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Diffstat (limited to 'Documentation/filesystems/f2fs.rst')
| -rw-r--r-- | Documentation/filesystems/f2fs.rst | 414 |
1 files changed, 317 insertions, 97 deletions
diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst index ec8d99703ecb..e5bb89452aff 100644 --- a/Documentation/filesystems/f2fs.rst +++ b/Documentation/filesystems/f2fs.rst @@ -25,10 +25,14 @@ 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: +For sending patches, please use the following mailing list: - linux-f2fs-devel@lists.sourceforge.net +For reporting bugs, please use the following f2fs bug tracker link: + +- https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs + Background and Design issues ============================ @@ -110,15 +114,19 @@ background_gc=%s Turn on/off cleaning operations, namely garbage on synchronous garbage collection running in background. Default value for this option is on. So garbage collection is on by default. +gc_merge When background_gc is on, this option can be enabled to + let background GC thread to handle foreground GC requests, + it can eliminate the sluggish issue caused by slow foreground + GC operation when GC is triggered from a process with limited + I/O and CPU resources. +nogc_merge Disable GC merge feature. disable_roll_forward Disable the roll-forward recovery routine norecovery Disable the roll-forward recovery routine, mounted read- only (i.e., -o ro,disable_roll_forward) discard/nodiscard Enable/disable real-time discard in f2fs, if discard is enabled, f2fs will 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. +heap/no_heap Deprecated. 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 @@ -127,14 +135,14 @@ 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. + is not aware of cold files such as media files. inline_xattr Enable the inline xattrs feature. noinline_xattr Disable the inline xattrs feature. inline_xattr_size=%u Support configuring inline xattr size, it depends on flexible inline xattr feature. -inline_data Enable the inline data feature: New created small(<~3.4k) +inline_data Enable the inline data feature: Newly created small (<~3.4k) files can be written into inode block. -inline_dentry Enable the inline dir feature: data in new created +inline_dentry Enable the inline dir feature: data in newly created directory entries can be written into inode block. The space of inode block which is used to store inline dentries is limited to ~3.4k. @@ -148,6 +156,8 @@ nobarrier This option can be used if underlying storage guarantees If this option is set, no cache_flush commands are issued but f2fs still guarantees the write ordering of all the data writes. +barrier If this option is set, cache_flush commands are allowed to be + issued. fastboot This option is used when a system wants to reduce mount time as much as possible, even though normal performance can be sacrificed. @@ -172,30 +182,55 @@ fault_type=%d Support configuring fault injection type, should be enabled with fault_injection option, fault type value is shown below, it supports single or combined type. - =================== =========== - Type_Name Type_Value - =================== =========== - FAULT_KMALLOC 0x000000001 - FAULT_KVMALLOC 0x000000002 - FAULT_PAGE_ALLOC 0x000000004 - FAULT_PAGE_GET 0x000000008 - FAULT_ALLOC_BIO 0x000000010 - FAULT_ALLOC_NID 0x000000020 - FAULT_ORPHAN 0x000000040 - FAULT_BLOCK 0x000000080 - FAULT_DIR_DEPTH 0x000000100 - FAULT_EVICT_INODE 0x000000200 - FAULT_TRUNCATE 0x000000400 - FAULT_READ_IO 0x000000800 - FAULT_CHECKPOINT 0x000001000 - FAULT_DISCARD 0x000002000 - FAULT_WRITE_IO 0x000004000 - =================== =========== + =========================== ========== + Type_Name Type_Value + =========================== ========== + FAULT_KMALLOC 0x00000001 + FAULT_KVMALLOC 0x00000002 + FAULT_PAGE_ALLOC 0x00000004 + FAULT_PAGE_GET 0x00000008 + FAULT_ALLOC_BIO 0x00000010 (obsolete) + FAULT_ALLOC_NID 0x00000020 + FAULT_ORPHAN 0x00000040 + FAULT_BLOCK 0x00000080 + FAULT_DIR_DEPTH 0x00000100 + FAULT_EVICT_INODE 0x00000200 + FAULT_TRUNCATE 0x00000400 + FAULT_READ_IO 0x00000800 + FAULT_CHECKPOINT 0x00001000 + FAULT_DISCARD 0x00002000 + FAULT_WRITE_IO 0x00004000 + FAULT_SLAB_ALLOC 0x00008000 + FAULT_DQUOT_INIT 0x00010000 + FAULT_LOCK_OP 0x00020000 + FAULT_BLKADDR_VALIDITY 0x00040000 + FAULT_BLKADDR_CONSISTENCE 0x00080000 + FAULT_NO_SEGMENT 0x00100000 + FAULT_INCONSISTENT_FOOTER 0x00200000 + FAULT_TIMEOUT 0x00400000 (1000ms) + FAULT_VMALLOC 0x00800000 + =========================== ========== mode=%s Control block allocation mode which supports "adaptive" and "lfs". In "lfs" mode, there should be no random writes towards main area. -io_bits=%u Set the bit size of write IO requests. It should be set - with "mode=lfs". + "fragment:segment" and "fragment:block" are newly added here. + These are developer options for experiments to simulate filesystem + fragmentation/after-GC situation itself. The developers use these + modes to understand filesystem fragmentation/after-GC condition well, + and eventually get some insights to handle them better. + In "fragment:segment", f2fs allocates a new segment in random + position. With this, we can simulate the after-GC condition. + In "fragment:block", we can scatter block allocation with + "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. + We added some randomness to both chunk and hole size to make + it close to realistic IO pattern. So, in this mode, f2fs will allocate + 1..