f2fs.txt 27 KB
Newer Older
Jaegeuk Kim's avatar
Jaegeuk Kim committed
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
================================================================================
WHAT IS Flash-Friendly File System (F2FS)?
================================================================================

NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
been equipped on a variety systems ranging from mobile to server systems. Since
they are known to have different characteristics from the conventional rotating
disks, a file system, an upper layer to the storage device, should adapt to the
changes from the sketch in the design level.

F2FS is a file system exploiting NAND flash memory-based storage devices, which
is based on Log-structured File System (LFS). The design has been focused on
addressing the fundamental issues in LFS, which are snowball effect of wandering
tree and high cleaning overhead.

Since a NAND flash memory-based storage device shows different characteristic
according to its internal geometry or flash memory management scheme, namely FTL,
F2FS and its tools support various parameters not only for configuring on-disk
layout, but also for selecting allocation and cleaning algorithms.

21 22
The following git tree provides the file system formatting tool (mkfs.f2fs),
a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
Jaegeuk Kim's avatar
Jaegeuk Kim committed
23 24 25 26
>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git

For reporting bugs and sending patches, please use the following mailing list:
>> linux-f2fs-devel@lists.sourceforge.net
Jaegeuk Kim's avatar
Jaegeuk Kim committed
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

================================================================================
BACKGROUND AND DESIGN ISSUES
================================================================================

Log-structured File System (LFS)
--------------------------------
"A log-structured file system writes all modifications to disk sequentially in
a log-like structure, thereby speeding up  both file writing and crash recovery.
The log is the only structure on disk; it contains indexing information so that
files can be read back from the log efficiently. In order to maintain large free
areas on disk for fast writing, we divide  the log into segments and use a
segment cleaner to compress the live information from heavily fragmented
segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
implementation of a log-structured file system", ACM Trans. Computer Systems
10, 1, 26–52.

Wandering Tree Problem
----------------------
In LFS, when a file data is updated and written to the end of log, its direct
pointer block is updated due to the changed location. Then the indirect pointer
block is also updated due to the direct pointer block update. In this manner,
the upper index structures such as inode, inode map, and checkpoint block are
also updated recursively. This problem is called as wandering tree problem [1],
and in order to enhance the performance, it should eliminate or relax the update
propagation as much as possible.

[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/

Cleaning Overhead
-----------------
Since LFS is based on out-of-place writes, it produces so many obsolete blocks
scattered across the whole storage. In order to serve new empty log space, it
needs to reclaim these obsolete blocks seamlessly to users. This job is called
as a cleaning process.

The process consists of three operations as follows.
1. A victim segment is selected through referencing segment usage table.
2. It loads parent index structures of all the data in the victim identified by
   segment summary blocks.
3. It checks the cross-reference between the data and its parent index structure.
4. It moves valid data selectively.

This cleaning job may cause unexpected long delays, so the most important goal
is to hide the latencies to users. And also definitely, it should reduce the
amount of valid data to be moved, and move them quickly as well.

================================================================================
KEY FEATURES
================================================================================

Flash Awareness
---------------
- Enlarge the random write area for better performance, but provide the high
  spatial locality
- Align FS data structures to the operational units in FTL as best efforts

Wandering Tree Problem
----------------------
- Use a term, “node”, that represents inodes as well as various pointer blocks
- Introduce Node Address Table (NAT) containing the locations of all the “node”
  blocks; this will cut off the update propagation.

Cleaning Overhead
-----------------
- Support a background cleaning process
- Support greedy and cost-benefit algorithms for victim selection policies
- Support multi-head logs for static/dynamic hot and cold data separation
- Introduce adaptive logging for efficient block allocation

================================================================================
MOUNT OPTIONS
================================================================================

