2012-11-02 16:05:42 +08:00
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================================================================================
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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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2013-07-04 16:12:47 +08:00
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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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2012-11-27 13:36:14 +08:00
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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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2012-11-02 16:05:42 +08:00
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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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2013-06-16 08:48:48 +08:00
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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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2015-11-16 19:46:28 +08:00
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will be turned off. If background_gc=sync, it will turn
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2015-10-06 02:02:54 +08:00
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on synchronous garbage collection running in background.
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2013-06-16 08:48:48 +08:00
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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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2012-11-02 16:05:42 +08:00
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disable_roll_forward Disable the roll-forward recovery routine
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2015-01-24 10:33:46 +08:00
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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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2016-07-03 22:05:14 +08:00
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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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2012-11-02 16:05:42 +08:00
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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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2013-11-01 10:20:05 +08:00
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inline_xattr Enable the inline xattrs feature.
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2017-02-15 10:34:45 +08:00
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noinline_xattr Disable the inline xattrs feature.
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2013-11-10 23:13:21 +08:00
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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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2014-09-24 18:20:23 +08:00
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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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2017-01-24 11:47:55 +08:00
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noinline_dentry Disable the inline dentry feature.
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2014-04-02 14:34:36 +08:00
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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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2014-07-24 00:57:31 +08:00
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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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2014-10-31 13:47:03 +08:00
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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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2015-02-05 17:55:51 +08:00
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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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2015-06-26 08:43:04 +08:00
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increasing the cache hit ratio. Set by default.
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2015-11-16 19:46:28 +08:00
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noextent_cache Disable an extent cache based on rb-tree explicitly, see
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2015-06-26 08:43:04 +08:00
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the above extent_cache mount option.
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2015-03-24 10:20:27 +08:00
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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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2015-12-16 13:12:16 +08:00
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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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2017-06-12 22:30:44 +08:00
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fault_injection=%d Enable fault injection in all supported types with
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specified injection rate.
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2018-08-08 17:36:41 +08:00
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fault_type=%d Support configuring fault injection type, should be
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enabled with fault_injection option, fault type value
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is shown below, it supports single or combined type.
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Type_Name Type_Value
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FAULT_KMALLOC 0x000000001
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FAULT_KVMALLOC 0x000000002
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FAULT_PAGE_ALLOC 0x000000004
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FAULT_PAGE_GET 0x000000008
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FAULT_ALLOC_BIO 0x000000010
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FAULT_ALLOC_NID 0x000000020
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FAULT_ORPHAN 0x000000040
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FAULT_BLOCK 0x000000080
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FAULT_DIR_DEPTH 0x000000100
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FAULT_EVICT_INODE 0x000000200
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FAULT_TRUNCATE 0x000000400
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FAULT_IO 0x000000800
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FAULT_CHECKPOINT 0x000001000
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FAULT_DISCARD 0x000002000
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2016-06-04 10:29:38 +08:00
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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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2016-12-22 09:09:19 +08:00
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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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2017-07-09 00:13:07 +08:00
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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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2017-07-26 00:01:41 +08:00
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prjquota Enable plain project quota accounting.
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2017-08-08 10:54:31 +08:00
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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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2018-01-31 10:36:59 +08:00
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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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2018-02-19 00:50:49 +08:00
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alloc_mode=%s Adjust block allocation policy, which supports "reuse"
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and "default".
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2018-05-26 09:02:58 +08:00
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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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2018-03-15 18:51:42 +08:00
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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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2012-11-02 16:05:42 +08:00
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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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2013-08-04 22:09:40 +08:00
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================================================================================
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SYSFS ENTRIES
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================================================================================
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2017-02-08 05:08:01 +08:00
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Information about mounted f2fs file systems can be found in
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2013-08-04 22:09:40 +08:00
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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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2013-08-04 22:10:15 +08:00
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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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2015-11-16 19:46:28 +08:00
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& setting gc_idle = 2 will select the greedy approach.
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2013-08-04 22:10:15 +08:00
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2017-08-07 13:09:00 +08:00
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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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2013-10-24 14:49:07 +08:00
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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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2014-03-19 13:17:21 +08:00
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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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2013-10-24 14:49:07 +08:00
|
|
|
|
|
2013-12-19 16:44:41 +08:00
|
|
|
|
max_small_discards This parameter controls the number of discard
|
|
|
|
|
commands that consist small blocks less than 2MB.
|
|
|
|
|
The candidates to be discarded are cached until
|
|
|
|
|
checkpoint is triggered, and issued during the
|
|
|
|
|
checkpoint. By default, it is disabled with 0.
|
|
|
|
|
|
2015-01-27 09:41:23 +08:00
|
|
|
|
trim_sections This parameter controls the number of sections
|
|
|
|
|
to be trimmed out in batch mode when FITRIM
|
|
|
|
|
conducts. 32 sections is set by default.
