diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/ABI/testing/sysfs-fs-f2fs | 280 | ||||
| -rw-r--r-- | Documentation/filesystems/f2fs.txt | 216 |
2 files changed, 219 insertions, 277 deletions
diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs index aedeae1e8ec1..1a6cd5397129 100644 --- a/Documentation/ABI/testing/sysfs-fs-f2fs +++ b/Documentation/ABI/testing/sysfs-fs-f2fs @@ -1,37 +1,40 @@ What: /sys/fs/f2fs/<disk>/gc_max_sleep_time Date: July 2013 Contact: "Namjae Jeon" <namjae.jeon@samsung.com> -Description: - Controls the maximun sleep time for gc_thread. Time - is in milliseconds. +Description: Controls the maximum sleep time for gc_thread. Time + is in milliseconds. What: /sys/fs/f2fs/<disk>/gc_min_sleep_time Date: July 2013 Contact: "Namjae Jeon" <namjae.jeon@samsung.com> -Description: - Controls the minimum sleep time for gc_thread. Time - is in milliseconds. +Description: Controls the minimum sleep time for gc_thread. Time + is in milliseconds. What: /sys/fs/f2fs/<disk>/gc_no_gc_sleep_time Date: July 2013 Contact: "Namjae Jeon" <namjae.jeon@samsung.com> -Description: - Controls the default sleep time for gc_thread. Time - is in milliseconds. +Description: Controls the default sleep time for gc_thread. Time + is in milliseconds. What: /sys/fs/f2fs/<disk>/gc_idle Date: July 2013 Contact: "Namjae Jeon" <namjae.jeon@samsung.com> -Description: - Controls the victim selection policy for garbage collection. +Description: Controls the victim selection 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 approach. What: /sys/fs/f2fs/<disk>/reclaim_segments Date: October 2013 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the issue rate of segment discard commands. - -What: /sys/fs/f2fs/<disk>/max_blkaddr +Description: 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. + +What: /sys/fs/f2fs/<disk>/main_blkaddr Date: November 2019 Contact: "Ramon Pantin" <pantin@google.com> Description: @@ -40,227 +43,278 @@ Description: What: /sys/fs/f2fs/<disk>/ipu_policy Date: November 2013 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the in-place-update policy. +Description: Controls the in-place-update policy. + updates in f2fs. User can set: + 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, + 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, + 0x10: F2FS_IPU_FSYNC, 0x20: F2FS_IPU_ASYNC, + 0x40: F2FS_IPU_NOCACHE. + Refer segment.h for details. What: /sys/fs/f2fs/<disk>/min_ipu_util Date: November 2013 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the FS utilization condition for the in-place-update - policies. +Description: Controls the FS utilization condition for the in-place-update + policies. It is used by F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies. What: /sys/fs/f2fs/<disk>/min_fsync_blocks Date: September 2014 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the dirty page count condition for the in-place-update - policies. +Description: Controls the dirty page count condition for the in-place-update + policies. What: /sys/fs/f2fs/<disk>/min_seq_blocks Date: August 2018 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the dirty page count condition for batched sequential - writes in ->writepages. - +Description: Controls the dirty page count condition for batched sequential + writes in writepages. What: /sys/fs/f2fs/<disk>/min_hot_blocks Date: March 2017 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the dirty page count condition for redefining hot data. +Description: Controls the dirty page count condition for redefining hot data. What: /sys/fs/f2fs/<disk>/min_ssr_sections Date: October 2017 Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Controls the fee section threshold to trigger SSR allocation. +Description: Controls the free section threshold to trigger SSR allocation. + If this is large, SSR mode will be enabled early. What: /sys/fs/f2fs/<disk>/max_small_discards Date: November 2013 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the issue rate of small discard commands. +Description: Controls the issue rate of discard commands that consist of 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. -What: /sys/fs/f2fs/<disk>/discard_granularity -Date: July 2017 -Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Controls discard granularity of inner discard thread, inner thread +What: /sys/fs/f2fs/<disk>/discard_granularity +Date: July 2017 +Contact: "Chao Yu" <yuchao0@huawei.com> +Description: Controls discard granularity of inner discard thread. Inner thread will not issue discards with size that is smaller than granularity. - The unit size is one block, now only support configuring in range - of [1, 512]. + The unit size is one block(4KB), now only support configuring + in range of [1, 512]. Default value is 4(=16KB). -What: /sys/fs/f2fs/<disk>/umount_discard_timeout -Date: January 2019 -Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Set timeout to issue discard commands during umount. - Default: 5 secs +What: /sys/fs/f2fs/<disk>/umount_discard_timeout +Date: January 2019 +Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> +Description: Set timeout to issue discard commands during umount. + Default: 5 secs What: /sys/fs/f2fs/<disk>/max_victim_search Date: January 2014 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the number of trials to find a victim segment. +Description: 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. What: /sys/fs/f2fs/<disk>/migration_granularity Date: October 2018 Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Controls migration granularity of garbage collection on large - section, it can let GC move partial segment{s} of one section - in one GC cycle, so that dispersing heavy overhead GC to - multiple lightweight one. +Description: Controls migration granularity of garbage collection on large + section, it can let GC move partial segment{s} of one section + in one GC cycle, so that dispersing heavy overhead GC to + multiple lightweight one. What: /sys/fs/f2fs/<disk>/dir_level Date: March 2014 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the directory level for large directory. +Description: Controls the directory level for 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. What: /sys/fs/f2fs/<disk>/ram_thresh Date: March 2014 Contact: "Jaegeuk Kim" <jaegeuk.kim@samsung.com> -Description: - Controls the memory footprint used by f2fs. +Description: Controls the memory footprint used by free nids and cached + nat entries. By default, 1 is set, which indicates + 10 MB / 1 GB RAM. What: /sys/fs/f2fs/<disk>/batched_trim_sections Date: February 2015 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the trimming rate in batch mode. - <deprecated> +Description: Controls the trimming rate in batch mode. + <deprecated> What: /sys/fs/f2fs/<disk>/cp_interval Date: October 2015 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the checkpoint timing. +Description: Controls the checkpoint timing, set to 60 seconds by default. What: /sys/fs/f2fs/<disk>/idle_interval Date: January 2016 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls the idle timing for all paths other than - discard and gc path. +Description: Controls the idle timing of system, if there is no FS operation + during given interval. + Set to 5 seconds by default. What: /sys/fs/f2fs/<disk>/discard_idle_interval Date: September 2018 Contact: "Chao Yu" <yuchao0@huawei.com> Contact: "Sahitya Tummala" <stummala@codeaurora.org> -Description: - Controls the idle timing for discard path. +Description: Controls the idle timing of discard thread given + this time interval. + Default is 5 secs. What: /sys/fs/f2fs/<disk>/gc_idle_interval Date: September 2018 Contact: "Chao Yu" <yuchao0@huawei.com> Contact: "Sahitya Tummala" <stummala@codeaurora.org> -Description: - Controls the idle timing for gc path. +Description: Controls the idle timing for gc path. Set to 5 seconds by default. What: /sys/fs/f2fs/<disk>/iostat_enable Date: August 2017 Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Controls to enable/disable IO stat. +Description: Controls to enable/disable IO stat. What: /sys/fs/f2fs/<disk>/ra_nid_pages Date: October 2015 Contact: "Chao Yu" <chao2.yu@samsung.com> -Description: - Controls the count of nid pages to be readaheaded. +Description: Controls the count of nid pages to be readaheaded. + When building free nids, F2FS reads NAT blocks ahead for + speed up. Default is 0. What: /sys/fs/f2fs/<disk>/dirty_nats_ratio Date: January 2016 Contact: "Chao Yu" <chao2.yu@samsung.com> -Description: - Controls dirty nat entries ratio threshold, if current - ratio exceeds configured threshold, checkpoint will - be triggered for flushing dirty nat entries. +Description: Controls dirty nat entries ratio threshold, if current + ratio exceeds configured threshold, checkpoint will + be triggered for flushing dirty nat entries. What: /sys/fs/f2fs/<disk>/lifetime_write_kbytes Date: January 2016 Contact: "Shuoran Liu" <liushuoran@huawei.com> -Description: - Shows total written kbytes issued to disk. +Description: Shows total written kbytes issued to disk. What: /sys/fs/f2fs/<disk>/features Date: July 2017 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Shows all enabled features in current device. +Description: Shows all enabled features in current device. What: /sys/fs/f2fs/<disk>/inject_rate Date: May 2016 Contact: "Sheng Yong" <shengyong1@huawei.com> -Description: - Controls the injection rate. +Description: Controls the injection rate of arbitrary faults. What: /sys/fs/f2fs/<disk>/inject_type Date: May 2016 Contact: "Sheng Yong" <shengyong1@huawei.com> -Description: - Controls the injection type. +Description: Controls the injection type of arbitrary faults. + +What: /sys/fs/f2fs/<disk>/dirty_segments +Date: October 2017 +Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> +Description: Shows the number of dirty segments. What: /sys/fs/f2fs/<disk>/reserved_blocks Date: June 2017 Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Controls target reserved blocks in system, the threshold - is soft, it could exceed current available user space. +Description: Controls target reserved blocks in system, the threshold + is soft, it could exceed current available user space. What: /sys/fs/f2fs/<disk>/current_reserved_blocks Date: October 2017 Contact: "Yunlong Song" <yunlong.song@huawei.com> Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Shows current reserved blocks in system, it may be temporarily - smaller than target_reserved_blocks, but will gradually - increase to target_reserved_blocks when more free blocks are - freed by user later. +Description: Shows current reserved blocks in system, it may be temporarily + smaller than target_reserved_blocks, but will gradually + increase to target_reserved_blocks when more free blocks are + freed by user