<max_fragment_chunk> blocks in a chunk and make a hole in the + length of 1..<max_fragment_hole> by turns. With this, the newly + allocated blocks will be scattered throughout the whole partition. + Note that "fragment:block" implicitly enables "fragment:segment" + option for more randomness. + Please, use these options for your experiments and we strongly + recommend to re-format the filesystem after using these options. usrquota Enable plain user disk quota accounting. grpquota Enable plain group disk quota accounting. prjquota Enable plain project quota accounting. @@ -203,17 +238,11 @@ usrjquota=<file> Appoint specified file and type during mount, so that quota grpjquota=<file> information can be properly updated during recovery flow, prjjquota=<file> <quota file>: must be in root directory; jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. -offusrjquota Turn off user journelled quota. -offgrpjquota Turn off group journelled quota. -offprjjquota Turn off project journelled quota. +usrjquota= Turn off user journalled quota. +grpjquota= Turn off group journalled quota. +prjjquota= Turn off project journalled quota. quota Enable plain user disk quota accounting. noquota Disable all plain disk quota option. -whint_mode=%s Control which write hints are passed down to block - layer. This supports "off", "user-based", and - "fs-based". In "off" mode (default), f2fs does not pass - down hints. In "user-based" mode, f2fs tries to pass - down hints given by users. And in "fs-based" mode, f2fs - passes down hints with its policy. alloc_mode=%s Adjust block allocation policy, which supports "reuse" and "default". fsync_mode=%s Control the policy of fsync. Currently supports "posix", @@ -232,11 +261,11 @@ test_dummy_encryption=%s The argument may be either "v1" or "v2", in order to select the corresponding fscrypt policy version. checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" - to reenable checkpointing. Is enabled by default. While + to re-enable checkpointing. Is enabled by default. While disabled, any unmounting or unexpected shutdowns will cause the filesystem contents to appear as they did when the filesystem was mounted with that option. - While mounting with checkpoint=disabled, the filesystem must + While mounting with checkpoint=disable, the filesystem must run garbage collection to ensure that all available space can be used. If this takes too much time, the mount may return EAGAIN. You may optionally add a value to indicate how much @@ -247,11 +276,26 @@ checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enabl hide up to all remaining free space. The actual space that would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable This space is reclaimed once checkpoint=enable. +checkpoint_merge When checkpoint is enabled, this can be used to create a kernel + daemon and make it to merge concurrent checkpoint requests as + much as possible to eliminate redundant checkpoint issues. Plus, + we can eliminate the sluggish issue caused by slow checkpoint + operation when the checkpoint is done in a process context in + a cgroup having low i/o budget and cpu shares. To make this + do better, we set the default i/o priority of the kernel daemon + to "3", to give one higher priority than other kernel threads. + This is the same way to give a I/O priority to the jbd2 + journaling thread of ext4 filesystem. +nocheckpoint_merge Disable checkpoint merge feature. compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", "lz4", "zstd" and "lzo-rle" algorithm. -compress_log_size=%u Support configuring compress cluster size, the size will - be 4KB * (1 << %u), 16KB is minimum size, also it's - default size. +compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only + "lz4" and "zstd" support compress level config. + algorithm level range + lz4 3 - 16 + zstd 1 - 22 +compress_log_size=%u Support configuring compress cluster size. The size will + be 4KB * (1 << %u). The default and minimum sizes are 16KB. compress_extension=%s Support adding specified extension, so that f2fs can enable compression on those corresponding files, e.g. if all files with '.ext' has high compression rate, we can set the '.ext' @@ -260,12 +304,72 @@ compress_extension=%s Support adding specified extension, so that f2fs can enab For other files, we can still enable compression via ioctl. Note that, there is one reserved special extension '*', it can be set to enable compression for all files. +nocompress_extension=%s Support adding specified extension, so that f2fs can disable + compression on those corresponding files, just contrary to compression extension. + If you know exactly which files cannot be compressed, you can use this. + The same extension name can't appear in both compress and