Documentation/filesystems/f2fs.txt

   1 ================================================================================
   2 WHAT IS Flash-Friendly File System (F2FS)?
   3 ================================================================================
   4
   5 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
   6 been equipped on a variety systems ranging from mobile to server systems. Since
   7 they are known to have different characteristics from the conventional rotating
   8 disks, a file system, an upper layer to the storage device, should adapt to the
   9 changes from the sketch in the design level.
  10
  11 F2FS is a file system exploiting NAND flash memory-based storage devices, which
  12 is based on Log-structured File System (LFS). The design has been focused on
  13 addressing the fundamental issues in LFS, which are snowball effect of wandering
  14 tree and high cleaning overhead.
  15
  16 Since a NAND flash memory-based storage device shows different characteristic
  17 according to its internal geometry or flash memory management scheme, namely FTL,
  18 F2FS and its tools support various parameters not only for configuring on-disk
  19 layout, but also for selecting allocation and cleaning algorithms.
  20
  21 The following git tree provides the file system formatting tool (mkfs.f2fs),
  22 a 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
  25 For reporting bugs and sending patches, please use the following mailing list:
  26 >> linux-f2fs-devel@lists.sourceforge.net
  27
  28 ================================================================================
  29 BACKGROUND AND DESIGN ISSUES
  30 ================================================================================
  31
  32 Log-structured File System (LFS)
  33 --------------------------------
  34 "A log-structured file system writes all modifications to disk sequentially in
  35 a log-like structure, thereby speeding up  both file writing and crash recovery.
  36 The log is the only structure on disk; it contains indexing information so that
  37 files can be read back from the log efficiently. In order to maintain large free
  38 areas on disk for fast writing, we divide  the log into segments and use a
  39 segment cleaner to compress the live information from heavily fragmented
  40 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
  41 implementation of a log-structured file system", ACM Trans. Computer Systems
  42 10, 1, 26–52.
  43
  44 Wandering Tree Problem
  45 ----------------------
  46 In LFS, when a file data is updated and written to the end of log, its direct
  47 pointer block is updated due to the changed location. Then the indirect pointer
  48 block is also updated due to the direct pointer block update. In this manner,
  49 the upper index structures such as inode, inode map, and checkpoint block are
  50 also updated recursively. This problem is called as wandering tree problem [1],
  51 and in order to enhance the performance, it should eliminate or relax the update
  52 propagation as much as possible.
  53
  54 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
  55
  56 Cleaning Overhead
  57 -----------------
  58 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
  59 scattered across the whole storage. In order to serve new empty log space, it
  60 needs to reclaim these obsolete blocks seamlessly to users. This job is called
  61 as a cleaning process.
  62
  63 The process consists of three operations as follows.
  64 1. A victim segment is selected through referencing segment usage table.
  65 2. It loads parent index structures of all the data in the victim identified by
  66    segment summary blocks.
  67 3. It checks the cross-reference between the data and its parent index structure.
  68 4. It moves valid data selectively.
  69
  70 This cleaning job may cause unexpected long delays, so the most important goal
  71 is to hide the latencies to users. And also definitely, it should reduce the
  72 amount of valid data to be moved, and move them quickly as well.
  73
  74 ================================================================================
  75 KEY FEATURES
  76 ================================================================================
  77
  78 Flash 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
  84 Wandering 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
  90 Cleaning 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 ================================================================================
  98 MOUNT OPTIONS
  99 ================================================================================
 100
 101 background_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 truned off.
 106                        Default value for this option is on. So garbage
 107                        collection is on by default.
 108 disable_roll_forward   Disable the roll-forward recovery routine
 109 discard                Issue discard/TRIM commands when a segment is cleaned.
 110 no_heap                Disable heap-style segment allocation which finds free
 111                        segments for data from the beginning of main area, while
 112                        for node from the end of main area.
 113 nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
 114                        by default if CONFIG_F2FS_FS_XATTR is selected.
 115 noacl                  Disable POSIX Access Control List. Note: acl is enabled
 116                        by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
 117 active_logs=%u         Support configuring the number of active logs. In the
 118                        current design, f2fs supports only 2, 4, and 6 logs.
