735 lines
35 KiB
Plaintext
735 lines
35 KiB
Plaintext
================================================================================
|
||
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.
|
||
|
||
Write-hint Policy
|
||
-----------------
|
||
|
||
1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
|
||
|
||
2) whint_mode=user-based. F2FS tries to pass down hints given by
|
||
users.
|
||
|
||
User F2FS Block
|
||
---- ---- -----
|
||
META WRITE_LIFE_NOT_SET
|
||
HOT_NODE "
|
||
WARM_NODE "
|
||
COLD_NODE "
|
||
*ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
|
||
*extension list " "
|
||
|
||
-- buffered io
|
||
WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
|
||
WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
|
||
WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
|
||
WRITE_LIFE_NONE " "
|
||
WRITE_LIFE_MEDIUM " "
|
||
WRITE_LIFE_LONG " "
|
||
|
||
-- direct io
|
||
WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
|
||
WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
|
||
WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
|
||
WRITE_LIFE_NONE " WRITE_LIFE_NONE
|
||
WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
|
||
WRITE_LIFE_LONG " WRITE_LIFE_LONG
|
||
|
||
3) whint_mode=fs-based. F2FS passes down hints with its policy.
|
||
|
||
User F2FS Block
|
||
---- ---- -----
|
||
META WRITE_LIFE_MEDIUM;
|
||
HOT_NODE WRITE_LIFE_NOT_SET
|
||
WARM_NODE "
|
||
COLD_NODE WRITE_LIFE_NONE
|
||
ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
|
||
extension list " "
|
||
|
||
-- buffered io
|
||
WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
|
||
WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
|
||
WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
|
||
WRITE_LIFE_NONE " "
|
||
WRITE_LIFE_MEDIUM " "
|
||
WRITE_LIFE_LONG " "
|
||
|
||
-- direct io
|
||
WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
|
||
WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
|
||
WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
|
||
WRITE_LIFE_NONE " WRITE_LIFE_NONE
|
||
WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
|
||
WRITE_LIFE_LONG " WRITE_LIFE_LONG
|
||
|
||
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 |
|
||
+-------------+-------------+----------+----------------------------+
|