nocompress + extension at the same time. + If the compress extension specifies all files, the types specified by the + nocompress extension will be treated as special cases and will not be compressed. + Don't allow use '*' to specifie all file in nocompress extension. + After add nocompress_extension, the priority should be: + dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. + See more in compression sections. + +compress_chksum Support verifying chksum of raw data in compressed cluster. +compress_mode=%s Control file compression mode. This supports "fs" and "user" + modes. In "fs" mode (default), f2fs does automatic compression + on the compression enabled files. In "user" mode, f2fs disables + the automaic compression and gives the user discretion of + choosing the target file and the timing. The user can do manual + compression/decompression on the compression enabled files using + ioctls. +compress_cache Support to use address space of a filesystem managed inode to + cache compressed block, in order to improve cache hit ratio of + random read. inlinecrypt When possible, encrypt/decrypt the contents of encrypted files using the blk-crypto framework rather than filesystem-layer encryption. This allows the use of inline encryption hardware. The on-disk format is unaffected. For more details, see Documentation/block/inline-encryption.rst. +atgc Enable age-threshold garbage collection, it provides high + effectiveness and efficiency on background GC. +discard_unit=%s Control discard unit, the argument can be "block", "segment" + and "section", issued discard command's offset/size will be + aligned to the unit, by default, "discard_unit=block" is set, + so that small discard functionality is enabled. + For blkzoned device, "discard_unit=section" will be set by + default, it is helpful for large sized SMR or ZNS devices to + reduce memory cost by getting rid of fs metadata supports small + discard. +memory=%s Control memory mode. This supports "normal" and "low" modes. + "low" mode is introduced to support low memory devices. + Because of the nature of low memory devices, in this mode, f2fs + will try to save memory sometimes by sacrificing performance. + "normal" mode is the default mode and same as before. +age_extent_cache Enable an age extent cache based on rb-tree. It records + data block update frequency of the extent per inode, in + order to provide better temperature hints for data block + allocation. +errors=%s Specify f2fs behavior on critical errors. This supports modes: + "panic", "continue" and "remount-ro", respectively, trigger + panic immediately, continue without doing anything, and remount + the partition in read-only mode. By default it uses "continue" + mode. + ====================== =============== =============== ======== + mode continue remount-ro panic + ====================== =============== =============== ======== + access ops normal normal N/A + syscall errors -EIO -EROFS N/A + mount option rw ro N/A + pending dir write keep keep N/A + pending non-dir write drop keep N/A + pending node write drop keep N/A + pending meta write keep keep N/A + ====================== =============== =============== ======== +nat_bits Enable nat_bits feature to enhance full/empty nat blocks access, + by default it's disabled. ======================== ============================================================ Debugfs Entries @@ -301,7 +405,7 @@ Usage # insmod f2fs.ko -3. Create a directory trying to mount:: +3. Create a directory to use when mounting:: # mkdir /mnt/f2fs @@ -315,7 +419,7 @@ 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: +The quick options consist of: =============== =========================================================== ``-l [label]`` Give a volume label, up to 512 unicode name. @@ -337,6 +441,8 @@ The options consist of: 1 is set by default, which conducts discard. =============== =========================================================== +Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. + fsck.f2fs --------- The fsck.f2fs is a tool to check the consistency of an f2fs-formatted @@ -344,10 +450,12 @@ 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:: +The quick options consist of:: -d debug level [default:0] +Note: please refer to the manpage of fsck.f2fs(8) to get full option list. + dump.f2fs --------- The dump.f2fs shows the information of specific inode and dumps SSA and SIT to @@ -371,6 +479,37 @@ Examples:: # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) +Note: please refer to the manpage of dump.f2fs(8) to get full option list. + +sload.f2fs +---------- +The sload.f2fs gives a way to insert files and directories in the existing disk +image. This tool is useful when building f2fs images given compiled files. + +Note: please refer to the manpage of sload.f2fs(8) to get full option list. + +resize.f2fs +----------- +The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving +all the files and directories stored in the image. + +Note: please refer to the manpage of resize.f2fs(8) to get full option list. + +defrag.f2fs +----------- +The defrag.f2fs can be used to defragment