|
|
|
|
|
|
2013-11-07 12:13:42 +08:00
|
|
|
|
ipu_policy This parameter controls the policy of in-place
|
|
|
|
|
updates in f2fs. There are five policies:
|
2014-09-17 09:30:54 +08:00
|
|
|
|
0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
|
|
|
|
|
0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
|
|
|
|
|
0x10: F2FS_IPU_FSYNC.
|
2013-11-07 12:13:42 +08:00
|
|
|
|
|
|
|
|
|
min_ipu_util This parameter controls the threshold to trigger
|
|
|
|
|
in-place-updates. The number indicates percentage
|
|
|
|
|
of the filesystem utilization, and used by
|
|
|
|
|
F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
|
|
|
|
|
|
2014-09-11 07:53:02 +08:00
|
|
|
|
min_fsync_blocks This parameter controls the threshold to trigger
|
|
|
|
|
in-place-updates when F2FS_IPU_FSYNC mode is set.
|
|
|
|
|
The number indicates the number of dirty pages
|
|
|
|
|
when fsync needs to flush on its call path. If
|
|
|
|
|
the number is less than this value, it triggers
|
|
|
|
|
in-place-updates.
|
|
|
|
|
|
2014-01-09 20:00:06 +08:00
|
|
|
|
max_victim_search This parameter controls the number of trials to
|
|
|
|
|
find a victim segment when conducting SSR and
|
|
|
|
|
cleaning operations. The default value is 4096
|
|
|
|
|
which covers 8GB block address range.
|
|
|
|
|
|
2014-02-27 19:09:05 +08:00
|
|
|
|
dir_level This parameter controls the directory level to
|
|
|
|
|
support large directory. If a directory has a
|
|
|
|
|
number of files, it can reduce the file lookup
|
|
|
|
|
latency by increasing this dir_level value.
|
|
|
|
|
Otherwise, it needs to decrease this value to
|
|
|
|
|
reduce the space overhead. The default value is 0.
|
|
|
|
|
|
2014-03-19 12:31:37 +08:00
|
|
|
|
ram_thresh This parameter controls the memory footprint used
|
|
|
|
|
by free nids and cached nat entries. By default,
|
|
|
|
|
10 is set, which indicates 10 MB / 1 GB RAM.
|
|
|
|
|
|
2012-11-02 16:05:42 +08:00
|
|
|
|
================================================================================
|
|
|
|
|
USAGE
|
|
|
|
|
================================================================================
|
|
|
|
|
|
|
|
|
|
1. Download userland tools and compile them.
|
|
|
|
|
|
|
|
|
|
2. Skip, if f2fs was compiled statically inside kernel.
|
|
|
|
|
Otherwise, insert the f2fs.ko module.
|
|
|
|
|
# insmod f2fs.ko
|
|
|
|
|
|
|
|
|
|
3. Create a 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
|
|
|
|
|
|
2013-07-04 16:12:47 +08:00
|
|
|
|
mkfs.f2fs
|
|
|
|
|
---------
|
|
|
|
|
The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
|
|
|
|
|
which builds a basic on-disk layout.
|
|
|
|
|
|
|
|
|
|
The options consist of:
|
2013-04-03 14:26:49 +08:00
|
|
|
|
-l [label] : Give a volume label, up to 512 unicode name.
|
2012-11-02 16:05:42 +08:00
|
|
|
|
-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"
|
2013-04-03 14:26:49 +08:00
|
|
|
|
-t [0 or 1] : Disable discard command or not.
|
|
|
|
|
1 is set by default, which conducts discard.
|
2012-11-02 16:05:42 +08:00
|
|
|
|
|
2013-07-04 16:12:47 +08:00
|
|
|
|
fsck.f2fs
|
|
|
|
|
---------
|
|
|
|
|
The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
|
|
|
|
|
partition, which examines whether the filesystem metadata and user-made data
|
|
|
|
|
are cross-referenced correctly or not.
|
|
|
|
|
Note that, initial version of the tool does not fix any inconsistency.
|
|
|
|
|
|
|
|
|
|
The options consist of:
|
|
|
|
|
-d debug level [default:0]
|
|
|
|
|
|
|
|
|
|
dump.f2fs
|
|
|
|
|
---------
|
|
|
|
|
The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
|
|
|
|
|
file. Each file is dump_ssa and dump_sit.
|
|
|
|
|
|
|
|
|
|
The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
|
2015-11-16 19:46:28 +08:00
|
|
|
|
It shows on-disk inode information recognized by a given inode number, and is
|
2013-07-04 16:12:47 +08:00
|
|
|
|
able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
|
|
|
|
|
./dump_sit respectively.