 119                        Default number is 6.
 120 disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
 121                        does not aware of cold files such as media files.
 122 inline_xattr           Enable the inline xattrs feature.
 123 inline_data            Enable the inline data feature: New created small(<~3.4k)
 124                        files can be written into inode block.
 125 inline_dentry          Enable the inline dir feature: data in new created
 126                        directory entries can be written into inode block. The
 127                        space of inode block which is used to store inline
 128                        dentries is limited to ~3.4k.
 129 flush_merge            Merge concurrent cache_flush commands as much as possible
 130                        to eliminate redundant command issues. If the underlying
 131                        device handles the cache_flush command relatively slowly,
 132                        recommend to enable this option.
 133 nobarrier              This option can be used if underlying storage guarantees
 134                        its cached data should be written to the novolatile area.
 135                        If this option is set, no cache_flush commands are issued
 136                        but f2fs still guarantees the write ordering of all the
 137                        data writes.
 138 fastboot               This option is used when a system wants to reduce mount
 139                        time as much as possible, even though normal performance
 140                        can be sacrificed.
 141
 142 ================================================================================
 143 DEBUGFS ENTRIES
 144 ================================================================================
 145
 146 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
 147 f2fs. Each file shows the whole f2fs information.
 148
 149 /sys/kernel/debug/f2fs/status includes:
 150  - major file system information managed by f2fs currently
 151  - average SIT information about whole segments
 152  - current memory footprint consumed by f2fs.
 153
 154 ================================================================================
 155 SYSFS ENTRIES
 156 ================================================================================
 157
 158 Information about mounted f2f2 file systems can be found in
 159 /sys/fs/f2fs.  Each mounted filesystem will have a directory in
 160 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
 161 The files in each per-device directory are shown in table below.
 162
 163 Files in /sys/fs/f2fs/<devname>
 164 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
 165 ..............................................................................
 166  File                         Content
 167
 168  gc_max_sleep_time            This tuning parameter controls the maximum sleep
 169                               time for the garbage collection thread. Time is
 170                               in milliseconds.
 171
 172  gc_min_sleep_time            This tuning parameter controls the minimum sleep
 173                               time for the garbage collection thread. Time is
 174                               in milliseconds.
 175
 176  gc_no_gc_sleep_time          This tuning parameter controls the default sleep
 177                               time for the garbage collection thread. Time is
 178                               in milliseconds.
 179
 180  gc_idle                      This parameter controls the selection of victim
 181                               policy for garbage collection. Setting gc_idle = 0
 182                               (default) will disable this option. Setting
 183                               gc_idle = 1 will select the Cost Benefit approach
 184                               & setting gc_idle = 2 will select the greedy aproach.
 185
 186  reclaim_segments             This parameter controls the number of prefree
 187                               segments to be reclaimed. If the number of prefree
 188                               segments is larger than the number of segments
 189                               in the proportion to the percentage over total
 190                               volume size, f2fs tries to conduct checkpoint to
 191                               reclaim the prefree segments to free segments.
 192                               By default, 5% over total # of segments.
 193
 194  max_small_discards           This parameter controls the number of discard
 195                               commands that consist small blocks less than 2MB.
 196                               The candidates to be discarded are cached until
 197                               checkpoint is triggered, and issued during the
 198                               checkpoint. By default, it is disabled with 0.
 199
 200  ipu_policy                   This parameter controls the policy of in-place
 201                               updates in f2fs. There are five policies:
 202                                0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
 203                                0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
 204                                0x10: F2FS_IPU_FSYNC.
 205
 206  min_ipu_util                 This parameter controls the threshold to trigger
 207                               in-place-updates. The number indicates percentage
 208                               of the filesystem utilization, and used by
 209                               F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
 210
 211  min_fsync_blocks             This parameter controls the threshold to trigger
 212                               in-place-updates when F2FS_IPU_FSYNC mode is set.
 213                               The number indicates the number of dirty pages
 214                               when fsync needs to flush on its call path. If
 215                               the number is less than this value, it triggers
 216                               in-place-updates.
