From: Yu Kuai <yuku...@huawei.com> Changes from v2: - add comments about KOBJECT_CHANGE uevent in patch 6; - convert llbitmap_suspend() to llbitmap_suspend_timeout() in patch 11; - add some comments in patch 11; - add review tag: - patch 3,4,5,9 from Hannes Changes from v1: - explain md_bitmap_fn in commit message, patch 3; - handle the case CONFIG_MD_BITMAP is disabled, patch 4; - split patch 7 from v1 into patch 5 + 6; - rewrite bitmap_type_store, patch 5; - fix dm-raid regerssion that md-bitmap sysfs entries should not be created under mddev kobject, patch 6 - merge llbitmap patches into one patch, with lots of cleanups; - add review tag: - patch 1,2,7,8,9,10 from Christoph - patch 1,2,7,8,10 from Hannes - patch 1,2,3,7 from Xiao
v2: https://lore.kernel.org/all/20250707165202.11073-12-yuk...@kernel.org/ v1: https://lore.kernel.org/all/20250524061320.370630-1-yuku...@huaweicloud.com/ RFC: https://lore.kernel.org/all/20250512011927.2809400-1-yuku...@huaweicloud.com/ Branch for review: https://git.kernel.org/pub/scm/linux/kernel/git/yukuai/linux.git/log/?h=yukuai/md-llbitmap-v3 #### Background Redundant data is used to enhance data fault tolerance, and the storage method for redundant data vary depending on the RAID levels. And it's important to maintain the consistency of redundant data. Bitmap is used to record which data blocks have been synchronized and which ones need to be resynchronized or recovered. Each bit in the bitmap represents a segment of data in the array. When a bit is set, it indicates that the multiple redundant copies of that data segment may not be consistent. Data synchronization can be performed based on the bitmap after power failure or readding a disk. If there is no bitmap, a full disk synchronization is required. #### Key Features - IO fastpath is lockless, if user issues lots of write IO to the same bitmap bit in a short time, only the first write have additional overhead to update bitmap bit, no additional overhead for the following writes; - support only resync or recover written data, means in the case creating new array or replacing with a new disk, there is no need to do a full disk resync/recovery; #### Key Concept ##### State Machine Each bit is one byte, contain 6 difference state, see llbitmap_state. And there are total 8 differenct actions, see llbitmap_action, can change state: llbitmap state machine: transitions between states | | Startwrite | Startsync | Endsync | Abortsync| | --------- | ---------- | --------- | ------- | ------- | | Unwritten | Dirty | x | x | x | | Clean | Dirty | x | x | x | | Dirty | x | x | x | x | | NeedSync | x | Syncing | x | x | | Syncing | x | Syncing | Dirty | NeedSync | | | Reload | Daemon | Discard | Stale | | --------- | -------- | ------ | --------- | --------- | | Unwritten | x | x | x | x | | Clean | x | x | Unwritten | NeedSync | | Dirty | NeedSync | Clean | Unwritten | NeedSync | | NeedSync | x | x | Unwritten | x | | Syncing | NeedSync | x | Unwritten | NeedSync | Typical scenarios: 1) Create new array All bits will be set to Unwritten by default, if --assume-clean is set, all bits will be set to Clean instead. 2) write data, raid1/raid10 have full copy of data, while raid456 doesn't and rely on xor data 2.1) write new data to raid1/raid10: Unwritten --StartWrite--> Dirty 2.2) write new data to raid456: Unwritten --StartWrite--> NeedSync Because the initial recover for raid456 is skipped, the xor data is not build yet, the bit must set to NeedSync first and after lazy initial recover is finished, the bit will finially set to Dirty(see 5.1 and 5.4); 2.3) cover write Clean --StartWrite--> Dirty 3) daemon, if the array is not degraded: Dirty --Daemon--> Clean For degraded array, the Dirty bit will never be cleared, prevent full disk recovery while readding a removed disk. 4) discard {Clean, Dirty, NeedSync, Syncing} --Discard--> Unwritten 5) resync and recover 5.1) common process NeedSync --Startsync--> Syncing --Endsync--> Dirty --Daemon--> Clean 5.2) resync after power failure Dirty --Reload--> NeedSync 5.3) recover while replacing with a new disk