btrfs-progs: docs: extra notes about read-only scrub on read-write fs #935
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[BUG] There is a bug report that read-only scrub on a read-write fs still causes writes into the fs, and that will be caught if there is a read-only block device among the storage stack. This will cause a kernel warning on failed transaction commit: BTRFS info (device dm-3): first mount of filesystem e18f0c40-88de-413f-9d7e-dcc8136ad6dd BTRFS info (device dm-3): using crc32c (crc32c-intel) checksum algorithm BTRFS info (device dm-3): using free-space-tree BTRFS info (device dm-3): scrub: started on devid 1 Trying to write to read-only block-device md127 btrfs_dev_stat_inc_and_print: 362 callbacks suppressed BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 1, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 2, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 3, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 4, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 5, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 6, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 7, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 8, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 9, rd 0, flush 0, corrupt 0, gen 0 BTRFS error (device dm-3): bdev /dev/mapper/data errs: wr 10, rd 0, flush 0, corrupt 0, gen 0 BTRFS: error (device dm-3) in btrfs_commit_transaction:2523: errno=-5 IO failure (Error while writing out transaction) BTRFS info (device dm-3 state E): forced readonly BTRFS warning (device dm-3 state E): Skipping commit of aborted transaction. BTRFS error (device dm-3 state EA): Transaction aborted (error -5) BTRFS: error (device dm-3 state EA) in cleanup_transaction:2017: errno=-5 IO failure BTRFS warning (device dm-3 state EA): failed setting block group ro: -5 BTRFS info (device dm-3 state EA): scrub: not finished on devid 1 with status: -5 [CAUSE] The root cause is inside btrfs_inc_block_group_ro(), where we need to hold a transaction handle, to prevent the transaction to be committed, until we hold ro_block_group_mutex. This will cause an empty transaction by itself, thus even if we can mark the block group read-only without any extra workload, we still need to commit the new and empty transaction. Unfortunately this means RO scrub on RW filesystem will always cause the fs to be updated. [FIX] The best fix is to make btrfs to avoid empty commit transaction, but even with that done, read-only scrub on rw mount can still cause real metadata updates (e.g. allocate new chunks and update device error statistics). It will be very complex to make read-only scrub to be fully read-only on a read-write btrfs. Thankfully read-only scrub on read-write mount with read-only device in the storage stack is pretty rare, thus a documentation update should be enough. Issue: kdave#934 Signed-off-by: Qu Wenruo <[email protected]>
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[BUG]
There is a bug report that read-only scrub on a read-write fs still causes writes into the fs, and that will be caught if there is a read-only block device among the storage stack.
This will cause a kernel warning on failed transaction commit:
[CAUSE]
The root cause is inside btrfs_inc_block_group_ro(), where we need to hold a transaction handle, to prevent the transaction to be committed, until we hold ro_block_group_mutex.
This will cause an empty transaction by itself, thus even if we can mark the block group read-only without any extra workload, we still need to commit the new and empty transaction.
Unfortunately this means RO scrub on RW filesystem will always cause the fs to be updated.
[FIX]
The best fix is to make btrfs to avoid empty commit transaction, but even with that done, read-only scrub on rw mount can still cause real metadata updates (e.g. allocate new chunks and update device error statistics).
It will be very complex to make read-only scrub to be fully read-only on a read-write btrfs.
Thankfully read-only scrub on read-write mount with read-only device in the storage stack is pretty rare, thus a documentation update should be enough.
Issue: #934