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@xxxxxxxxxxx> > --- > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