|
|
|
|
|
|
|
|
|
|
The options consist of:
|
|
|
|
|
-d debug level [default:0]
|
|
|
|
|
-i inode no (hex)
|
|
|
|
|
-s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
|
|
|
|
|
-a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
|
|
|
|
|
|
|
|
|
|
Examples:
|
|
|
|
|
# dump.f2fs -i [ino] /dev/sdx
|
|
|
|
|
# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
|
|
|
|
|
# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
|
|
|
|
|
|
2012-11-02 16:05:42 +08:00
|
|
|
|
================================================================================
|
|
|
|
|
DESIGN
|
|
|
|
|
================================================================================
|
|
|
|
|
|
|
|
|
|
On-disk Layout
|
|
|
|
|
--------------
|
|
|
|
|
|
|
|
|
|
F2FS divides the whole volume into a number of segments, each of which is fixed
|
|
|
|
|
to 2MB in size. A section is composed of consecutive segments, and a zone
|
|
|
|
|
consists of a set of sections. By default, section and zone sizes are set to one
|
|
|
|
|
segment size identically, but users can easily modify the sizes by mkfs.
|
|
|
|
|
|
|
|
|
|
F2FS splits the entire volume into six areas, and all the areas except superblock
|
|
|
|
|
consists of multiple segments as described below.
|
|
|
|
|
|
|
|
|
|
align with the zone size <-|
|
|
|
|
|
|-> align with the segment size
|
|
|
|
|
_________________________________________________________________________
|
2012-12-31 13:59:04 +08:00
|
|
|
|
| | | Segment | Node | Segment | |
|
|
|
|
|
| Superblock | Checkpoint | Info. | Address | Summary | Main |
|
|
|
|
|
| (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
|
2012-11-02 16:05:42 +08:00
|
|
|
|
|____________|_____2______|______N______|______N______|______N_____|__N___|
|
|
|
|
|
. .
|
|
|
|
|
. .
|
|
|
|
|
. .
|
|
|
|
|
._________________________________________.
|
|
|
|
|
|_Segment_|_..._|_Segment_|_..._|_Segment_|
|
|
|
|
|
. .
|
|
|
|
|
._________._________
|
|
|
|
|
|_section_|__...__|_
|
|
|
|
|
. .
|
|
|
|
|
.________.
|
|
|
|
|
|__zone__|
|
|
|
|
|
|
|
|
|
|
- Superblock (SB)
|
|
|
|
|
: It is located at the beginning of the partition, and there exist two copies
|
|
|
|
|
to avoid file system crash. It contains basic partition information and some
|
|
|
|
|
default parameters of f2fs.
|
|
|
|
|
|
|
|
|
|
- Checkpoint (CP)
|
|
|
|
|
: It contains file system information, bitmaps for valid NAT/SIT sets, orphan
|
|
|
|
|
inode lists, and summary entries of current active segments.
|
|
|
|
|
|
|
|
|
|
- Segment Information Table (SIT)
|
|
|
|
|
: It contains segment information such as valid block count and bitmap for the
|
|
|
|
|
validity of all the blocks.
|
|
|
|
|
|
2012-12-31 13:59:04 +08:00
|
|
|
|
- Node Address Table (NAT)
|
|
|
|
|
: It is composed of a block address table for all the node blocks stored in
|
|
|
|
|
Main area.
|
|
|
|
|
|
2012-11-02 16:05:42 +08:00
|
|
|
|
- Segment Summary Area (SSA)
|
|
|
|
|
: It contains summary entries which contains the owner information of all the
|
|
|
|
|
data and node blocks stored in Main area.
|
|
|
|
|
|
|
|
|
|
- Main Area
|
|
|
|
|
: It contains file and directory data including their indices.
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
Reference the following survey for additional technical details.
|
|
|
|
|
https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
|
|
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
|
|
|
|
+--------+----------+---------+
|
2012-12-31 13:59:04 +08:00
|
|
|
|
| CP | SIT | NAT |
|
2012-11-02 16:05:42 +08:00
|
|
|
|
+--------+----------+---------+
|
|
|
|
|
. . . .
|
|
|
|
|
. . . .
|
|
|
|
|
. . . .
