582 строки
27 KiB
Plaintext
582 строки
27 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 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.
|
|
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.
|
|
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.
|
|
|
|
================================================================================
|
|
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 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.
|
|
|
|
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
|
|
& setting gc_idle = 2 will select the greedy approach.
|
|
|
|
reclaim_segments This parameter controls the number of prefree
|
|
segments to be reclaimed. If the number of prefree
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
ipu_policy This parameter controls the policy of in-place
|
|
updates in f2fs. There are five policies:
|
|
0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
|
|
0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
|
|
0x10: F2FS_IPU_FSYNC.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
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.
|
|
|
|
================================================================================
|
|
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.
|