| ================================================================================ |
| 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 file system formatting tool, "mkfs.f2fs", is available from the following |
| git tree: |
| >> 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_off Turn off cleaning operations, namely garbage collection, |
| triggered in background when I/O subsystem is idle. |
| 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. |
| |
| ================================================================================ |
| 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. |
| |
| ================================================================================ |
| 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 |
| |
| Format options |
| -------------- |
| -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. |
| |
| ================================================================================ |
| 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, if n < MAX_DIR_HASH_DEPTH / 2, |
| # of buckets in level #n = | |
| `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise |
| |
| When F2FS finds a file name in a directory, at first a hash value of the file |
| name is calculated. Then, F2FS scans the hash table in level #0 to find the |
| dentry consisting of the file name and its inode number. If not found, F2FS |
| scans the next hash table in level #1. In this way, F2FS scans hash tables in |
| each levels incrementally from 1 to N. In each levels F2FS needs to scan only |
| one bucket determined by the 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. |
