630 lines
30 KiB
Plaintext
630 lines
30 KiB
Plaintext
================================================================================
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WHAT IS Flash-Friendly File System (F2FS)?
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================================================================================
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NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
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been equipped on a variety systems ranging from mobile to server systems. Since
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they are known to have different characteristics from the conventional rotating
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disks, a file system, an upper layer to the storage device, should adapt to the
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changes from the sketch in the design level.
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F2FS is a file system exploiting NAND flash memory-based storage devices, which
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is based on Log-structured File System (LFS). The design has been focused on
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addressing the fundamental issues in LFS, which are snowball effect of wandering
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tree and high cleaning overhead.
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Since a NAND flash memory-based storage device shows different characteristic
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according to its internal geometry or flash memory management scheme, namely FTL,
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F2FS and its tools support various parameters not only for configuring on-disk
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layout, but also for selecting allocation and cleaning algorithms.
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The following git tree provides the file system formatting tool (mkfs.f2fs),
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a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
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>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
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For reporting bugs and sending patches, please use the following mailing list:
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>> linux-f2fs-devel@lists.sourceforge.net
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================================================================================
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BACKGROUND AND DESIGN ISSUES
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================================================================================
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Log-structured File System (LFS)
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--------------------------------
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"A log-structured file system writes all modifications to disk sequentially in
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a log-like structure, thereby speeding up both file writing and crash recovery.
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The log is the only structure on disk; it contains indexing information so that
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files can be read back from the log efficiently. In order to maintain large free
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areas on disk for fast writing, we divide the log into segments and use a
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segment cleaner to compress the live information from heavily fragmented
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segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
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implementation of a log-structured file system", ACM Trans. Computer Systems
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10, 1, 26–52.
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Wandering Tree Problem
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----------------------
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In LFS, when a file data is updated and written to the end of log, its direct
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pointer block is updated due to the changed location. Then the indirect pointer
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block is also updated due to the direct pointer block update. In this manner,
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the upper index structures such as inode, inode map, and checkpoint block are
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also updated recursively. This problem is called as wandering tree problem [1],
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and in order to enhance the performance, it should eliminate or relax the update
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propagation as much as possible.
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[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
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Cleaning Overhead
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-----------------
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Since LFS is based on out-of-place writes, it produces so many obsolete blocks
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scattered across the whole storage. In order to serve new empty log space, it
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needs to reclaim these obsolete blocks seamlessly to users. This job is called
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as a cleaning process.
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The process consists of three operations as follows.
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1. A victim segment is selected through referencing segment usage table.
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2. It loads parent index structures of all the data in the victim identified by
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segment summary blocks.
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3. It checks the cross-reference between the data and its parent index structure.
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4. It moves valid data selectively.
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This cleaning job may cause unexpected long delays, so the most important goal
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is to hide the latencies to users. And also definitely, it should reduce the
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amount of valid data to be moved, and move them quickly as well.
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================================================================================
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KEY FEATURES
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================================================================================
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Flash Awareness
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---------------
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- Enlarge the random write area for better performance, but provide the high
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spatial locality
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- Align FS data structures to the operational units in FTL as best efforts
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Wandering Tree Problem
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----------------------
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- Use a term, “node”, that represents inodes as well as various pointer blocks
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- Introduce Node Address Table (NAT) containing the locations of all the “node”
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blocks; this will cut off the update propagation.
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Cleaning Overhead
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-----------------
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- Support a background cleaning process
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- Support greedy and cost-benefit algorithms for victim selection policies
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- Support multi-head logs for static/dynamic hot and cold data separation
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- Introduce adaptive logging for efficient block allocation
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================================================================================
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MOUNT OPTIONS
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================================================================================
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background_gc=%s Turn on/off cleaning operations, namely garbage
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collection, triggered in background when I/O subsystem is
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idle. If background_gc=on, it will turn on the garbage
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collection and if background_gc=off, garbage collection
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will be turned off. If background_gc=sync, it will turn
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on synchronous garbage collection running in background.
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Default value for this option is on. So garbage
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collection is on by default.
