422 строки
19 KiB
Plaintext
422 строки
19 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 file system formatting tool, "mkfs.f2fs", is available from the following
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git tree:
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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_off Turn off cleaning operations, namely garbage collection,
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triggered in background when I/O subsystem is idle.
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disable_roll_forward Disable the roll-forward recovery routine
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discard Issue discard/TRIM commands when a 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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================================================================================
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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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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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Format options
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--------------
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-l [label] : Give a volume label, up to 256 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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================================================================================
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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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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:
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4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
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Inode block (4KB)
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|- data (923)
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|- direct node (2)
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| `- data (1018)
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|- indirect node (2)
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| `- direct node (1018)
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| `- data (1018)
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`- double indirect node (1)
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`- indirect node (1018)
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`- direct node (1018)
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`- data (1018)
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Note that, all the node blocks are mapped by NAT which means the location of
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each node is translated by the NAT table. In the consideration of the wandering
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tree problem, F2FS is able to cut off the propagation of node updates caused by
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leaf data writes.
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Directory Structure
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-------------------
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A directory entry occupies 11 bytes, which consists of the following attributes.
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- hash hash value of the file name
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- ino inode number
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- len the length of file name
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- type file type such as directory, symlink, etc
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A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
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used to represent whether each dentry is valid or not. A dentry block occupies
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4KB with the following composition.
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Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
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dentries(11 * 214 bytes) + file name (8 * 214 bytes)
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[Bucket]
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+--------------------------------+
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|dentry block 1 | dentry block 2 |
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+--------------------------------+
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. .
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. .
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. [Dentry Block Structure: 4KB] .
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+--------+----------+----------+------------+
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| bitmap | reserved | dentries | file names |
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+--------+----------+----------+------------+
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[Dentry Block: 4KB] . .
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. .
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. .
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+------+------+-----+------+
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| hash | ino | len | type |
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+------+------+-----+------+
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[Dentry Structure: 11 bytes]
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F2FS implements multi-level hash tables for directory structure. Each level has
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a hash table with dedicated number of hash buckets as shown below. Note that
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"A(2B)" means a bucket includes 2 data blocks.
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----------------------
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A : bucket
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B : block
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N : MAX_DIR_HASH_DEPTH
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----------------------
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level #0 | A(2B)
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level #1 | A(2B) - A(2B)
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level #2 | A(2B) - A(2B) - A(2B) - A(2B)
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. | . . . .
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level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
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. | . . . .
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level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
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The number of blocks and buckets are determined by,
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,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
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# of blocks in level #n = |
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`- 4, Otherwise
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,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
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# of buckets in level #n = |
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`- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
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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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