================================================================================
  WHAT IS Flash-Friendly File System (F2FS)?
  ================================================================================
  
  NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
  been equipped on a variety systems ranging from mobile to server systems. Since
  they are known to have different characteristics from the conventional rotating
  disks, a file system, an upper layer to the storage device, should adapt to the
  changes from the sketch in the design level.
  
  F2FS is a file system exploiting NAND flash memory-based storage devices, which
  is based on Log-structured File System (LFS). The design has been focused on
  addressing the fundamental issues in LFS, which are snowball effect of wandering
  tree and high cleaning overhead.
  
  Since a NAND flash memory-based storage device shows different characteristic
  according to its internal geometry or flash memory management scheme, namely FTL,
  F2FS and its tools support various parameters not only for configuring on-disk
  layout, but also for selecting allocation and cleaning algorithms.

  The following git tree provides the file system formatting tool (mkfs.f2fs),
  a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).

  >> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
  
  For reporting bugs and sending patches, please use the following mailing list:
  >> linux-f2fs-devel@lists.sourceforge.net

  
  ================================================================================
  BACKGROUND AND DESIGN ISSUES
  ================================================================================
  
  Log-structured File System (LFS)
  --------------------------------
  "A log-structured file system writes all modifications to disk sequentially in
  a log-like structure, thereby speeding up  both file writing and crash recovery.
  The log is the only structure on disk; it contains indexing information so that
  files can be read back from the log efficiently. In order to maintain large free
  areas on disk for fast writing, we divide  the log into segments and use a
  segment cleaner to compress the live information from heavily fragmented
  segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
  implementation of a log-structured file system", ACM Trans. Computer Systems
  10, 1, 26–52.
  
  Wandering Tree Problem
  ----------------------
  In LFS, when a file data is updated and written to the end of log, its direct
  pointer block is updated due to the changed location. Then the indirect pointer
  block is also updated due to the direct pointer block update. In this manner,
  the upper index structures such as inode, inode map, and checkpoint block are
  also updated recursively. This problem is called as wandering tree problem [1],
  and in order to enhance the performance, it should eliminate or relax the update
  propagation as much as possible.
  
  [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
  
  Cleaning Overhead
  -----------------
  Since LFS is based on out-of-place writes, it produces so many obsolete blocks
  scattered across the whole storage. In order to serve new empty log space, it
  needs to reclaim these obsolete blocks seamlessly to users. This job is called
  as a cleaning process.
  
  The process consists of three operations as follows.
  1. A victim segment is selected through referencing segment usage table.
  2. It loads parent index structures of all the data in the victim identified by
     segment summary blocks.
  3. It checks the cross-reference between the data and its parent index structure.
  4. It moves valid data selectively.
  
  This cleaning job may cause unexpected long delays, so the most important goal
  is to hide the latencies to users. And also definitely, it should reduce the
  amount of valid data to be moved, and move them quickly as well.
  
  ================================================================================
  KEY FEATURES
  ================================================================================
  
  Flash Awareness
  ---------------
  - Enlarge the random write area for better performance, but provide the high
    spatial locality
  - Align FS data structures to the operational units in FTL as best efforts
  
  Wandering Tree Problem
  ----------------------
  - Use a term, “node”, that represents inodes as well as various pointer blocks
  - Introduce Node Address Table (NAT) containing the locations of all the “node”
    blocks; this will cut off the update propagation.
  
  Cleaning Overhead
  -----------------
  - Support a background cleaning process
  - Support greedy and cost-benefit algorithms for victim selection policies
  - Support multi-head logs for static/dynamic hot and cold data separation
  - Introduce adaptive logging for efficient block allocation
  
  ================================================================================
  MOUNT OPTIONS
  ================================================================================

  background_gc=%s       Turn on/off cleaning operations, namely garbage
                         collection, triggered in background when I/O subsystem is
                         idle. If background_gc=on, it will turn on the garbage
                         collection and if background_gc=off, garbage collection
                         will be truned off.
                         Default value for this option is on. So garbage
                         collection is on by default.

  disable_roll_forward   Disable the roll-forward recovery routine
  discard                Issue discard/TRIM commands when a segment is cleaned.
  no_heap                Disable heap-style segment allocation which finds free
                         segments for data from the beginning of main area, while
  		       for node from the end of main area.
  nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
                         by default if CONFIG_F2FS_FS_XATTR is selected.
  noacl                  Disable POSIX Access Control List. Note: acl is enabled
                         by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
  active_logs=%u         Support configuring the number of active logs. In the
                         current design, f2fs supports only 2, 4, and 6 logs.
                         Default number is 6.
  disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
                         does not aware of cold files such as media files.

  inline_xattr           Enable the inline xattrs feature.

  inline_data            Enable the inline data feature: New created small(<~3.4k)
                         files can be written into inode block.

  flush_merge	       Merge concurrent cache_flush commands as much as possible
                         to eliminate redundant command issues. If the underlying
  		       device handles the cache_flush command relatively slowly,
  		       recommend to enable this option.

  nobarrier              This option can be used if underlying storage guarantees
                         its cached data should be written to the novolatile area.
  		       If this option is set, no cache_flush commands are issued
  		       but f2fs still guarantees the write ordering of all the
  		       data writes.