By default, the old bitmap framework will recover all data, and llbitmap implement this by a new helper, see llbitmap_skip_sync_blocks: skip recover for bits other than dirty or clean; 5.4) lazy initial recover for raid5: By default, the old bitmap framework will only allow new recover when there are spares(new disk), a new recovery flag MD_RECOVERY_LAZY_RECOVER is add to perform raid456 lazy recover for set bits(from 2.2). ##### Bitmap IO ##### Chunksize The default bitmap size is 128k, incluing 1k bitmap super block, and the default size of segment of data in the array each bit(chunksize) is 64k, and chunksize will adjust to twice the old size each time if the total number bits is not less than 127k.(see llbitmap_init) ##### READ While creating bitmap, all pages will be allocated and read for llbitmap, there won't be read afterwards ##### WRITE WRITE IO is divided into logical_block_size of the array, the dirty state of each block is tracked independently, for example: each page is 4k, contain 8 blocks; each block is 512 bytes contain 512 bit; | page0 | page1 | ... | page 31 | | | | \-----------------------\ | | | block0 | block1 | ... | block 8| | | | \-----------------\ | | | bit0 | bit1 | ... | bit511 | >From IO path, if one bit is changed to Dirty or NeedSync, the corresponding subpage will be marked dirty, such block must write first before the IO is issued. This behaviour will affect IO performance, to reduce the impact, if multiple bits are changed in the same block in a short time, all bits in this block will be changed to Dirty/NeedSync, so that there won't be any overhead until daemon clears dirty bits. ##### Dirty Bits syncronization IO fast path will set bits to dirty, and those dirty bits will be cleared by daemon after IO is done. llbitmap_page_ctl is used to synchronize between IO path and daemon; IO path: 1) try to grab a reference, if succeed, set expire time after 5s and return; 2) if failed to grab a reference, wait for daemon to finish clearing dirty bits; Daemon(Daemon will be waken up every daemon_sleep seconds): For each page: 1) check if page expired, if not skip this page; for expired page: 2) suspend the page and wait for inflight write IO to be done; 3) change dirty page to clean; 4) resume the page; Performance Test: Simple fio randwrite test to build array with 20GB ramdisk in my VM: | | none | bitmap | llbitmap | | -------------------- | --------- | --------- | --------- | | raid1 | 13.7MiB/s | 9696KiB/s | 19.5MiB/s | | raid1(assume clean) | 19.5MiB/s | 11.9MiB/s | 19.5MiB/s | | raid10 | 21.9MiB/s | 11.6MiB/s | 27.8MiB/s | | raid10(assume clean) | 27.8MiB/s | 15.4MiB/s | 27.8MiB/s | | raid5 | 14.0MiB/s | 11.6MiB/s | 12.9MiB/s | | raid5(assume clean) | 17.8MiB/s | 13.4MiB/s | 13.9MiB/s | For raid1/raid10 llbitmap can be better than none bitmap with background initial resync, and it's the same as none bitmap without it. Noted that llbitmap performance improvement for raid5 is not obvious, this is due to raid5 has many other performance bottleneck, perf results still shows that bitmap overhead will be much less. Yu Kuai (11): md: add a new parameter 'offset' to md_super_write() md: factor out a helper raid_is_456() md/md-bitmap: support discard for bitmap ops md: add a new mddev field 'bitmap_id' md/md-bitmap: add a new sysfs api bitmap_type md/md-bitmap: delay registration of bitmap_ops until creating bitmap md/md-bitmap: add a new method skip_sync_blocks() in bitmap_operations md/md-bitmap: add a new method blocks_synced() in bitmap_operations md: add a new recovery_flag MD_RECOVERY_LAZY_RECOVER md/md-bitmap: make method bitmap_ops->daemon_work optional md/md-llbitmap: introduce new lockless bitmap Documentation/admin-guide/md.rst | 84 +- drivers/md/Kconfig | 11 + drivers/md/Makefile | 1 + drivers/md/md-bitmap.c | 15 +- drivers/md/md-bitmap.h | 45 +- drivers/md/md-llbitmap.c | 1564 ++++++++++++++++++++++++++++++ drivers/md/md.c | 289 ++++-- drivers/md/md.h | 20 +- drivers/md/raid5.c | 6 + 9 files changed, 1927 insertions(+), 108 deletions(-) create mode 100644 drivers/md/md-llbitmap.c -- 2.39.2
