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Date: Fri, 05 Oct 2012 12:56:23 -0700
From: Randy Dunlap <rdunlap@...otime.net>
To: 김재극 <jaegeuk.kim@...sung.com>
CC: viro@...iv.linux.org.uk, 'Theodore Ts'o' <tytso@....edu>,
gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
chur.lee@...sung.com, cm224.lee@...sung.com,
jooyoung.hwang@...sung.com
Subject: Re: [PATCH 01/16] f2fs: add document
On 10/05/2012 04:56 AM, 김재극 wrote:
> This adds a document describing the mount options, proc entries, usage, and
> design of Flash-Friendly File System, namely F2FS.
>
> Signed-off-by: Jaegeuk Kim <jaegeuk.kim@...sung.com>
> ---
> Documentation/filesystems/00-INDEX | 2 +
> Documentation/filesystems/f2fs.txt | 314 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 316 insertions(+)
> create mode 100644 Documentation/filesystems/f2fs.txt
>
> diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
> index 8c624a1..ce5fd46 100644
> --- a/Documentation/filesystems/00-INDEX
> +++ b/Documentation/filesystems/00-INDEX
> @@ -48,6 +48,8 @@ ext4.txt
> - info, mount options and specifications for the Ext4 filesystem.
> files.txt
> - info on file management in the Linux kernel.
> +f2fs.txt
> + - info and mount options for the F2FS filesystem.
> fuse.txt
> - info on the Filesystem in User SpacE including mount options.
> gfs2.txt
> diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt
> new file mode 100644
> index 0000000..cd3f846
> --- /dev/null
> +++ b/Documentation/filesystems/f2fs.txt
> @@ -0,0 +1,314 @@
> +================================================================================
> +WHAT IS Flash-Friendly File System (F2FS)?
> +================================================================================
> +
> +NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
> +been widely being used for ranging from mobile to server systems. Since they are
been widely used for devices ranging from
or
been widely used for storage ranging from
> +known to have different characteristics from the conventional rotational disks,
> +a file system, an upper layer to the storage device, should adapt to the changes
> +from the sketch.
from the start.
?
> +
> +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 aka FTL,
> +F2FS and its tools support various parameters not only for configuring on-disk
> +layout, but also for selecting allocation and cleaning algorithms.
> +
> +The file system formatting tool, "mkfs.f2fs", is available from the following
> +download page: http://sourceforge.net/projects/f2fs-tools/
> +
> +================================================================================
> +MOUNT OPTIONS
> +================================================================================
> +
> +background_gc_off Turn off the cleaning operation, aka garbage collection,
Some people won't know what "aka" means, so how about:
Turn off the cleaning operation [garbage collection]
> + in background triggered when I/O subsystem is idle.
> +disable_roll_forward Disable the roll-forward recovery routine during SPOR.
what is SPOR?
> +discard Issue discard/TRIM commands when a segment is cleaned.
> +no_heap Disable heap-style segment allocation in which finds free
drop: in
> + segments for data from the beginning of main area, while
> + for node from the end of main area.
> +nouser_xattr Disable Extened User Attributes. Note: xattr is enabled
Extended
> + 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.
> +
> +================================================================================
> +PROC ENTRIES
> +================================================================================
> +
> +/proc/fs/f2fs/ contains information about partitions mounted as f2fs. For each
> +partition, a corresponding directory, named as its device name, is provided with
> +the following proc entries.
> +
> +- f2fs_stat major file system information managed by f2fs currently
> +- f2fs_sit_stat average SIT information about whole segments
what is SIT?
> +- f2fs_mem_stat current memory footprint consumed by f2fs
> +
> +e.g., in /proc/fs/f2fs/sdb1/
> +
> +================================================================================
> +USAGE
> +================================================================================
> +
> +1. Download userland tools
> +
> +2. Insmod f2fs.ko module:
> + # insmod f2fs.ko
> +
> +3. Check the 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
> +
> +================================================================================
> +DESIGN
> +================================================================================
> +
> +On-disk Layout
> +--------------
> +
> +F2FS divides whole volume into a number of segments each of which size is 2MB by
divides the whole volume into a number of segments, each of which is
2MB in size by default.