  
  ================================================================================
  DEBUGFS ENTRIES
  ================================================================================
  
  /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
  f2fs. Each file shows the whole f2fs information.
  
  /sys/kernel/debug/f2fs/status includes:
   - major file system information managed by f2fs currently
   - average SIT information about whole segments
   - current memory footprint consumed by f2fs.
  
  ================================================================================

  SYSFS ENTRIES
  ================================================================================
  
  Information about mounted f2f2 file systems can be found in
  /sys/fs/f2fs.  Each mounted filesystem will have a directory in
  /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
  The files in each per-device directory are shown in table below.
  
  Files in /sys/fs/f2fs/<devname>
  (see also Documentation/ABI/testing/sysfs-fs-f2fs)
  ..............................................................................
   File                         Content
  
   gc_max_sleep_time            This tuning parameter controls the maximum sleep
                                time for the garbage collection thread. Time is
                                in milliseconds.
  
   gc_min_sleep_time            This tuning parameter controls the minimum sleep
                                time for the garbage collection thread. Time is
                                in milliseconds.
  
   gc_no_gc_sleep_time          This tuning parameter controls the default sleep
                                time for the garbage collection thread. Time is
                                in milliseconds.

   gc_idle                      This parameter controls the selection of victim
                                policy for garbage collection. Setting gc_idle = 0
                                (default) will disable this option. Setting
                                gc_idle = 1 will select the Cost Benefit approach
                                & setting gc_idle = 2 will select the greedy aproach.

   reclaim_segments             This parameter controls the number of prefree
                                segments to be reclaimed. If the number of prefree

  			      segments is larger than the number of segments
  			      in the proportion to the percentage over total
  			      volume size, f2fs tries to conduct checkpoint to
  			      reclaim the prefree segments to free segments.
  			      By default, 5% over total # of segments.

   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.

   ipu_policy                   This parameter controls the policy of in-place
                                updates in f2fs. There are five policies:
                                 0: F2FS_IPU_FORCE, 1: F2FS_IPU_SSR,
                                 2: F2FS_IPU_UTIL,  3: F2FS_IPU_SSR_UTIL,

                                 4: F2FS_IPU_FSYNC, 5: F2FS_IPU_DISABLE.

  
   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.

   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.

   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.

   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.

   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.

  ================================================================================

  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

  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:

  -l [label]   : Give a volume label, up to 512 unicode name.

  -a [0 or 1]  : Split start location of each area for heap-based allocation.
                 1 is set by default, which performs this.
  -o [int]     : Set overprovision ratio in percent over volume size.
                 5 is set by default.
  -s [int]     : Set the number of segments per section.
                 1 is set by default.
  -z [int]     : Set the number of sections per zone.
                 1 is set by default.
  -e [str]     : Set basic extension list. e.g. "mp3,gif,mov"

  -t [0 or 1]  : Disable discard command or not.
                 1 is set by default, which conducts discard.

  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.
  It shows on-disk inode information reconized by a given inode number, and is
  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)

  ================================================================================
  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
       _________________________________________________________________________

      |            |            |   Segment   |    Node     |   Segment  |      |
      | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
      |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |

      |____________|_____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.

  - Node Address Table (NAT)
   : It is composed of a block address table for all the node blocks stored in
     Main area.

  - 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.
  
    +--------+----------+---------+

    |   CP   |    SIT   |   NAT   |

    +--------+----------+---------+
    .         .          .          .
    .            .              .              .
    .               .                 .                 .
    +-------+-------+--------+--------+--------+--------+

    | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |

    +-------+-------+--------+--------+--------+--------+
       |             ^                          ^
       |             |                          |
       `----------------------------------------'
  
  Index Structure
  ---------------
  
  The key data structure to manage the data locations is a "node". Similar to
  traditional file structures, F2FS has three types of node: inode, direct node,

  indirect node. F2FS assigns 4KB to an inode block which contains 923 data block

  indices, two direct node pointers, two indirect node pointers, and one double
  indirect node pointer as described below. One direct node block contains 1018
  data blocks, and one indirect node block contains also 1018 node blocks. Thus,
  one inode block (i.e., a file) covers:
  
    4KB * (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.