On Fri, Jun 24, 2016 at 2:50 AM, Hugo Mills <hugo@xxxxxxxxxxxxx> wrote:
> Checksums are not parity, correct. However, every data block
> (including, I think, the parity) is checksummed and put into the csum
> tree.
I don't see how parity is checksummed. It definitely is not in the
csum tree. Two file systems, one raid5, one single, each with a single
identical file:
raid5
item 0 key (EXTENT_CSUM EXTENT_CSUM 12009865216) itemoff 16155 itemsize 128
extent csum item
single
item 0 key (EXTENT_CSUM EXTENT_CSUM 2168717312) itemoff 16155 itemsize 128
extent csum item
That's the only entry in the csum tree. The raid5 one is not 33.33%
bigger to account for the extra parity being checksummed.
Now, if parity is used to reconstruction of data, that data *is*
checksummed so if it fails checksum after reconstruction the
information is available to determine it was incorrectly
reconstructed. The notes in btrfs/raid56.c recognize the possibility
of parity corruption and how to handle it. But I think that corruption
is inferred. Maybe the parity csums are in some other metadata item,
but I don't see how it's in the csum tree.
--
Chris Murphy
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