place-updates when F2FS_IPU_FSYNC mode is set. 299 The number indicates the number of dirty pages 300 when fsync needs to flush on its call path. If 301 the number is less than this value, it triggers 302 in-place-updates. 303 304 max_victim_search This parameter controls the number of trials to 305 find a victim segment when conducting SSR and 306 cleaning operations. The default value is 4096 307 which covers 8GB block address range. 308 309 dir_level This parameter controls the directory level to 310 support large directory. If a directory has a 311 number of files, it can reduce the file lookup 312 latency by increasing this dir_level value. 313 Otherwise, it needs to decrease this value to 314 reduce the space overhead. The default value is 0. 315 316 ram_thresh This parameter controls the memory footprint used 317 by free nids and cached nat entries. By default, 318 10 is set, which indicates 10 MB / 1 GB RAM. 319 320================================================================================ 321USAGE 322================================================================================ 323 3241. Download userland tools and compile them. 325 3262. Skip, if f2fs was compiled statically inside kernel. 327 Otherwise, insert the f2fs.ko module. 328 # insmod f2fs.ko 329 3303. Create a directory trying to mount 331 # mkdir /mnt/f2fs 332 3334. Format the block device, and then mount as f2fs 334 # mkfs.f2fs -l label /dev/block_device 335 # mount -t f2fs /dev/block_device /mnt/f2fs 336 337mkfs.f2fs 338--------- 339The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 340which builds a basic on-disk layout. 341 342The options consist of: 343-l [label] : Give a volume label, up to 512 unicode name. 344-a [0 or 1] : Split start location of each area for heap-based allocation. 345 1 is set by default, which performs this. 346-o [int] : Set overprovision ratio in percent over volume size. 347 5 is set by default. 348-s [int] : Set the number of segments per section. 349 1 is set by default. 350-z [int] : Set the number of sections per zone. 351 1 is set by default. 352-e [str] : Set basic extension list. e.g. "mp3,gif,mov" 353-t [0 or 1] : Disable discard command or not. 354 1 is set by default, which conducts discard. 355 356fsck.f2fs 357--------- 358The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 359partition, which examines whether the filesystem metadata and user-made data 360are cross-referenced correctly or not. 361Note that, initial version of the tool does not fix any inconsistency. 362 363The options consist of: 364 -d debug level [default:0] 365 366dump.f2fs 367--------- 368The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 369file. Each file is dump_ssa and dump_sit. 370 371The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 372It shows on-disk inode information recognized by a given inode number, and is 373able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 374./dump_sit respectively. 375 376The options consist of: 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# dump.f2fs -i [ino] /dev/sdx 384# dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 385# dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 386 387================================================================================ 388DESIGN 389================================================================================ 390 391On-disk Layout 392-------------- 393 394F2FS divides the whole volume into a number of segments, each of which is fixed 395to 2MB in size. A section is composed of consecutive segments, and a zone 396consists of a set of sections. By default, section and zone sizes are set to one 397segment size identically, but users can easily modify the sizes by mkfs. 398 399F2FS splits the entire volume into six areas, and all the areas except superblock 400consists of multiple segments as described below. 401 402 align with the zone size <-| 403 |-> align with the segment size 404 _________________________________________________________________________ 405 | | | Segment | Node | Segment | | 406 | Superblock | Checkpoint | Info. | Address | Summary | Main | 407 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 408 |____________|_____2______|______N______|______N______|______N_____|__N___| 409 . . 410 . . 411 . . 412 ._________________________________________. 413 |_Segment_|_..._|_Segment_|_..._|_Segment_| 414 . . 415 ._________._________ 416 |_section_|__...__|_ 417 . . 418 .________. 419 |__zone__| 420 421- Superblock (SB) 422 : It is located at the beginning of the partition, and there exist two copies 423 to avoid file system crash. It contains basic partition information and some 424 default parameters of f2fs. 425 426- Checkpoint (CP) 427 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan 428 inode lists, and summary entries of current active segments. 429 430- Segment Information Table (SIT) 431 : It contains segment information such as valid block count and bitmap for the 432 validity of all the blocks. 