scattered written data as well as +filesystem metadata across the disk. This can improve the write speed by giving +more free consecutive space. + +Note: please refer to the manpage of defrag.f2fs(8) to get full option list. + +f2fs_io +------- +The f2fs_io is a simple tool to issue various filesystem APIs as well as +f2fs-specific ones, which is very useful for QA tests. + +Note: please refer to the manpage of f2fs_io(8) to get full option list. + Design ====== @@ -383,7 +522,7 @@ 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:: +consist of multiple segments as described below:: align with the zone size <-| |-> align with the segment size @@ -486,7 +625,7 @@ one inode block (i.e., a file) covers:: `- direct node (1018) `- data (1018) -Note that, all the node blocks are mapped by NAT which means the location of +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. @@ -566,7 +705,7 @@ 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 +each levels incrementally from 1 to N. In each level F2FS needs to scan only one bucket determined by the following equation, which shows O(log(# of files)) complexity:: @@ -643,57 +782,22 @@ bitmap is composed of a bit stream covering whole blocks in main area. Write-hint Policy ----------------- -1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. - -2) whint_mode=user-based. F2FS tries to pass down hints given by -users. +F2FS sets the whint all the time with the below policy. ===================== ======================== =================== User F2FS Block ===================== ======================== =================== - META WRITE_LIFE_NOT_SET - HOT_NODE " - WARM_NODE " - COLD_NODE " +N/A META WRITE_LIFE_NONE|REQ_META +N/A HOT_NODE WRITE_LIFE_NONE +N/A WARM_NODE WRITE_LIFE_MEDIUM +N/A COLD_NODE WRITE_LIFE_LONG ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME extension list " " -- buffered io -WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME -WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT -WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET -WRITE_LIFE_NONE " " -WRITE_LIFE_MEDIUM " " -WRITE_LIFE_LONG " " - --- direct io -WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME -WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT -WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET -WRITE_LIFE_NONE " WRITE_LIFE_NONE -WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM -WRITE_LIFE_LONG " WRITE_LIFE_LONG -===================== ======================== =================== - -3) whint_mode=fs-based. F2FS passes down hints with its policy. - -===================== ======================== =================== -User F2FS Block -===================== ======================== =================== - META WRITE_LIFE_MEDIUM; - HOT_NODE WRITE_LIFE_NOT_SET - WARM_NODE " - COLD_NODE WRITE_LIFE_NONE -ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME -extension list " " - --- buffered io -WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME -WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT -WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG -WRITE_LIFE_NONE " " -WRITE_LIFE_MEDIUM " " -WRITE_LIFE_LONG " " +N/A COLD_DATA WRITE_LIFE_EXTREME +N/A HOT_DATA WRITE_LIFE_SHORT +N/A WARM_DATA WRITE_LIFE_NOT_SET -- direct io WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME @@ -707,7 +811,7 @@ WRITE_LIFE_LONG " WRITE_LIFE_LONG Fallocate(2) Policy ------------------- -The default policy follows the below posix rule. +The default policy follows the below POSIX rule. Allocating disk space The default operation (i.e., mode is zero) of fallocate() allocates @@ -720,7 +824,7 @@ Allocating disk space as a method of optimally implementing that function. However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to -fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having +fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having zero or random data, which is useful to the below scenario where: 1. create(fd) @@ -739,7 +843,7 @@ Compression implementation cluster can be compressed or not. - In cluster metadata layout, one special block address is used to indicate - cluster is compressed one or normal one, for compressed cluster, following + a cluster is a compressed one or normal one; for compressed cluster, following metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs stores data including compress header and compressed data. @@ -748,11 +852,40 @@ Compression implementation all logical blocks in cluster contain valid data and compress ratio of cluster data is lower than specified threshold. -- To enable compression on regular inode, there are three ways: +- To enable compression on regular inode, there are four ways: * chattr +c file * chattr +c dir; touch dir/file * mount w/ -o compress_extension=ext; touch file.ext + * mount w/ -o compress_extension=*; touch any_file + +- To disable compression on regular inode, there are two ways: + + * chattr -c file + * mount w/ -o nocompress_extension=ext; touch file.ext + +- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: + + * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch + dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt + should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip + can enable compress on bar.zip. + * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch + dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be + compresse, bar.zip and baz.txt should be non-compressed. + chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip + and baz.txt. + +- At this point, compression feature doesn't expose compressed space to user + directly in order to guarantee potential data updates later to the space. + Instead, the main goal is to reduce data writes to flash disk as much as + possible, resulting in extending disk life time as well as relaxing IO + congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) + interface to reclaim compressed space and show it to user after setting a + special flag to the inode. Once the compressed space is released, the flag + will block writing data to the file until either the compressed space is + reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is + truncated to zero. Compress metadata layout:: @@ -761,14 +894,101 @@ Compress metadata layout:: | cluster 1 | cluster 2 | ......... | cluster N | +-----------------------------------------------+ . . . . - . . . . + . . . . . Compressed Cluster . . Normal Cluster . +----------+---------+---------+---------+ +---------+---------+---------+---------+ |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | +----------+---------+---------+---------+ +---------+---------+---------+---------+ - . . + . . . . . . +-------------+-------------+----------+----------------------------+ | data length | data chksum | reserved | compressed data | +-------------+-------------+----------+----------------------------+ + +Compression mode +-------------------------- + +f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. +With this option, f2fs provides a choice to select the way how to compress the +compression enabled files (refer to "Compression implementation" section for how to +enable compression on a regular inode). + +1) compress_mode=fs +This is the default option. f2fs does automatic compression in the writeback of the +compression enabled files. + +2) compress_mode=user +This disables the automatic compression and gives the user discretion of choosing the +target file and the timing. The user can do manual compression/decompression on the +compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE +ioctls like the below. + +To decompress a file, + +fd = open(filename, O_WRONLY, 0); +ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); + +To compress a file, + +fd = open(filename, O_WRONLY, 0); +ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); + +NVMe Zoned Namespace devices +---------------------------- + +- ZNS defines a per-zone capacity which can be equal or less than the + zone-size. Zone-capacity is the number of usable blocks in the zone. + F2FS checks if zone-capacity is less than zone-size, if it is, then any + segment which starts after the zone-capacity is marked as not-free in + the free segment bitmap at initial mount time. These segments are marked + as permanently used so they are not allocated for writes and + consequently are not needed to be garbage collected. In case the + zone-capacity is not aligned to default segment size(2MB), then a segment + can start before the zone-capacity and span across zone-capacity boundary. + Such spanning segments are also considered as usable segments. All blocks + past the zone-capacity are considered unusable in these segments. + +Device aliasing feature +----------------------- + +f2fs can utilize a special file called a "device aliasing file." This file allows +the entire storage device to be mapped with a single, large extent, not using +the usual f2fs node structures. This mapped area is pinned and primarily intended +for holding the space. + +Essentially, this mechanism allows a portion of the f2fs area to be temporarily +reserved and used by another filesystem or for different purposes. Once that +external usage is complete, the device aliasing file can be deleted, releasing +the reserved space back to F2FS for its own use. + +<use-case> + +# ls /dev/vd* +/dev/vdb (32GB) /dev/vdc (32GB) +# mkfs.ext4 /dev/vdc +# mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb +# mount /dev/vdb /mnt/f2fs +# ls -l /mnt/f2fs +vdc.file +# df -h +/dev/vdb 64G 33G 32G 52% /mnt/f2fs + +# mount -o loop /dev/vdc /mnt/ext4 +# df -h +/dev/vdb 64G 33G 32G 52% /mnt/f2fs +/dev/loop7 32G 24K 30G 1% /mnt/ext4 +# umount /mnt/ext4 + +# f2fs_io getflags /mnt/f2fs/vdc.file +get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable +# f2fs_io setflags noimmutable /mnt/f2fs/vdc.file +get a flag on noimmutable ret=0, flags=800010 +set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable +# rm /mnt/f2fs/vdc.file +# df -h +/dev/vdb 64G 753M 64G 2% /mnt/f2fs + +So, the key idea is, user can do any file operations on /dev/vdc, and +reclaim the space after the use, while the space is counted as /data. +That doesn't require modifying partition size and filesystem format. |