> +default. A section is composed of consecutive segments, and a zone consists of a
> +set of sections.
> +
> +F2FS maintains logically six log areas. Except SB, all the log areas are managed
> +in a unit of multiple segments. SB is located at the beggining of the partition,
beginning
> +and there exist two superblocks to avoid file system crash. Other file system
> +metadata such as CP, NAT, SIT, and SSA are located in front part of the volume.
in the front part
> +Main area contains file and directory data including their indices.
> +
> +Each area manages the following contents.
> +- CP File system information, bitmaps for valid NAT/SIT sets, orphan
> + inode lists, and summary entries of current active segments.
> +- NAT Block address table for all the node blocks stored in Main area.
> +- SIT Segment information such as valid block count and bitmap for the
> + validity of all the blocks.
> +- SSA Summary entries which contains the owner information of all the
> + data and node blocks stored in Main area.
> +- Main Node and data blocks.
> +
> +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.
> +
> + align with the zone size <-|
> + |-> align with the segment size
> + _________________________________________________________________________
> + | | | Node | Segment | Segment | |
> + | Superblock | Checkpoint | Address | Info. | Summary | Main |
> + | (SB) | (CP) | Table (NAT) | Table (SIT) | Area (SSA) | |
> + |____________|_____2______|______N______|______N______|______N_____|__N___|
> + . .
> + . .
> + . .
> + ._________________________________________.
> + |_Segment_|_..._|_Segment_|_..._|_Segment_|
> + . .
> + ._________._________
> + |_section_|__...__|_
> + . .
> + .________.
> + |__zone__|
> +
> +
> +File System Metadata Structure
> +------------------------------
> +
> +F2FS adopts the checkpointing scheme to maintain file system consistency. At the
drop: the
> +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 adopts the shadow copy mechanism.
adopt
> +
> +For file system consistency, each CP points which NAT and SIT copies are valid,
points to which
> +as shown as below.
> +
> + +--------+----------+---------+
> + | CP | NAT | SIT |
> + +--------+----------+---------+
> + . . . .
> + . . . .
> + . . . .
> + +-------+-------+--------+--------+--------+--------+
> + | CP #0 | CP #1 | NAT #0 | NAT #1 | SIT #0 | SIT #1 |
> + +-------+-------+--------+--------+--------+--------+
> + | ^ ^
> + | | |
> + `----------------------------------------'
> +
> +Index Structure
> +---------------
> +
> +The key data structure to manage the data locations is a "node". As similar as
Similar to
> +traditional file structures, F2FS has three types of node: inode, direct node,
> +indirect node. F2FS assigns 4KB to an inode block where contains 929 data block
which
> +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:
one
> + 4KB * (929 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
> +
> + Inode block (4KB)
> + |- data (929)
> + |- direct node (2)
> + | `- data (1018)
> + |- indirect node (2)
> + | `- direct node (1018)
> + | `- data (1018)
> + `- triple 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
drop: ,
> +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. There-in bitmap is
maybe: 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,
drop: ,
> +"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, if n < 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
level level,
> +one bucket determined by the follow equation, which shows O(log(# of files))
following
> +complexity.
> +
> + bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
> +
> +In the case of file creation, F2FS finds an empty consecutive slots that covers
drop: an 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
> +------------------------
> +
> +In runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
At runtime,
> +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, aka cleaning, is well-suited for devices
"aka" usage is not nice IMO.
> +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 storages, F2FS allocates a
storage,
> +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 sections of the active
section
> +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 greedy algorithm, F2FS selects a victim segment having the smallest number of
In the greedy
> +valid blocks. In cost-benefit algorithm, F2FS selects a victim segment according
In the cost-benefit
> +to the segment age and the number of valid blocks in order to address log block
> +thrashing problem in greedy algorithm. F2FS adopts greedy algorithm for on-demand
in the greedy adopts the greedy
> +cleaner, while background cleaner adopts cost-benefit algorithm.
adopts the
> +
> +In order to identify what the data in the victim segment are valid or not, F2FS
if the data
or
whether the data
> +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.
--
~Randy
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