Re: RAID56 discussion related to RST. (Was "Re: [RFC ONLY 0/8] btrfs: introduce raid-stripe-tree")

Chris Murphy
2022-07-15T20:14:36Z

On Fri, Jul 15, 2022 at 1:55 PM Goffredo Baroncelli <kreijack@libero.it> wrote:
>
> On 14/07/2022 09.46, Johannes Thumshirn wrote:
> > On 14.07.22 09:32, Qu Wenruo wrote:
> >>[...]
> >
> > Again if you're doing sub-stripe size writes, you're asking stupid things and
> > then there's no reason to not give the user stupid answers.
> >
>
> Qu is right, if we consider only full stripe write the "raid hole" problem
> disappear, because if a "full stripe" is not fully written it is not
> referenced either.
>
>
> Personally I think that the ZFS variable stripe size, may be interesting
> to evaluate. Moreover, because the BTRFS disk format is quite flexible,
> we can store different BG with different number of disks. Let me to make an
> example: if we have 10 disks, we could allocate:
> 1 BG RAID1
> 1 BG RAID5, spread over 4 disks only
> 1 BG RAID5, spread over 8 disks only
> 1 BG RAID5, spread over 10 disks
>
> So if we have short writes, we could put the extents in the RAID1 BG; for longer
> writes we could use a RAID5 BG with 4 or 8 or 10 disks depending by length
> of the data.
>
> Yes this would require a sort of garbage collector to move the data to the biggest
> raid5 BG, but this would avoid (or reduce) the fragmentation which affect the
> variable stripe size.
>
> Doing so we don't need any disk format change and it would be backward compatible.

My 2 cents...

Regarding the current raid56 support, in order of preference:

a. Fix the current bugs, without changing format. Zygo has an extensive list.
b. Mostly fix the write hole, also without changing the format, by
only doing COW with full stripe writes. Yes you could somehow get
corrupt parity still and not know it until degraded operation produces
a bad reconstruction of data - but checksum will still catch that.
This kind of "unreplicated corruption" is not quite the same thing as
the write hole, because it isn't pernicious like the write hole.
c. A new de-clustered parity raid56 implementation that is not
backwards compatible.

Ergo, I think it's best to not break the format twice. Even if a new
raid implementation is years off.

Metadata centric workloads suck on parity raid anyway. If Btrfs always
does full stripe COW won't matter even if the performance is worse
because no one should use parity raid for this workload anyway.

--
Chris Murphy