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disable_roll_forward Disable the roll-forward recovery routine
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norecovery Disable the roll-forward recovery routine, mounted read-
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only (i.e., -o ro,disable_roll_forward)
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discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
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enabled, f2fs will issue discard/TRIM commands when a
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segment is cleaned.
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no_heap Disable heap-style segment allocation which finds free
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segments for data from the beginning of main area, while
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for node from the end of main area.
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nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
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by default if CONFIG_F2FS_FS_XATTR is selected.
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noacl Disable POSIX Access Control List. Note: acl is enabled
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by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
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active_logs=%u Support configuring the number of active logs. In the
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current design, f2fs supports only 2, 4, and 6 logs.
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Default number is 6.
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disable_ext_identify Disable the extension list configured by mkfs, so f2fs
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does not aware of cold files such as media files.
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inline_xattr Enable the inline xattrs feature.
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inline_data Enable the inline data feature: New created small(<~3.4k)
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files can be written into inode block.
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inline_dentry Enable the inline dir feature: data in new created
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directory entries can be written into inode block. The
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space of inode block which is used to store inline
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dentries is limited to ~3.4k.
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noinline_dentry Diable the inline dentry feature.
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flush_merge Merge concurrent cache_flush commands as much as possible
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to eliminate redundant command issues. If the underlying
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device handles the cache_flush command relatively slowly,
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recommend to enable this option.
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nobarrier This option can be used if underlying storage guarantees
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its cached data should be written to the novolatile area.
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If this option is set, no cache_flush commands are issued
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but f2fs still guarantees the write ordering of all the
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data writes.
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fastboot This option is used when a system wants to reduce mount
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time as much as possible, even though normal performance
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can be sacrificed.
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extent_cache Enable an extent cache based on rb-tree, it can cache
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as many as extent which map between contiguous logical
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address and physical address per inode, resulting in
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increasing the cache hit ratio. Set by default.
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noextent_cache Disable an extent cache based on rb-tree explicitly, see
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the above extent_cache mount option.
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noinline_data Disable the inline data feature, inline data feature is
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enabled by default.
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data_flush Enable data flushing before checkpoint in order to
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persist data of regular and symlink.
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mode=%s Control block allocation mode which supports "adaptive"
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and "lfs". In "lfs" mode, there should be no random
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writes towards main area.
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io_bits=%u Set the bit size of write IO requests. It should be set
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with "mode=lfs".
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usrquota Enable plain user disk quota accounting.
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grpquota Enable plain group disk quota accounting.
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prjquota Enable plain project quota accounting.
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usrjquota=<file> Appoint specified file and type during mount, so that quota
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grpjquota=<file> information can be properly updated during recovery flow,
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prjjquota=<file> <quota file>: must be in root directory;
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jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
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offusrjquota Turn off user journelled quota.
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offgrpjquota Turn off group journelled quota.
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offprjjquota Turn off project journelled quota.
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quota Enable plain user disk quota accounting.
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noquota Disable all plain disk quota option.
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whint_mode=%s Control which write hints are passed down to block
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layer. This supports "off", "user-based", and
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"fs-based". In "off" mode (default), f2fs does not pass
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down hints. In "user-based" mode, f2fs tries to pass
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down hints given by users. And in "fs-based" mode, f2fs
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passes down hints with its policy.
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alloc_mode=%s Adjust block allocation policy, which supports "reuse"
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and "default".
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fsync_mode=%s Control the policy of fsync. Currently supports "posix",
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"strict", and "nobarrier". In "posix" mode, which is
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default, fsync will follow POSIX semantics and does a
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light operation to improve the filesystem performance.
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In "strict" mode, fsync will be heavy and behaves in line
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with xfs, ext4 and btrfs, where xfstest generic/342 will
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pass, but the performance will regress. "nobarrier" is
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based on "posix", but doesn't issue flush command for
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non-atomic files likewise "nobarrier" mount option.
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test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt
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context. The fake fscrypt context is used by xfstests.
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================================================================================
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DEBUGFS ENTRIES
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================================================================================
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/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
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f2fs. Each file shows the whole f2fs information.
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/sys/kernel/debug/f2fs/status includes:
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- major file system information managed by f2fs currently
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- average SIT information about whole segments
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- current memory footprint consumed by f2fs.