101 102 103 104
background_gc=%s       Turn on/off cleaning operations, namely garbage
                       collection, triggered in background when I/O subsystem is
                       idle. If background_gc=on, it will turn on the garbage
                       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.
107 108
                       Default value for this option is on. So garbage
                       collection is on by default.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
109
disable_roll_forward   Disable the roll-forward recovery routine
110 111
norecovery             Disable the roll-forward recovery routine, mounted read-
                       only (i.e., -o ro,disable_roll_forward)
Jaegeuk Kim's avatar
Jaegeuk Kim committed
112 113 114 115 116 117 118 119 120 121 122 123 124
discard                Issue discard/TRIM commands when a segment is cleaned.
no_heap                Disable heap-style segment allocation which finds free
                       segments for data from the beginning of main area, while
		       for node from the end of main area.
nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
                       by default if CONFIG_F2FS_FS_XATTR is selected.
noacl                  Disable POSIX Access Control List. Note: acl is enabled
                       by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
active_logs=%u         Support configuring the number of active logs. In the
                       current design, f2fs supports only 2, 4, and 6 logs.
                       Default number is 6.
disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
                       does not aware of cold files such as media files.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
125
inline_xattr           Enable the inline xattrs feature.
Huajun Li's avatar
Huajun Li committed
126 127
inline_data            Enable the inline data feature: New created small(<~3.4k)
                       files can be written into inode block.
128 129 130 131
inline_dentry          Enable the inline dir feature: data in new created
                       directory entries can be written into inode block. The
                       space of inode block which is used to store inline
                       dentries is limited to ~3.4k.
132 133 134 135
flush_merge	       Merge concurrent cache_flush commands as much as possible
                       to eliminate redundant command issues. If the underlying
		       device handles the cache_flush command relatively slowly,
		       recommend to enable this option.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
136 137 138 139 140
nobarrier              This option can be used if underlying storage guarantees
                       its cached data should be written to the novolatile area.
		       If this option is set, no cache_flush commands are issued
		       but f2fs still guarantees the write ordering of all the
		       data writes.
141 142 143
fastboot               This option is used when a system wants to reduce mount
                       time as much as possible, even though normal performance
		       can be sacrificed.
144 145 146
extent_cache           Enable an extent cache based on rb-tree, it can cache
                       as many as extent which map between contiguous logical
                       address and physical address per inode, resulting in
147
                       increasing the cache hit ratio. Set by default.
148
noextent_cache         Disable an extent cache based on rb-tree explicitly, see
149
                       the above extent_cache mount option.
150 151
noinline_data          Disable the inline data feature, inline data feature is
                       enabled by default.
152 153
data_flush             Enable data flushing before checkpoint in order to
                       persist data of regular and symlink.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
154 155 156 157 158 159 160 161 162 163 164 165 166

================================================================================
DEBUGFS ENTRIES
================================================================================

/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
f2fs. Each file shows the whole f2fs information.

/sys/kernel/debug/f2fs/status includes:
 - major file system information managed by f2fs currently
 - average SIT information about whole segments
 - current memory footprint consumed by f2fs.

167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192
================================================================================
SYSFS ENTRIES
================================================================================

Information about mounted f2f2 file systems can be found in
/sys/fs/f2fs.  Each mounted filesystem will have a directory in
/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
The files in each per-device directory are shown in table below.

Files in /sys/fs/f2fs/<devname>
(see also Documentation/ABI/testing/sysfs-fs-f2fs)
..............................................................................
 File                         Content

 gc_max_sleep_time            This tuning parameter controls the maximum sleep
                              time for the garbage collection thread. Time is
                              in milliseconds.

 gc_min_sleep_time            This tuning parameter controls the minimum sleep
                              time for the garbage collection thread. Time is
                              in milliseconds.

 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
                              time for the garbage collection thread. Time is
                              in milliseconds.

193 194 195 196
 gc_idle                      This parameter controls the selection of victim
                              policy for garbage collection. Setting gc_idle = 0
                              (default) will disable this option. Setting
                              gc_idle = 1 will select the Cost Benefit approach
197
                              & setting gc_idle = 2 will select the greedy approach.
198

199 200
 reclaim_segments             This parameter controls the number of prefree
                              segments to be reclaimed. If the number of prefree
201 202 203 204 205
			      segments is larger than the number of segments
			      in the proportion to the percentage over total
			      volume size, f2fs tries to conduct checkpoint to
			      reclaim the prefree segments to free segments.
			      By default, 5% over total # of segments.
206

207 208 209 210 211 212
 max_small_discards	      This parameter controls the number of discard
			      commands that consist small blocks less than 2MB.
			      The candidates to be discarded are cached until
			      checkpoint is triggered, and issued during the
			      checkpoint. By default, it is disabled with 0.