433 434- Node Address Table (NAT) 435 : It is composed of a block address table for all the node blocks stored in 436 Main area. 437 438- Segment Summary Area (SSA) 439 : It contains summary entries which contains the owner information of all the 440 data and node blocks stored in Main area. 441 442- Main Area 443 : It contains file and directory data including their indices. 444 445In order to avoid misalignment between file system and flash-based storage, F2FS 446aligns the start block address of CP with the segment size. Also, it aligns the 447start block address of Main area with the zone size by reserving some segments 448in SSA area. 449 450Reference the following survey for additional technical details. 451https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 452 453File System Metadata Structure 454------------------------------ 455 456F2FS adopts the checkpointing scheme to maintain file system consistency. At 457mount time, F2FS first tries to find the last valid checkpoint data by scanning 458CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 459One of them always indicates the last valid data, which is called as shadow copy 460mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 461 462For file system consistency, each CP points to which NAT and SIT copies are 463valid, as shown as below. 464 465 +--------+----------+---------+ 466 | CP | SIT | NAT | 467 +--------+----------+---------+ 468 . . . . 469 . . . . 470 . . . . 471 +-------+-------+--------+--------+--------+--------+ 472 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 473 +-------+-------+--------+--------+--------+--------+ 474 | ^ ^ 475 | | | 476 `----------------------------------------' 477 478Index Structure 479--------------- 480 481The key data structure to manage the data locations is a "node". Similar to 482traditional file structures, F2FS has three types of node: inode, direct node, 483indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 484indices, two direct node pointers, two indirect node pointers, and one double 485indirect node pointer as described below. One direct node block contains 1018 486data blocks, and one indirect node block contains also 1018 node blocks. Thus, 487one inode block (i.e., a file) covers: 488 489 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 490 491 Inode block (4KB) 492 |- data (923) 493 |- direct node (2) 494 | `- data (1018) 495 |- indirect node (2) 496 | `- direct node (1018) 497 | `- data (1018) 498 `- double indirect node (1) 499 `- indirect node (1018) 500 `- direct node (1018) 501 `- data (1018) 502 503Note that, all the node blocks are mapped by NAT which means the location of 504each node is translated by the NAT table. In the consideration of the wandering 505tree problem, F2FS is able to cut off the propagation of node updates caused by 506leaf data writes. 507 508Directory Structure 509------------------- 510 511A directory entry occupies 11 bytes, which consists of the following attributes. 512 513- hash hash value of the file name 514- ino inode number 515- len the length of file name 516- type file type such as directory, symlink, etc 517 518A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 519used to represent whether each dentry is valid or not. A dentry block occupies 5204KB with the following composition. 521 522 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 523 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 524 525 [Bucket] 526 +--------------------------------+ 527 |dentry block 1 | dentry block 2 | 528 +--------------------------------+ 529 . . 530 . . 531 . [Dentry Block Structure: 4KB] . 532 +--------+----------+----------+------------+ 533 | bitmap | reserved | dentries | file names | 534 +--------+----------+----------+------------+ 535 [Dentry Block: 4KB] . . 536 . . 537 . . 538 +------+------+-----+------+ 539 | hash | ino | len | type | 540 +------+------+-----+------+ 541 [Dentry Structure: 11 bytes] 542 543F2FS implements multi-level hash tables for directory structure. Each level has 544a hash table with dedicated number of hash buckets as shown below. Note that 545"A(2B)" means a bucket includes 2 data blocks. 546 547---------------------- 548A : bucket 549B : block 550N : MAX_DIR_HASH_DEPTH 551---------------------- 552 553level #0 | A(2B) 554 | 555level #1 | A(2B) - A(2B) 556 | 557level #2 | A(2B) - A(2B) - A(2B) - A(2B) 558 . | . . . . 559level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 560 . | . . . . 561level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 562 563The number of blocks and buckets are determined by, 564 565 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 566 # of blocks in level #n = | 567 `- 4, Otherwise 568 569 ,- 2^(n + dir_level), 570 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 571 # of buckets in level #n = | 572 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 573 Otherwise 574 575When F2FS finds a file name in a directory, at first a hash value of the file 576name is calculated. Then, F2FS scans the hash table in level #0 to find the 577dentry consisting of the file name and its inode number. If not found, F2FS 578scans the next hash table in level #1. In this way, F2FS scans hash tables in 579each levels incrementally from 1 to N. In each levels F2FS needs to scan only 580one bucket determined by the following equation, which shows O(log(# of files)) 581complexity. 