Andrei Borzenkov wrote:
02.05.2018 21:17, waxhead пишет:
Goffredo Baroncelli wrote:
On 05/02/2018 06:55 PM, waxhead wrote:
So again, which problem would solve having the parity checksummed ?
On the best of my knowledge nothing. In any case the data is
checksummed so it is impossible to return corrupted data (modulo bug
:-) ).
I am not a BTRFS dev , but this should be quite easy to answer.
Unless you checksum the parity there is no way to verify that that
the data (parity) you use to reconstruct other data is correct.
In any case you could catch that the compute data is wrong, because
the data is always checksummed. And in any case you must check the
data against their checksum.
What if you lost an entire disk?
How does it matter exactly? RAID is per chunk anyway.
It does not matter. I was wrong, got bitten by thinking about BTRFS
"RAID5" as normal RAID5. Again a good reason to change the naming for it
I think...
or had corruption for both data AND checksum?
By the same logic you may have corrupted parity and its checksum.
Yup. Indeed
How do you plan to safely reconstruct that without checksummed
parity?
Define "safely". The main problem of current RAID56 implementation is
that stripe is not updated atomically (at least, that is what I
understood from the past discussions) and this is not solved by having
extra parity checksum. So how exactly "safety" is improved here? You
still need overall checksum to verify result of reconstruction, what
exactly extra parity checksum buys you?
> [...]
Again - please describe when having parity checksum will be beneficial
over current implementation. You do not reconstruct anything as long as
all data strips are there, so parity checksum will not be used. If one
data strip fails (including checksum) it will be reconstructed and
verified. If parity itself is corrupted, checksum verification fails
(hopefully). How is it different from verifying parity checksum before
reconstructing? In both cases data cannot be reconstructed, end of story.
Ok, before attempting and answer I have to admit that I do not know
enough about how RAID56 is laid out on disk in BTRFS terms. Is data
checksummed pr. stripe or pr. disk? Is parity calculated on the data
only or is it calculated on the data+checksum ?!
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