 217
 218  max_victim_search            This parameter controls the number of trials to
 219                               find a victim segment when conducting SSR and
 220                               cleaning operations. The default value is 4096
 221                               which covers 8GB block address range.
 222
 223  dir_level                    This parameter controls the directory level to
 224                               support large directory. If a directory has a
 225                               number of files, it can reduce the file lookup
 226                               latency by increasing this dir_level value.
 227                               Otherwise, it needs to decrease this value to
 228                               reduce the space overhead. The default value is 0.
 229
 230  ram_thresh                   This parameter controls the memory footprint used
 231                               by free nids and cached nat entries. By default,
 232                               10 is set, which indicates 10 MB / 1 GB RAM.
 233
 234 ================================================================================
 235 USAGE
 236 ================================================================================
 237
 238 1. Download userland tools and compile them.
 239
 240 2. Skip, if f2fs was compiled statically inside kernel.
 241    Otherwise, insert the f2fs.ko module.
 242  # insmod f2fs.ko
 243
 244 3. Create a directory trying to mount
 245  # mkdir /mnt/f2fs
 246
 247 4. Format the block device, and then mount as f2fs
 248  # mkfs.f2fs -l label /dev/block_device
 249  # mount -t f2fs /dev/block_device /mnt/f2fs
 250
 251 mkfs.f2fs
 252 ---------
 253 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
 254 which builds a basic on-disk layout.
 255
 256 The options consist of:
 257 -l [label]   : Give a volume label, up to 512 unicode name.
 258 -a [0 or 1]  : Split start location of each area for heap-based allocation.
 259                1 is set by default, which performs this.
 260 -o [int]     : Set overprovision ratio in percent over volume size.
 261                5 is set by default.
 262 -s [int]     : Set the number of segments per section.
 263                1 is set by default.
 264 -z [int]     : Set the number of sections per zone.
 265                1 is set by default.
 266 -e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
 267 -t [0 or 1]  : Disable discard command or not.
 268                1 is set by default, which conducts discard.
 269
 270 fsck.f2fs
 271 ---------
 272 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
 273 partition, which examines whether the filesystem metadata and user-made data
 274 are cross-referenced correctly or not.
 275 Note that, initial version of the tool does not fix any inconsistency.
 276
 277 The options consist of:
 278   -d debug level [default:0]
 279
 280 dump.f2fs
 281 ---------
 282 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
 283 file. Each file is dump_ssa and dump_sit.
 284
 285 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
 286 It shows on-disk inode information reconized by a given inode number, and is
 287 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
 288 ./dump_sit respectively.
 289
 290 The options consist of:
 291   -d debug level [default:0]
 292   -i inode no (hex)
 293   -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
 294   -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
 295
 296 Examples:
 297 # dump.f2fs -i [ino] /dev/sdx
 298 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
 299 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
 300
 301 ================================================================================
 302 DESIGN
 303 ================================================================================
 304
 305 On-disk Layout
 306 --------------
 307
 308 F2FS divides the whole volume into a number of segments, each of which is fixed
 309 to 2MB in size. A section is composed of consecutive segments, and a zone
 310 consists of a set of sections. By default, section and zone sizes are set to one
 311 segment size identically, but users can easily modify the sizes by mkfs.
 312
 313 F2FS splits the entire volume into six areas, and all the areas except superblock
 314 consists of multiple segments as described below.
 315
 316                                             align with the zone size <-|
 317                  |-> align with the segment size
 318      _________________________________________________________________________
 319     |            |            |   Segment   |    Node     |   Segment  |      |
 320     | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
 321     |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
 322     |____________|_____2______|______N______|______N______|______N_____|__N___|
 323                                                                        .      .
 324                                                              .                .
 325                                                  .                            .
 326                                     ._________________________________________.
 327                                     |_Segment_|_..._|_Segment_|_..._|_Segment_|
 328                                     .           .
 329                                     ._________._________
 330                                     |_section_|__...__|_
 331                                     .            .
 332                                     .________.