|
|
|
|
|
+-------+-------+--------+--------+--------+--------+
|
2012-12-31 13:59:04 +08:00
|
|
|
|
| CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
|
2012-11-02 16:05:42 +08:00
|
|
|
|
+-------+-------+--------+--------+--------+--------+
|
|
|
|
|
| ^ ^
|
|
|
|
|
| | |
|
|
|
|
|
`----------------------------------------'
|
|
|
|
|
|
|
|
|
|
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,
|
2012-12-05 16:45:32 +08:00
|
|
|
|
indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
|
2012-11-02 16:05:42 +08:00
|
|
|
|
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 * (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
|
|
|
|
|
|
2014-05-28 08:56:09 +08:00
|
|
|
|
,- 2^(n + dir_level),
|
|
|
|
|
| if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
|
2012-11-02 16:05:42 +08:00
|
|
|
|
# of buckets in level #n = |
|
2014-05-28 08:56:09 +08:00
|
|
|
|
`- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
|
|
|
|
|
Otherwise
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2012-11-02 16:05:42 +08:00
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When F2FS finds a file name in a directory, at first a hash value of the file
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name is calculated. Then, F2FS scans the hash table in level #0 to find the
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dentry consisting of the file name and its inode number. If not found, F2FS
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scans the next hash table in level #1. In this way, F2FS scans hash tables in
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each levels incrementally from 1 to N. In each levels F2FS needs to scan only
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one bucket determined by the following equation, which shows O(log(# of files))
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complexity.
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bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
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In the case of file creation, F2FS finds empty consecutive slots that cover the
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file name. F2FS searches the empty slots in the hash tables of whole levels from
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1 to N in the same way as the lookup operation.
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The following figure shows an example of two cases holding children.
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--------------> Dir <--------------
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child child
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child - child [hole] - child
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child - child - child [hole] - [hole] - child
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Case 1: Case 2:
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Number of children = 6, Number of children = 3,
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File size = 7 File size = 7
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Default Block Allocation
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------------------------
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At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
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and Hot/Warm/Cold data.
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- Hot node contains direct node blocks of directories.
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- Warm node contains direct node blocks except hot node blocks.
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- Cold node contains indirect node blocks
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- Hot data contains dentry blocks
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- Warm data contains data blocks except hot and cold data blocks
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- Cold data contains multimedia data or migrated data blocks
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LFS has two schemes for free space management: threaded log and copy-and-compac-
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tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
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for devices showing very good sequential write performance, since free segments
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are served all the time for writing new data. However, it suffers from cleaning
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overhead under high utilization. Contrarily, the threaded log scheme suffers
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from random writes, but no cleaning process is needed. F2FS adopts a hybrid
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scheme where the copy-and-compaction scheme is adopted by default, but the
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policy is dynamically changed to the threaded log scheme according to the file
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system status.
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In order to align F2FS with underlying flash-based storage, F2FS allocates a
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segment in a unit of section. F2FS expects that the section size would be the
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same as the unit size of garbage collection in FTL. Furthermore, with respect
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to the mapping granularity in FTL, F2FS allocates each section of the active
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logs from different zones as much as possible, since FTL can write the data in
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the active logs into one allocation unit according to its mapping granularity.
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Cleaning process
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----------------
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F2FS does cleaning both on demand and in the background. On-demand cleaning is
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triggered when there are not enough free segments to serve VFS calls. Background
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cleaner is operated by a kernel thread, and triggers the cleaning job when the
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system is idle.
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F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
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In the greedy algorithm, F2FS selects a victim segment having the smallest number
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of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
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according to the segment age and the number of valid blocks in order to address
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log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
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algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
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algorithm.
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In order to identify whether the data in the victim segment are valid or not,
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F2FS manages a bitmap. Each bit represents the validity of a block, and the
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bitmap is composed of a bit stream covering whole blocks in main area.
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2018-01-31 10:36:59 +08:00
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Write-hint Policy
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-----------------
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1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
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2) whint_mode=user-based. F2FS tries to pass down hints given by
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users.
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User F2FS Block
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---- ---- -----
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META WRITE_LIFE_NOT_SET
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HOT_NODE "
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WARM_NODE "
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COLD_NODE "
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*ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
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*extension list " "
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-- buffered io
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WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
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WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
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WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
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WRITE_LIFE_NONE " "
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WRITE_LIFE_MEDIUM " "
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WRITE_LIFE_LONG " "
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-- direct io
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WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
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WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
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WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
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WRITE_LIFE_NONE " WRITE_LIFE_NONE
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WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
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WRITE_LIFE_LONG " WRITE_LIFE_LONG
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3) whint_mode=fs-based. F2FS passes down hints with its policy.
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User F2FS Block
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---- ---- -----
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META WRITE_LIFE_MEDIUM;
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HOT_NODE WRITE_LIFE_NOT_SET
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WARM_NODE "
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COLD_NODE WRITE_LIFE_NONE
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ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
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extension list " "
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-- buffered io
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WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
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WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
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WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
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WRITE_LIFE_NONE " "
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WRITE_LIFE_MEDIUM " "
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WRITE_LIFE_LONG " "
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-- direct io
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WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
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WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
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WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
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WRITE_LIFE_NONE " WRITE_LIFE_NONE
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WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
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WRITE_LIFE_LONG " WRITE_LIFE_LONG
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