Re: How does btrfs behave on checksum mismatch?

[Hugo Mills <hugo@carfax.org.uk>]

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On Sat, Oct 27, 2012 at 09:56:45PM +0000, Michael Kjörling wrote:
> I came across the tidbit that ZFS has a contract guarantee that the
> data read back will either be correct (the checksum computed over the
> data read from the disk matches the checksum stored on disk), or you
> get an I/O error. Obviously, this greatly reduces the probability that
> the data is invalid. (Particularly when taken in combination with the
> disk firmware's own ECC and checksumming.)
> 
> With the default options, does btrfs make any similar guarantees? If
> not, then are there any options to force it to make such guarantees?

   It does indeed do the same thing: if the checksum doesn't match the
block, then the alternative block is read (if one exists, e.g. RAID-1,
RAID-10). If that does not exist, or also has a checksum failure, then
EIO is returned.

   Hugo.

> I'm interested in this both from a specification and an implementation
> point of view.
> 
> The last thing anyone wants is probably undetected bit rot, and with
> today's large drives, even with the quite low bit rot numbers it can
> be a real concern. If even the act of simply successfully reading a
> file guarantees, to the extent of the checksumming algorithm's ability
> to detect changes, that the data read is the same as was once written,
> that would be a major selling point for btrfs for me personally.
> 
> The closest I was able to find was that btrfs uses crc32c currently
> for data and metadata checksumming and that this can be turned off if
> so desired (using the "nodatasum" mount option), but nothing about
> what the file system code does or is supposed to do in the face of a
> checksum mismatch.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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