 333                                     |__zone__|
 334
 335 - Superblock (SB)
 336  : It is located at the beginning of the partition, and there exist two copies
 337    to avoid file system crash. It contains basic partition information and some
 338    default parameters of f2fs.
 339
 340 - Checkpoint (CP)
 341  : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
 342    inode lists, and summary entries of current active segments.
 343
 344 - Segment Information Table (SIT)
 345  : It contains segment information such as valid block count and bitmap for the
 346    validity of all the blocks.
 347
 348 - Node Address Table (NAT)
 349  : It is composed of a block address table for all the node blocks stored in
 350    Main area.
 351
 352 - Segment Summary Area (SSA)
 353  : It contains summary entries which contains the owner information of all the
 354    data and node blocks stored in Main area.
 355
 356 - Main Area
 357  : It contains file and directory data including their indices.
 358
 359 In order to avoid misalignment between file system and flash-based storage, F2FS
 360 aligns the start block address of CP with the segment size. Also, it aligns the
 361 start block address of Main area with the zone size by reserving some segments
 362 in SSA area.
 363
 364 Reference the following survey for additional technical details.
 365 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
 366
 367 File System Metadata Structure
 368 ------------------------------
 369
 370 F2FS adopts the checkpointing scheme to maintain file system consistency. At
 371 mount time, F2FS first tries to find the last valid checkpoint data by scanning
 372 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
 373 One of them always indicates the last valid data, which is called as shadow copy
 374 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
 375
 376 For file system consistency, each CP points to which NAT and SIT copies are
 377 valid, as shown as below.
 378
 379   +--------+----------+---------+
 380   |   CP   |    SIT   |   NAT   |
 381   +--------+----------+---------+
 382   .         .          .          .
 383   .            .              .              .
 384   .               .                 .                 .
 385   +-------+-------+--------+--------+--------+--------+
 386   | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
 387   +-------+-------+--------+--------+--------+--------+
 388      |             ^                          ^
 389      |             |                          |
 390      `----------------------------------------'
 391
 392 Index Structure
 393 ---------------
 394
 395 The key data structure to manage the data locations is a "node". Similar to
 396 traditional file structures, F2FS has three types of node: inode, direct node,
 397 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
 398 indices, two direct node pointers, two indirect node pointers, and one double
 399 indirect node pointer as described below. One direct node block contains 1018
 400 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
 401 one inode block (i.e., a file) covers:
 402
 403   4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
 404
 405    Inode block (4KB)
 406      |- data (923)
 407      |- direct node (2)
 408      |          `- data (1018)
 409      |- indirect node (2)
 410      |            `- direct node (1018)
 411      |                       `- data (1018)
 412      `- double indirect node (1)
 413                          `- indirect node (1018)
 414                                       `- direct node (1018)
 415                                                  `- data (1018)
 416
 417 Note that, all the node blocks are mapped by NAT which means the location of
 418 each node is translated by the NAT table. In the consideration of the wandering
 419 tree problem, F2FS is able to cut off the propagation of node updates caused by
 420 leaf data writes.
 421
 422 Directory Structure
 423 -------------------
 424
 425 A directory entry occupies 11 bytes, which consists of the following attributes.
 426
 427 - hash          hash value of the file name
 428 - ino           inode number
 429 - len           the length of file name
 430 - type          file type such as directory, symlink, etc
 431
 432 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
 433 used to represent whether each dentry is valid or not. A dentry block occupies
 434 4KB with the following composition.
 435
 436   Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
 437                       dentries(11 * 214 bytes) + file name (8 * 214 bytes)
 438
 439                          [Bucket]
 440              +--------------------------------+
 441              |dentry block 1 | dentry block 2 |
 442              +--------------------------------+
 443              .               .
 444        .                             .
 445   .       [Dentry Block Structure: 4KB]       .
 446   +--------+----------+----------+------------+
 447   | bitmap | reserved | dentries | file names |
 448   +--------+----------+----------+------------+
 449   [Dentry Block: 4KB] .   .
 450                  .               .
 451             .                          .
