[PATCH 01/16] f2fs: add document
Date: Fri Oct 05 2012 - 07:56:01 EST
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 | 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
@@ -48,6 +48,8 @@ ext4.txt
- info, mount options and specifications for the Ext4 filesystem.
- info on file management in the Linux kernel.
+ - info and mount options for the F2FS filesystem.
- info on the Filesystem in User SpacE including mount options.
diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt
new file mode 100644
@@ -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
+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.
+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/
+background_gc_off Turn off the cleaning operation, aka garbage collection,
+ in background triggered when I/O subsystem is idle.
+disable_roll_forward Disable the roll-forward recovery routine during SPOR.
+discard Issue discard/TRIM commands when a segment is cleaned.
+no_heap Disable heap-style segment allocation in which finds free
+ 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
+ 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/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
+- f2fs_mem_stat current memory footprint consumed by f2fs
+e.g., in /proc/fs/f2fs/sdb1/
+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
+F2FS divides whole volume into a number of segments each of which size is 2MB by
+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,
+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.
+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) | |
+ . .
+ . .
+ . .
+ . .
+ . .
+File System Metadata Structure
+F2FS adopts the checkpointing scheme to maintain file system consistency. At 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.
+For file system consistency, each CP points 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 |
+ | ^ ^
+ | | |
+The key data structure to manage the data locations is a "node". As similar as
+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
+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 * (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
+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.
+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
+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)
+ |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 follow equation, which shows O(log(# of files))
+ 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
+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
+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
+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
+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
+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.
+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
+valid blocks. In 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 greedy algorithm. F2FS adopts greedy algorithm for on-demand
+cleaner, while background cleaner adopts cost-benefit algorithm.
+In order to identify what 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.
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