Jaegeuk Kim's avatar
Jaegeuk Kim committed
213 214 215 216
 trim_sections                This parameter controls the number of sections
                              to be trimmed out in batch mode when FITRIM
                              conducts. 32 sections is set by default.

217 218
 ipu_policy                   This parameter controls the policy of in-place
                              updates in f2fs. There are five policies:
219 220 221
                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
                               0x10: F2FS_IPU_FSYNC.
222 223 224 225 226 227

 min_ipu_util                 This parameter controls the threshold to trigger
                              in-place-updates. The number indicates percentage
                              of the filesystem utilization, and used by
                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.

228 229 230 231 232 233 234
 min_fsync_blocks             This parameter controls the threshold to trigger
                              in-place-updates when F2FS_IPU_FSYNC mode is set.
			      The number indicates the number of dirty pages
			      when fsync needs to flush on its call path. If
			      the number is less than this value, it triggers
			      in-place-updates.

235 236 237 238 239
 max_victim_search	      This parameter controls the number of trials to
			      find a victim segment when conducting SSR and
			      cleaning operations. The default value is 4096
			      which covers 8GB block address range.

240 241 242 243 244 245 246
 dir_level                    This parameter controls the directory level to
			      support large directory. If a directory has a
			      number of files, it can reduce the file lookup
			      latency by increasing this dir_level value.
			      Otherwise, it needs to decrease this value to
			      reduce the space overhead. The default value is 0.

247 248 249 250
 ram_thresh                   This parameter controls the memory footprint used
			      by free nids and cached nat entries. By default,
			      10 is set, which indicates 10 MB / 1 GB RAM.

Jaegeuk Kim's avatar
Jaegeuk Kim committed
251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267
================================================================================
USAGE
================================================================================

1. Download userland tools and compile them.

2. Skip, if f2fs was compiled statically inside kernel.
   Otherwise, insert the f2fs.ko module.
 # insmod f2fs.ko

3. Create a directory trying to mount
 # mkdir /mnt/f2fs

4. Format the block device, and then mount as f2fs
 # mkfs.f2fs -l label /dev/block_device
 # mount -t f2fs /dev/block_device /mnt/f2fs

268 269 270 271 272 273
mkfs.f2fs
---------
The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
which builds a basic on-disk layout.

The options consist of:
274
-l [label]   : Give a volume label, up to 512 unicode name.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
275 276 277 278 279 280 281 282 283
-a [0 or 1]  : Split start location of each area for heap-based allocation.
               1 is set by default, which performs this.
-o [int]     : Set overprovision ratio in percent over volume size.
               5 is set by default.
-s [int]     : Set the number of segments per section.
               1 is set by default.
-z [int]     : Set the number of sections per zone.
               1 is set by default.
-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
284 285
-t [0 or 1]  : Disable discard command or not.
               1 is set by default, which conducts discard.
Jaegeuk Kim's avatar
Jaegeuk Kim committed
286

287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302
fsck.f2fs
---------
The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
partition, which examines whether the filesystem metadata and user-made data
are cross-referenced correctly or not.
Note that, initial version of the tool does not fix any inconsistency.

The options consist of:
  -d debug level [default:0]

dump.f2fs
---------
The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
file. Each file is dump_ssa and dump_sit.

The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
303
It shows on-disk inode information recognized by a given inode number, and is
304 305 306 307 308 309 310 311 312 313 314 315 316 317
able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
./dump_sit respectively.

The options consist of:
  -d debug level [default:0]
  -i inode no (hex)
  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]

Examples:
# dump.f2fs -i [ino] /dev/sdx
# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)

Jaegeuk Kim's avatar
Jaegeuk Kim committed
318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335
================================================================================
DESIGN
================================================================================

On-disk Layout
--------------

F2FS divides the whole volume into a number of segments, each of which is fixed
to 2MB in size. A section is composed of consecutive segments, and a zone
consists of a set of sections. By default, section and zone sizes are set to one
segment size identically, but users can easily modify the sizes by mkfs.

