Re: Behavior for crash recovery when it detects a corrupt WAL record

On 10.10.2012 17:37, Amit Kapila wrote:
> On Tuesday, October 09, 2012 7:38 PM Heikki Linnakangas wrote:
>> We rely on the CRC to detect end of WAL during recovery. If the
>> system crashes while the WAL is being flushed to disk, it's normal that
>> there's a corrupt (ie. partially written) record at the end of the WAL.
>> This is a common technique used by pretty much every system with a
>> transaction log / journal.
>
> Yeah, Can't we check if there is a next valid page, then it can be
> derived that current page has some corruption and not a partial page
> write problem.

No. The OS or disk controller can flush the pages out-of-order, so on
recovery, it's entirely possible that the next page is valid even if the
previous one is not.

BTW, this means that the CRC on WAL records can *not* be used to detect
random corruption of the WAL, because if will be confused with
end-of-WAL. I don't think many people realize that. You will have to use
a filesystem with checksums if you want to detect random bit errors etc.
in the WAL. In crash recovery, anyway; in archive recovery or
replication you can make more assumptions.

- Heikki