 452             +------+------+-----+------+
 453             | hash | ino  | len | type |
 454             +------+------+-----+------+
 455             [Dentry Structure: 11 bytes]
 456
 457 F2FS implements multi-level hash tables for directory structure. Each level has
 458 a hash table with dedicated number of hash buckets as shown below. Note that
 459 "A(2B)" means a bucket includes 2 data blocks.
 460
 461 ----------------------
 462 A : bucket
 463 B : block
 464 N : MAX_DIR_HASH_DEPTH
 465 ----------------------
 466
 467 level #0   | A(2B)
 468            |
 469 level #1   | A(2B) - A(2B)
 470            |
 471 level #2   | A(2B) - A(2B) - A(2B) - A(2B)
 472      .     |   .       .       .       .
 473 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
 474      .     |   .       .       .       .
 475 level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
 476
 477 The number of blocks and buckets are determined by,
 478
 479                             ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
 480   # of blocks in level #n = |
 481                             `- 4, Otherwise
 482
 483                              ,- 2^(n + dir_level),
 484                              |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
 485   # of buckets in level #n = |
 486                              `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
 487                                       Otherwise
 488
 489 When F2FS finds a file name in a directory, at first a hash value of the file
 490 name is calculated. Then, F2FS scans the hash table in level #0 to find the
 491 dentry consisting of the file name and its inode number. If not found, F2FS
 492 scans the next hash table in level #1. In this way, F2FS scans hash tables in
 493 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
 494 one bucket determined by the following equation, which shows O(log(# of files))
 495 complexity.
 496
 497   bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
 498
 499 In the case of file creation, F2FS finds empty consecutive slots that cover the
 500 file name. F2FS searches the empty slots in the hash tables of whole levels from
 501 1 to N in the same way as the lookup operation.
 502
 503 The following figure shows an example of two cases holding children.
 504        --------------> Dir <--------------
 505        |                                 |
 506     child                             child
 507
 508     child - child                     [hole] - child
 509
 510     child - child - child             [hole] - [hole] - child
 511
 512    Case 1:                           Case 2:
 513    Number of children = 6,           Number of children = 3,
 514    File size = 7                     File size = 7
 515
 516 Default Block Allocation
 517 ------------------------
 518
 519 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
 520 and Hot/Warm/Cold data.
 521
 522 - Hot node      contains direct node blocks of directories.
 523 - Warm node     contains direct node blocks except hot node blocks.
 524 - Cold node     contains indirect node blocks
 525 - Hot data      contains dentry blocks
 526 - Warm data     contains data blocks except hot and cold data blocks
 527 - Cold data     contains multimedia data or migrated data blocks
 528
 529 LFS has two schemes for free space management: threaded log and copy-and-compac-
 530 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
 531 for devices showing very good sequential write performance, since free segments
 532 are served all the time for writing new data. However, it suffers from cleaning
 533 overhead under high utilization. Contrarily, the threaded log scheme suffers
 534 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
 535 scheme where the copy-and-compaction scheme is adopted by default, but the
 536 policy is dynamically changed to the threaded log scheme according to the file
 537 system status.
 538
 539 In order to align F2FS with underlying flash-based storage, F2FS allocates a
 540 segment in a unit of section. F2FS expects that the section size would be the
 541 same as the unit size of garbage collection in FTL. Furthermore, with respect
 542 to the mapping granularity in FTL, F2FS allocates each section of the active
 543 logs from different zones as much as possible, since FTL can write the data in
 544 the active logs into one allocation unit according to its mapping granularity.
 545
 546 Cleaning process
 547 ----------------
 548
 549 F2FS does cleaning both on demand and in the background. On-demand cleaning is
 550 triggered when there are not enough free segments to serve VFS calls. Background
 551 cleaner is operated by a kernel thread, and triggers the cleaning job when the
 552 system is idle.
 553
 554 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
 555 In the greedy algorithm, F2FS selects a victim segment having the smallest number
 556 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
 557 according to the segment age and the number of valid blocks in order to address
 558 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
 559 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
 560 algorithm.
 561
 562 In order to identify whether the data in the victim segment are valid or not,
 563 F2FS manages a bitmap. Each bit represents the validity of a block, and the
 564 bitmap is composed of a bit stream covering whole blocks in main area.