Date:	Sun, 07 Oct 2012 03:27:03 +0900
From:	Jaegeuk Kim <jaegeuk.kim@...il.com>
To:	Randy Dunlap <rdunlap@...otime.net>
Cc:	±èÀç±Ø <jaegeuk.kim@...sung.com>,
	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

I'll apply this.
Thanks,

2012-10-05 (금), 12:56 -0700, Randy Dunlap:
> 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.
> 
> 
> 
> 
> 

-- 
Jaegeuk Kim
Samsung

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