Re: RAID-6 and write hole with write-intent bitmap

antlists
2020-11-24T21:51:17Z

On 24/11/2020 18:50, Mukund Sivaraman wrote:
> (a) With RAID-5, assuming there are 4 member disks A, B, C, D, a write
> operation with its data on disk A and stripe's parity on disk B may
> involve:

Close
>
> 1. a read of the stripe
> 2. update of data in the stripe
2.5 write A^C^D
> 3. computation and update of parity A^C^D on B
>
> These are not atomic updates. If power is lost between steps 2 and 3,
> upon recovery the mismatch between data and parity for the stripe would
> be found and the parity can be updated on B. The data chunk written to A
> may be incomplete if power is lost during step 2, but the ext4's journal
> would return the FS to a consistent state. Moreover, there should not be
> any modification/corruption of data in the stripe on disks C and D
> (assuming the disks are OK).

I *don't* think that's necessarily true. Yes it probably is true, but
it's not guaranteed ...
>
> (b) With RAID-6, assuming there are 5 member disks A, B, C, D, E, a
> write operation with its data on disk A and stripe's parity on disks
> B(p) and C(q) would involve:
>
> 1. a read of the stripe
> 2. update of data in stripe
2.5 write A^D^E
> 3. computation and update of parity on B(p)
> 4. update of parity on C(q)

Cheers,
Wol