F2FS splits the entire volume into six areas, and all the areas except superblock
consists of multiple segments as described below.

                                            align with the zone size <-|
                 |-> align with the segment size
     _________________________________________________________________________
336 337 338
    |            |            |   Segment   |    Node     |   Segment  |      |
    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
Jaegeuk Kim's avatar
Jaegeuk Kim committed
339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364
    |____________|_____2______|______N______|______N______|______N_____|__N___|
                                                                       .      .
                                                             .                .
                                                 .                            .
                                    ._________________________________________.
                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
                                    .           .
                                    ._________._________
                                    |_section_|__...__|_
                                    .            .
		                    .________.
	                            |__zone__|

- Superblock (SB)
 : It is located at the beginning of the partition, and there exist two copies
   to avoid file system crash. It contains basic partition information and some
   default parameters of f2fs.

- Checkpoint (CP)
 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
   inode lists, and summary entries of current active segments.

- Segment Information Table (SIT)
 : It contains segment information such as valid block count and bitmap for the
   validity of all the blocks.

365 366 367 368
- Node Address Table (NAT)
 : It is composed of a block address table for all the node blocks stored in
   Main area.

Jaegeuk Kim's avatar
Jaegeuk Kim committed
369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396
- Segment Summary Area (SSA)
 : It contains summary entries which contains the owner information of all the
   data and node blocks stored in Main area.

- Main Area
 : It contains file and directory data including their indices.

In order to avoid misalignment between file system and flash-based storage, F2FS
aligns the start block address of CP with the segment size. Also, it aligns the
start block address of Main area with the zone size by reserving some segments
in SSA area.

Reference the following survey for additional technical details.
https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey

File System Metadata Structure
------------------------------

F2FS adopts the checkpointing scheme to maintain file system consistency. At
mount time, F2FS first tries to find the last valid checkpoint data by scanning
CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
One of them always indicates the last valid data, which is called as shadow copy
mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.

For file system consistency, each CP points to which NAT and SIT copies are
valid, as shown as below.

  +--------+----------+---------+
397
  |   CP   |    SIT   |   NAT   |
Jaegeuk Kim's avatar
Jaegeuk Kim committed
398 399 400 401 402
  +--------+----------+---------+
  .         .          .          .
  .            .              .              .
  .               .                 .                 .
  +-------+-------+--------+--------+--------+--------+
403
  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
Jaegeuk Kim's avatar
Jaegeuk Kim committed
404 405 406 407 408 409 410 411 412 413
  +-------+-------+--------+--------+--------+--------+
     |             ^                          ^
     |             |                          |
     `----------------------------------------'

Index Structure
---------------

The key data structure to manage the data locations is a "node". Similar to
traditional file structures, F2FS has three types of node: inode, direct node,
414
indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
Jaegeuk Kim's avatar
Jaegeuk Kim committed
415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499
indices, two direct node pointers, two indirect node pointers, and one double
indirect node pointer as described below. One direct node block contains 1018
data blocks, and one indirect node block contains also 1018 node blocks. Thus,
one inode block (i.e., a file) covers:

  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.

   Inode block (4KB)
     |- data (923)
     |- direct node (2)
     |          `- data (1018)
     |- indirect node (2)
     |            `- direct node (1018)
     |                       `- data (1018)
     `- double indirect node (1)
                         `- indirect node (1018)
			              `- direct node (1018)
	                                         `- data (1018)

Note that, all the node blocks are mapped by NAT which means the location of
each node is translated by the NAT table. In the consideration of the wandering
tree problem, F2FS is able to cut off the propagation of node updates caused by
leaf data writes.

Directory Structure
-------------------

A directory entry occupies 11 bytes, which consists of the following attributes.

- hash		hash value of the file name
- ino		inode number
- len		the length of file name
- type		file type such as directory, symlink, etc

A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
used to represent whether each dentry is valid or not. A dentry block occupies
4KB with the following composition.

