Re: Block-level CRC checks

Heikki Linnakangas wrote:
> Alvaro Herrera wrote:
>> XFS, for example, zeroes out during recovery any block
>> that was written to but not fsync'ed before a crash. This means that if
>> we change a hint bit after a checkpoing and mark the page dirty, the
>> system can write the page. Suppose we crash at this point. On
>> recovery, XFS will zero out the block, but there will be nothing with
>> which to recovery it, because there's no backup block ...
>
> Really? That would mean that you're prone to lose data if you run
> PostgreSQL on XFS, even without the CRC patch.
>
> I doubt that's true, though. Google found this:
>
> http://marc.info/?l=linux-xfs&m=122549156102504&w=2

Ah, there's no problem here then. This email mentions another one by
"Eric" which is this one:
http://marc.info/?l=linux-xfs&m=122546510218150&w=2
It contains more information about the problem.

> Although, Florian Weimer suggested earlier in this thread that IBM DTLA
> disks have exactly that problem; a sector could be zero-filled if the
> write is interrupted.

Hmm.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support