later. What: /sys/fs/f2fs/<disk>/gc_urgent Date: August 2017 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Do background GC agressively +Description: Do background GC agressively when set. When gc_urgent = 1, + background thread starts to do GC by given gc_urgent_sleep_time + interval. It is set to 0 by default. What: /sys/fs/f2fs/<disk>/gc_urgent_sleep_time Date: August 2017 Contact: "Jaegeuk Kim" <jaegeuk@kernel.org> -Description: - Controls sleep time of GC urgent mode +Description: Controls sleep time of GC urgent mode. Set to 500ms by default. What: /sys/fs/f2fs/<disk>/readdir_ra Date: November 2017 Contact: "Sheng Yong" <shengyong1@huawei.com> -Description: - Controls readahead inode block in readdir. +Description: Controls readahead inode block in readdir. Enabled by default. + +What: /sys/fs/f2fs/<disk>/gc_pin_file_thresh +Date: January 2018 +Contact: Jaegeuk Kim <jaegeuk@kernel.org> +Description: This indicates how many GC can be failed for the pinned + file. If it exceeds this, F2FS doesn't guarantee its pinning + state. 2048 trials is set by default. What: /sys/fs/f2fs/<disk>/extension_list Date: Feburary 2018 Contact: "Chao Yu" <yuchao0@huawei.com> -Description: - Used to control configure extension list: - - Query: cat /sys/fs/f2fs/<disk>/extension_list - - Add: echo '[h/c]extension' > /sys/fs/f2fs/<disk>/extension_list - - Del: echo '[h/c]!extension' > /sys/fs/f2fs/<disk>/extension_list - - [h] means add/del hot file extension - - [c] means add/del cold file extension +Description: Used to control configure extension list: + - Query: cat /sys/fs/f2fs/<disk>/extension_list + - Add: echo '[h/c]extension' > /sys/fs/f2fs/<disk>/extension_list + - Del: echo '[h/c]!extension' > /sys/fs/f2fs/<disk>/extension_list + - [h] means add/del hot file extension + - [c] means add/del cold file extension What: /sys/fs/f2fs/<disk>/unusable Date April 2019 Contact: "Daniel Rosenberg" <drosen@google.com> -Description: - If checkpoint=disable, it displays the number of blocks that are unusable. - If checkpoint=enable it displays the enumber of blocks that would be unusable - if checkpoint=disable were to be set. +Description: If checkpoint=disable, it displays the number of blocks that + are unusable. + If checkpoint=enable it displays the enumber of blocks that + would be unusable if checkpoint=disable were to be set. What: /sys/fs/f2fs/<disk>/encoding Date July 2019 Contact: "Daniel Rosenberg" <drosen@google.com> -Description: - Displays name and version of the encoding set for the filesystem. - If no encoding is set, displays (none) +Description: Displays name and version of the encoding set for the filesystem. + If no encoding is set, displays (none) + +What: /sys/fs/f2fs/<disk>/free_segments +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of free segments in disk. + +What: /sys/fs/f2fs/<disk>/cp_foreground_calls +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of checkpoint operations performed on demand. Available when + CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/cp_background_calls +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of checkpoint operations performed in the background to + free segments. Available when CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/gc_foreground_calls +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of garbage collection operations performed on demand. + Available when CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/gc_background_calls +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of garbage collection operations triggered in background. + Available when CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/moved_blocks_foreground +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of blocks moved by garbage collection in foreground. + Available when CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/moved_blocks_background +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Number of blocks moved by garbage collection in background. + Available when CONFIG_F2FS_STAT_FS=y. + +What: /sys/fs/f2fs/<disk>/avg_vblocks +Date: September 2019 +Contact: "Hridya Valsaraju" <hridya@google.com> +Description: Average number of valid blocks. + Available when CONFIG_F2FS_STAT_FS=y. diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt index 3135b80df6da..4eb3e2ddd00e 100644 --- a/Documentation/filesystems/f2fs.txt +++ b/Documentation/filesystems/f2fs.txt @@ -235,6 +235,17 @@ checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "en 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. +compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo" + and "lz4" 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_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' + on compression extension list and enable compression on + these file by default rather than to enable it via ioctl. + For other files, we can still enable compression via ioctl. ================================================================================ DEBUGFS ENTRIES @@ -259,170 +270,6 @@ 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_urgent_sleep_time This parameter controls sleep time for gc_urgent. - 500 ms is set by default. See above gc_urgent. - - gc_min_sleep_time This tuning parameter controls the minimum sleep - time for the garbage collection thread. Time is - in milliseconds. - - gc_max_sleep_time This tuning parameter controls the maximum 