582 583 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 584 585In the case of file creation, F2FS finds empty consecutive slots that cover the 586file name. F2FS searches the empty slots in the hash tables of whole levels from 5871 to N in the same way as the lookup operation. 588 589The following figure shows an example of two cases holding children. 590 --------------> Dir <-------------- 591 | | 592 child child 593 594 child - child [hole] - child 595 596 child - child - child [hole] - [hole] - child 597 598 Case 1: Case 2: 599 Number of children = 6, Number of children = 3, 600 File size = 7 File size = 7 601 602Default Block Allocation 603------------------------ 604 605At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 606and Hot/Warm/Cold data. 607 608- Hot node contains direct node blocks of directories. 609- Warm node contains direct node blocks except hot node blocks. 610- Cold node contains indirect node blocks 611- Hot data contains dentry blocks 612- Warm data contains data blocks except hot and cold data blocks 613- Cold data contains multimedia data or migrated data blocks 614 615LFS has two schemes for free space management: threaded log and copy-and-compac- 616tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 617for devices showing very good sequential write performance, since free segments 618are served all the time for writing new data. However, it suffers from cleaning 619overhead under high utilization. Contrarily, the threaded log scheme suffers 620from random writes, but no cleaning process is needed. F2FS adopts a hybrid 621scheme where the copy-and-compaction scheme is adopted by default, but the 622policy is dynamically changed to the threaded log scheme according to the file 623system status. 624 625In order to align F2FS with underlying flash-based storage, F2FS allocates a 626segment in a unit of section. F2FS expects that the section size would be the 627same as the unit size of garbage collection in FTL. Furthermore, with respect 628to the mapping granularity in FTL, F2FS allocates each section of the active 629logs from different zones as much as possible, since FTL can write the data in 630the active logs into one allocation unit according to its mapping granularity. 631 632Cleaning process 633---------------- 634 635F2FS does cleaning both on demand and in the background. On-demand cleaning is 636triggered when there are not enough free segments to serve VFS calls. Background 637cleaner is operated by a kernel thread, and triggers the cleaning job when the 638system is idle. 639 640F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 641In the greedy algorithm, F2FS selects a victim segment having the smallest number 642of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 643according to the segment age and the number of valid blocks in order to address 644log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 645algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 646algorithm. 647 648In order to identify whether the data in the victim segment are valid or not, 649F2FS manages a bitmap. Each bit represents the validity of a block, and the 650bitmap is composed of a bit stream covering whole blocks in main area. 651 652Write-hint Policy 653----------------- 654 6551) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. 656 6572) whint_mode=user-based. F2FS tries to pass down hints given by 658users. 659 660User F2FS Block 661---- ---- ----- 662 META WRITE_LIFE_NOT_SET 663 HOT_NODE " 664 WARM_NODE " 665 COLD_NODE " 666*ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 667*extension list " " 668 669-- buffered io 670WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 671WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 672WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 673WRITE_LIFE_NONE " " 674WRITE_LIFE_MEDIUM " " 675WRITE_LIFE_LONG " " 676 677-- direct io 678WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 679WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 680WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 681WRITE_LIFE_NONE " WRITE_LIFE_NONE 682WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 683WRITE_LIFE_LONG " WRITE_LIFE_LONG 684 6853) whint_mode=fs-based. F2FS passes down hints with its policy. 686 687User F2FS Block 688---- ---- ----- 689 META WRITE_LIFE_MEDIUM; 690 HOT_NODE WRITE_LIFE_NOT_SET 691 WARM_NODE " 692 COLD_NODE WRITE_LIFE_NONE 693ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME 694extension list " " 695 696-- buffered io 697WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 698WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 699WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG 700WRITE_LIFE_NONE " " 701WRITE_LIFE_MEDIUM " " 702WRITE_LIFE_LONG " " 703 704-- direct io 705WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME 706WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT 707WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET 708WRITE_LIFE_NONE " WRITE_LIFE_NONE 709WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM 710WRITE_LIFE_LONG " WRITE_LIFE_LONG 711