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================================================================================
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SYSFS ENTRIES
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================================================================================
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Information about mounted f2f2 file systems can be found in
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/sys/fs/f2fs. Each mounted filesystem will have a directory in
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/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
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The files in each per-device directory are shown in table below.
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Files in /sys/fs/f2fs/<devname>
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(see also Documentation/ABI/testing/sysfs-fs-f2fs)
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..............................................................................
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File Content
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gc_max_sleep_time This tuning parameter controls the maximum sleep
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time for the garbage collection thread. Time is
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in milliseconds.
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gc_min_sleep_time This tuning parameter controls the minimum sleep
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time for the garbage collection thread. Time is
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in milliseconds.
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gc_no_gc_sleep_time This tuning parameter controls the default sleep
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time for the garbage collection thread. Time is
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in milliseconds.
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gc_idle This parameter controls the selection of victim
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policy for garbage collection. Setting gc_idle = 0
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(default) will disable this option. Setting
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gc_idle = 1 will select the Cost Benefit approach
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& setting gc_idle = 2 will select the greedy approach.
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gc_urgent This parameter controls triggering background GCs
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urgently or not. Setting gc_urgent = 0 [default]
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makes back to default behavior, while if it is set
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to 1, background thread starts to do GC by given
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gc_urgent_sleep_time interval.
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gc_urgent_sleep_time This parameter controls sleep time for gc_urgent.
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500 ms is set by default. See above gc_urgent.
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reclaim_segments This parameter controls the number of prefree
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segments to be reclaimed. If the number of prefree
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segments is larger than the number of segments
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in the proportion to the percentage over total
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volume size, f2fs tries to conduct checkpoint to
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reclaim the prefree segments to free segments.
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By default, 5% over total # of segments.
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max_small_discards This parameter controls the number of discard
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commands that consist small blocks less than 2MB.
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The candidates to be discarded are cached until
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checkpoint is triggered, and issued during the
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checkpoint. By default, it is disabled with 0.
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trim_sections This parameter controls the number of sections
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to be trimmed out in batch mode when FITRIM
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conducts. 32 sections is set by default.
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ipu_policy This parameter controls the policy of in-place
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updates in f2fs. There are five policies:
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0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
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0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
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0x10: F2FS_IPU_FSYNC.
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min_ipu_util This parameter controls the threshold to trigger
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in-place-updates. The number indicates percentage
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of the filesystem utilization, and used by
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F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
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min_fsync_blocks This parameter controls the threshold to trigger
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in-place-updates when F2FS_IPU_FSYNC mode is set.
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The number indicates the number of dirty pages
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when fsync needs to flush on its call path. If
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the number is less than this value, it triggers
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in-place-updates.
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max_victim_search This parameter controls the number of trials to
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find a victim segment when conducting SSR and
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cleaning operations. The default value is 4096
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which covers 8GB block address range.
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dir_level This parameter controls the directory level to
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support large directory. If a directory has a
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number of files, it can reduce the file lookup
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latency by increasing this dir_level value.
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Otherwise, it needs to decrease this value to
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reduce the space overhead. The default value is 0.
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ram_thresh This parameter controls the memory footprint used
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by free nids and cached nat entries. By default,
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10 is set, which indicates 10 MB / 1 GB RAM.
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================================================================================
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USAGE
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================================================================================
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1. Download userland tools and compile them.
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2. Skip, if f2fs was compiled statically inside kernel.
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Otherwise, insert the f2fs.ko module.
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# insmod f2fs.ko
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3. Create a directory trying to mount
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# mkdir /mnt/f2fs
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4. Format the block device, and then mount as f2fs
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# mkfs.f2fs -l label /dev/block_device
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# mount -t f2fs /dev/block_device /mnt/f2fs
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mkfs.f2fs
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---------
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The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
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which builds a basic on-disk layout.
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The options consist of:
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-l [label] : Give a volume label, up to 512 unicode name.
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-a [0 or 1] : Split start location of each area for heap-based allocation.
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1 is set by default, which performs this.
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-o [int] : Set overprovision ratio in percent over volume size.
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5 is set by default.