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 approach. - - gc_urgent This parameter controls triggering background GCs - urgently or not. Setting gc_urgent = 0 [default] - makes back to default behavior, while if it is set - to 1, background thread starts to do GC by given - gc_urgent_sleep_time interval. - - 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. - - main_blkaddr This value gives the first block address of - MAIN area in the partition. - - 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. - - discard_granularity This parameter controls the granularity of discard - command size. It will issue discard commands iif - the size is larger than given granularity. Its - unit size is 4KB, and 4 (=16KB) is set by default. - The maximum value is 128 (=512KB). - - reserved_blocks This parameter indicates the number of blocks that - f2fs reserves internally for root. - - batched_trim_sections This parameter controls the number of sections - to be trimmed out in batch mode when FITRIM - conducts. 32 sections is set by default. - - ipu_policy This parameter controls the policy of in-place - updates in f2fs. There are five policies: - 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, - 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, - 0x10: F2FS_IPU_FSYNC. - - 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. - - min_seq_blocks This parameter controls the threshold to serialize - write IOs issued by multiple threads in parallel. - - min_hot_blocks This parameter controls the threshold to allocate - a hot data log for pending data blocks to write. - - min_ssr_sections This parameter adds the threshold when deciding - SSR block allocation. If this is large, SSR mode - will be enabled early. - - ram_thresh This parameter controls the memory footprint used - by free nids and cached nat entries. By default, - 1 is set, which indicates 10 MB / 1 GB RAM. - - ra_nid_pages When building free nids, F2FS reads NAT blocks - ahead for speed up. Default is 0. - - dirty_nats_ratio Given dirty ratio of cached nat entries, F2FS - determines flushing them in background. - - 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. - - migration_granularity For large-sized sections, F2FS can stop GC given - this granularity instead of reclaiming entire - section. - - 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. - - cp_interval F2FS tries to do checkpoint periodically, 60 secs - by default. - - idle_interval F2FS detects system is idle, if there's no F2FS - operations during given interval, 5 secs by - default. - - discard_idle_interval F2FS detects the discard thread is idle, given - time interval. Default is 5 secs. - - gc_idle_interval F2FS detects the GC thread is idle, given time - interval. Default is 5 secs. - - umount_discard_timeout When unmounting the disk, F2FS waits for finishing - queued discard commands which can take huge time. - This gives time out for it, 5 secs by default. - - iostat_enable This controls to enable/disable iostat in F2FS. - - readdir_ra This enables/disabled readahead of inode blocks - in readdir, and default is enabled. - - gc_pin_file_thresh This indicates how many GC can be failed for the - pinned file. If it exceeds this, F2FS doesn't - guarantee its pinning state. 2048 trials is set - by default. - - extension_list This enables to change extension_list for hot/cold - files in runtime. - - inject_rate This controls injection rate of arbitrary faults. - - inject_type This controls injection type of arbitrary faults. - - dirty_segments This shows # of dirty segments. - - lifetime_write_kbytes This shows # of data written to the disk. - - features This shows current features enabled on F2FS. - - current_reserved_blocks This shows # of blocks currently reserved. - - unusable If checkpoint=disable, this shows the number of - blocks that are unusable. - If checkpoint=enable it shows the number of blocks - that would be unusable if checkpoint=disable were - to be set. - -encoding This shows the encoding used for casefolding. - If casefolding is not enabled, returns (none) ================================================================================ USAGE @@ -840,3 +687,44 @@ zero or random data, which is useful to the below scenario where: 4. address = fibmap(fd, offset) 5. open(blkdev) 6. write(blkdev, address) + +Compression implementation +-------------------------- + +- New term named cluster is defined as basic unit of compression, file can +be divided into multiple clusters logically. One cluster includes 4 << n +(n >= 0) logical pages, compression size is also cluster size, each of +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 +metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs +stores data including compress header and compressed data. + +- In order to eliminate write amplification during overwrite, F2FS only +support compression on write-once file, data can be compressed only when +all logical blocks in file are valid and cluster compress ratio is lower +than specified threshold. + +- To enable compression on regular inode, there are three ways: +* chattr +c file +* chattr +c dir; touch dir/file +* mount w/ -o compress_extension=ext; touch file.ext + +Compress metadata layout: + [Dnode Structure] + +-----------------------------------------------+ + | 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 | + +-------------+-------------+----------+----------------------------+ |