  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
	              dentries(11 * 214 bytes) + file name (8 * 214 bytes)

                         [Bucket]
             +--------------------------------+
             |dentry block 1 | dentry block 2 |
             +--------------------------------+
             .               .
       .                             .
  .       [Dentry Block Structure: 4KB]       .
  +--------+----------+----------+------------+
  | bitmap | reserved | dentries | file names |
  +--------+----------+----------+------------+
  [Dentry Block: 4KB] .   .
		 .               .
            .                          .
            +------+------+-----+------+
            | hash | ino  | len | type |
            +------+------+-----+------+
            [Dentry Structure: 11 bytes]

F2FS implements multi-level hash tables for directory structure. Each level has
a hash table with dedicated number of hash buckets as shown below. Note that
"A(2B)" means a bucket includes 2 data blocks.

----------------------
A : bucket
B : block
N : MAX_DIR_HASH_DEPTH
----------------------

level #0   | A(2B)
           |
level #1   | A(2B) - A(2B)
           |
level #2   | A(2B) - A(2B) - A(2B) - A(2B)
     .     |   .       .       .       .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
     .     |   .       .       .       .
level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)

The number of blocks and buckets are determined by,

                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
  # of blocks in level #n = |
                            `- 4, Otherwise

500 501
                             ,- 2^(n + dir_level),
			     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
Jaegeuk Kim's avatar
Jaegeuk Kim committed
502
  # of buckets in level #n = |
503 504
                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
			              Otherwise
Jaegeuk Kim's avatar
Jaegeuk Kim committed
505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581

When F2FS finds a file name in a directory, at first a hash value of the file
name is calculated. Then, F2FS scans the hash table in level #0 to find the
dentry consisting of the file name and its inode number. If not found, F2FS
scans the next hash table in level #1. In this way, F2FS scans hash tables in
each levels incrementally from 1 to N. In each levels F2FS needs to scan only
one bucket determined by the following equation, which shows O(log(# of files))
complexity.

  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)

In the case of file creation, F2FS finds empty consecutive slots that cover the
file name. F2FS searches the empty slots in the hash tables of whole levels from
1 to N in the same way as the lookup operation.

The following figure shows an example of two cases holding children.
       --------------> Dir <--------------
       |                                 |
    child                             child

    child - child                     [hole] - child

    child - child - child             [hole] - [hole] - child

   Case 1:                           Case 2:
   Number of children = 6,           Number of children = 3,
   File size = 7                     File size = 7

Default Block Allocation
------------------------

At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
and Hot/Warm/Cold data.

- Hot node	contains direct node blocks of directories.
- Warm node	contains direct node blocks except hot node blocks.
- Cold node	contains indirect node blocks
- Hot data	contains dentry blocks
- Warm data	contains data blocks except hot and cold data blocks
- Cold data	contains multimedia data or migrated data blocks

LFS has two schemes for free space management: threaded log and copy-and-compac-
tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
for devices showing very good sequential write performance, since free segments
are served all the time for writing new data. However, it suffers from cleaning
overhead under high utilization. Contrarily, the threaded log scheme suffers
from random writes, but no cleaning process is needed. F2FS adopts a hybrid
scheme where the copy-and-compaction scheme is adopted by default, but the
policy is dynamically changed to the threaded log scheme according to the file
system status.

In order to align F2FS with underlying flash-based storage, F2FS allocates a
segment in a unit of section. F2FS expects that the section size would be the
same as the unit size of garbage collection in FTL. Furthermore, with respect
to the mapping granularity in FTL, F2FS allocates each section of the active
logs from different zones as much as possible, since FTL can write the data in
the active logs into one allocation unit according to its mapping granularity.

Cleaning process
----------------

F2FS does cleaning both on demand and in the background. On-demand cleaning is
triggered when there are not enough free segments to serve VFS calls. Background
cleaner is operated by a kernel thread, and triggers the cleaning job when the
system is idle.

F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
In the greedy algorithm, F2FS selects a victim segment having the smallest number
of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
according to the segment age and the number of valid blocks in order to address
log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
algorithm.

In order to identify whether the data in the victim segment are valid or not,
F2FS manages a bitmap. Each bit represents the validity of a block, and the
bitmap is composed of a bit stream covering whole blocks in main area.