================================================================================
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.

The following git tree provides the file system formatting tool (mkfs.f2fs),
a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
>> 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

================================================================================
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
================================================================================

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
                       will be turned off. If background_gc=sync, it will turn
                       on synchronous garbage collection running in background.
                       Default value for this option is on. So garbage
                       collection is on by default.
disable_roll_forward   Disable the roll-forward recovery routine
norecovery             Disable the roll-forward recovery routine, mounted read-
                       only (i.e., -o ro,disable_roll_forward)
discard/nodiscard      Enable/disable real-time discard in f2fs, if discard is
                       enabled, f2fs will 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.
inline_xattr           Enable the inline xattrs feature.
noinline_xattr         Disable the inline xattrs feature.
inline_xattr_size=%u   Support configuring inline xattr size, it depends on
		       flexible inline xattr feature.
inline_data            Enable the inline data feature: New created small(<~3.4k)
                       files can be written into inode block.
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.
noinline_dentry        Disable the inline dentry feature.
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.
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.
fastboot               This option is used when a system wants to reduce mount
                       time as much as possible, even though normal performance
		       can be sacrificed.
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
                       increasing the cache hit ratio. Set by default.
noextent_cache         Disable an extent cache based on rb-tree explicitly, see
                       the above extent_cache mount option.
noinline_data          Disable the inline data feature, inline data feature is
                       enabled by default.
data_flush             Enable data flushing before checkpoint in order to
                       persist data of regular and symlink.
reserve_root=%d        Support configuring reserved space which is used for
                       allocation from a privileged user with specified uid or
                       gid, unit: 4KB, the default limit is 0.2% of user blocks.
resuid=%d              The user ID which may use the reserved blocks.
resgid=%d              The group ID which may use the reserved blocks.
fault_injection=%d     Enable fault injection in all supported types with
                       specified injection rate.
fault_type=%d          Support configuring fault injection type, should be
                       enabled with fault_injection option, fault type value
                       is shown below, it supports single or combined type.
                       Type_Name		Type_Value
                       FAULT_KMALLOC		0x000000001
                       FAULT_KVMALLOC		0x000000002
                       FAULT_PAGE_ALLOC		0x000000004
                       FAULT_PAGE_GET		0x000000008
                       FAULT_ALLOC_BIO		0x000000010
                       FAULT_ALLOC_NID		0x000000020
                       FAULT_ORPHAN		0x000000040
                       FAULT_BLOCK		0x000000080
                       FAULT_DIR_DEPTH		0x000000100
                       FAULT_EVICT_INODE	0x000000200
                       FAULT_TRUNCATE		0x000000400
                       FAULT_READ_IO		0x000000800
                       FAULT_CHECKPOINT		0x000001000
                       FAULT_DISCARD		0x000002000
                       FAULT_WRITE_IO		0x000004000
mode=%s                Control block allocation mode which supports "adaptive"
                       and "lfs". In "lfs" mode, there should be no random
                       writes towards main area.
io_bits=%u             Set the bit size of write IO requests. It should be set
                       with "mode=lfs".
usrquota               Enable plain user disk quota accounting.
grpquota               Enable plain group disk quota accounting.
prjquota               Enable plain project quota accounting.
usrjquota=<file>       Appoint specified file and type during mount, so that quota
grpjquota=<file>       information can be properly updated during recovery flow,
prjjquota=<file>       <quota file>: must be in root directory;
jqfmt=<quota type>     <quota type>: [vfsold,vfsv0,vfsv1].
offusrjquota           Turn off user journelled quota.
offgrpjquota           Turn off group journelled quota.
offprjjquota           Turn off project journelled quota.
quota                  Enable plain user disk quota accounting.
noquota                Disable all plain disk quota option.
whint_mode=%s          Control which write hints are passed down to block
                       layer. This supports "off", "user-based", and
                       "fs-based".  In "off" mode (default), f2fs does not pass
                       down hints. In "user-based" mode, f2fs tries to pass
                       down hints given by users. And in "fs-based" mode, f2fs
                       passes down hints with its policy.
alloc_mode=%s          Adjust block allocation policy, which supports "reuse"
                       and "default".
fsync_mode=%s          Control the policy of fsync. Currently supports "posix",
                       "strict", and "nobarrier". In "posix" mode, which is
                       default, fsync will follow POSIX semantics and does a
                       light operation to improve the filesystem performance.
                       In "strict" mode, fsync will be heavy and behaves in line
                       with xfs, ext4 and btrfs, where xfstest generic/342 will
                       pass, but the performance will regress. "nobarrier" is
                       based on "posix", but doesn't issue flush command for
                       non-atomic files likewise "nobarrier" mount option.
test_dummy_encryption
test_dummy_encryption=%s
                       Enable dummy encryption, which provides a fake fscrypt
                       context. The fake fscrypt context is used by xfstests.
                       The argument may be either "v1" or "v2", in order to
                       select the corresponding fscrypt policy version.
checkpoint=%s[:%u[%]]     Set to "disable" to turn off checkpointing. Set to "enable"
                       to reenable checkpointing. Is enabled by default. While
                       disabled, any unmounting or unexpected shutdowns will cause
                       the filesystem contents to appear as they did when the
                       filesystem was mounted with that option.
                       While mounting with checkpoint=disabled, the filesystem must
                       run garbage collection to ensure that all available space can
                       be used. If this takes too much time, the mount may return
                       EAGAIN. You may optionally add a value to indicate how much
                       of the disk you would be willing to temporarily give up to
                       avoid additional garbage collection. This can be given as a
                       number of blocks, or as a percent. For instance, mounting
                       with checkpoint=disable:100% would always succeed, but it may
                       hide up to all remaining free space. The actual space that
                       would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
                       This space is reclaimed once checkpoint=enable.
compress_algorithm=%s  Control compress algorithm, currently f2fs supports "lzo",
                       "lz4" and "zstd" algorithm.
compress_log_size=%u   Support configuring compress cluster size, the size will
                       be 4KB * (1 << %u), 16KB is minimum size, also it's
                       default size.
compress_extension=%s  Support adding specified extension, so that f2fs can enable
                       compression on those corresponding files, e.g. if all files
                       with '.ext' has high compression rate, we can set the '.ext'
                       on compression extension list and enable compression on
                       these file by default rather than to enable it via ioctl.
                       For other files, we can still enable compression via ioctl.

