Date:	Fri, 26 Oct 2012 07:14:35 +0900
From:	Jaegeuk Kim <jaegeuk.kim@...sung.com>
To:	'Vyacheslav Dubeyko' <slava@...eyko.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org, viro@...iv.linux.org.uk, arnd@...db.de,
	tytso@....edu, chur.lee@...sung.com, cm224.lee@...sung.com,
	jooyoung.hwang@...sung.com
Subject: RE: [PATCH 01/16 v2] f2fs: add document

I'll enhance the document as much as possible according to your
recommendation.
Thank you for intensive review. :)

---
Jaegeuk Kim
Samsung

> On Tue, 2012-10-23 at 11:25 +0900, Jaegeuk Kim 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 |  404 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 406 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..f2b4fde
> > --- /dev/null
> > +++ b/Documentation/filesystems/f2fs.txt
> > @@ -0,0 +1,404 @@
> > +================================================================================
> > +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 storage ranging from mobile to server systems. Since
> 
> Maybe, it needs to reformulate "... have been widely being used ..."?
> 
> > +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 file system formatting tool, "mkfs.f2fs", is available from the following
> > +download page:		http://sourceforge.net/projects/f2fs-tools/
> > +
> > +================================================================================
> > +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_off      Turn off cleaning operations, namely garbage collection,
> > +		       triggered in background when I/O subsystem is idle.
> > +disable_roll_forward   Disable the roll-forward recovery routine
> > +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.
> > +
> > +================================================================================
> > +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 utilization information of the whole segments
> > +- 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
> > +
> 
> What about the case of static compilation of f2fs in the kernel?
> 
> > +3. Check the directory trying to mount
> > + # mkdir /mnt/f2fs
> > +
> 
> Create or check?
> 
> > +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
> > +
> > +Mount options
> 
> Sorry, is it really mount options? Maybe, I misunderstand possibility to
> set volume label during mount.
> 
> > +-------------
> > +-l [label]   : Give a volume label, up to 256 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"
> > +
> > +================================================================================
> > +DESIGN
> > +================================================================================
> > +
> > +On-disk Layout
> > +--------------
> > +
> > +F2FS divides the whole volume into a number of segments, each of which is 2MB in
> > +size by default. A section is composed of consecutive segments, and a zone
> > +consists of a set of sections.
> > +
> 
> Maybe, it makes sense to describe here possible sizes of sections and
> zones?
> 
> > +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 beginning of the partition,
> > +and there exist two superblocks to avoid file system crash. Other file system
> > +metadata such as CP, NAT, SIT, and SSA are located in the front part of the
> > +volume. Main area contains file and directory data including their indices.
> > +
> 
> I feel necessity to know more details about log concept here. Could you
> add slightly more description about log?
> 
> > +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.
> > +
> 
> Could you add definition of abbreviations here also (for example, NAT
> Node Address Table: <description>)?
> 
> > +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.
> 
> Maybe, it makes sense to add some technical details about aligning
> procedure here?
> 
> > +
> > +                                            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
> > +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   |    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". 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 929 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 * (927 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
> > +
> > +   Inode block (4KB)
> > +     |- data (927)
> > +     |- 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, 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
> > +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.
> 
> With the best regards,
> Vyacheslav Dubeyko.


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