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-s [int] : Set the number of segments per section.
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1 is set by default.
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-z [int] : Set the number of sections per zone.
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1 is set by default.
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-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
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-t [0 or 1] : Disable discard command or not.
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1 is set by default, which conducts discard.
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fsck.f2fs
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---------
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The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
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partition, which examines whether the filesystem metadata and user-made data
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are cross-referenced correctly or not.
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Note that, initial version of the tool does not fix any inconsistency.
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The options consist of:
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-d debug level [default:0]
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dump.f2fs
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---------
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The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
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file. Each file is dump_ssa and dump_sit.
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The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
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It shows on-disk inode information recognized by a given inode number, and is
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able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
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./dump_sit respectively.
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The options consist of:
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-d debug level [default:0]
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-i inode no (hex)
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-s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
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-a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
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Examples:
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# dump.f2fs -i [ino] /dev/sdx
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# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
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# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
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================================================================================
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DESIGN
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================================================================================
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On-disk Layout
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--------------
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F2FS divides the whole volume into a number of segments, each of which is fixed
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to 2MB in size. A section is composed of consecutive segments, and a zone
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consists of a set of sections. By default, section and zone sizes are set to one
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segment size identically, but users can easily modify the sizes by mkfs.
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F2FS splits the entire volume into six areas, and all the areas except superblock
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consists of multiple segments as described below.
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align with the zone size <-|
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|-> align with the segment size
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_________________________________________________________________________
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| | | Segment | Node | Segment | |
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| Superblock | Checkpoint | Info. | Address | Summary | Main |
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| (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
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|____________|_____2______|______N______|______N______|______N_____|__N___|
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. .
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. .
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. .
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._________________________________________.
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|_Segment_|_..._|_Segment_|_..._|_Segment_|
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. .
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._________._________
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|_section_|__...__|_
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. .
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.________.
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|__zone__|
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- Superblock (SB)
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: It is located at the beginning of the partition, and there exist two copies
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to avoid file system crash. It contains basic partition information and some
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default parameters of f2fs.
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- Checkpoint (CP)
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: It contains file system information, bitmaps for valid NAT/SIT sets, orphan
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inode lists, and summary entries of current active segments.
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- Segment Information Table (SIT)
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: It contains segment information such as valid block count and bitmap for the
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validity of all the blocks.
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- Node Address Table (NAT)
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: It is composed of a block address table for all the node blocks stored in
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Main area.
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- Segment Summary Area (SSA)
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: It contains summary entries which contains the owner information of all the
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data and node blocks stored in Main area.
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- Main Area
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: It contains file and directory data including their indices.
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In order to avoid misalignment between file system and flash-based storage, F2FS
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aligns the start block address of CP with the segment size. Also, it aligns the
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start block address of Main area with the zone size by reserving some segments
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in SSA area.
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Reference the following survey for additional technical details.
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https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
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File System Metadata Structure
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------------------------------
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F2FS adopts the checkpointing scheme to maintain file system consistency. At
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mount time, F2FS first tries to find the last valid checkpoint data by scanning
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CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
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One of them always indicates the last valid data, which is called as shadow copy
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mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
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For file system consistency, each CP points to which NAT and SIT copies are
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valid, as shown as below.
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+--------+----------+---------+
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| CP | SIT | NAT |
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+--------+----------+---------+
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. . . .
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. . . .
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. . . .
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+-------+-------+--------+--------+--------+--------+
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| CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
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+-------+-------+--------+--------+--------+--------+
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| ^ ^
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| | |
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`----------------------------------------'
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Index Structure
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---------------
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The key data structure to manage the data locations is a "node". Similar to
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traditional file structures, F2FS has three types of node: inode, direct node,
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indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
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indices, two direct node pointers, two indirect node pointers, and one double
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indirect node pointer as described below. One direct node block contains 1018
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data blocks, and one indirect node block contains also 1018 node blocks. Thus,
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||
one inode block (i.e., a file) covers:
|
||
|
||
4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
|
||
|
||
Inode block (4KB)
|
||
|- data (923)
|
||
|- 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 + dir_level),
|
||
| if n + dir_level < 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.
|