================================================================================
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.

================================================================================
SYSFS ENTRIES
================================================================================

Information about mounted f2fs 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)

================================================================================
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

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:
-l [label]   : Give a volume label, up to 512 unicode name.
-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"
-t [0 or 1]  : Disable discard command or not.
               1 is set by default, which conducts discard.

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.
It shows on-disk inode information recognized by a given inode number, and is
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)

================================================================================
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
     _________________________________________________________________________
    |            |            |   Segment   |    Node     |   Segment  |      |
    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
    |____________|_____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.

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

- 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.

  +--------+----------+---------+
  |   CP   |    SIT   |   NAT   |
  +--------+----------+---------+
  .         .          .          .
  .            .              .              .
  .               .                 .                 .
  +-------+-------+--------+--------+--------+--------+
  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
  +-------+-------+--------+--------+--------+--------+
     |             ^                          ^
     |             |                          |
     `----------------------------------------'

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,
indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
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

                             ,- 2^(n + dir_level),
			     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
  # of buckets in level #n = |
                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
			              Otherwise

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.

Fallocate(2) Policy
-------------------

The default policy follows the below posix rule.

Allocating disk space
    The default operation (i.e., mode is zero) of fallocate() allocates
    the disk space within the range specified by offset and len.  The
    file size (as reported by stat(2)) will be changed if offset+len is
    greater than the file size.  Any subregion within the range specified
    by offset and len that did not contain data before the call will be
    initialized to zero.  This default behavior closely resembles the
    behavior of the posix_fallocate(3) library function, and is intended
    as a method of optimally implementing that function.

However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having
zero or random data, which is useful to the below scenario where:
 1. create(fd)
 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
 3. fallocate(fd, 0, 0, size)
 4. address = fibmap(fd, offset)
 5. open(blkdev)
 6. write(blkdev, address)

Compression implementation
--------------------------

- New term named cluster is defined as basic unit of compression, file can
be divided into multiple clusters logically. One cluster includes 4 << n
(n >= 0) logical pages, compression size is also cluster size, each of
cluster can be compressed or not.

- In cluster metadata layout, one special block address is used to indicate
cluster is compressed one or normal one, for compressed cluster, following
metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
stores data including compress header and compressed data.

- In order to eliminate write amplification during overwrite, F2FS only
support compression on write-once file, data can be compressed only when
all logical blocks in file are valid and cluster compress ratio is lower
than specified threshold.

- To enable compression on regular inode, there are three ways:
* chattr +c file
* chattr +c dir; touch dir/file
* mount w/ -o compress_extension=ext; touch file.ext

Compress metadata layout:
                             [Dnode Structure]
             +-----------------------------------------------+
             | cluster 1 | cluster 2 | ......... | cluster N |
             +-----------------------------------------------+
             .           .                       .           .
       .                       .                .                      .
  .         Compressed Cluster       .        .        Normal Cluster            .
+----------+---------+---------+---------+  +---------+---------+---------+---------+
|compr flag| block 1 | block 2 | block 3 |  | block 1 | block 2 | block 3 | block 4 |
+----------+---------+---------+---------+  +---------+---------+---------+---------+
           .                             .
         .                                           .
       .                                                           .
      +-------------+-------------+----------+----------------------------+
      | data length | data chksum | reserved |      compressed data       |
      +-------------+-------------+----------+----------------------------+