From: | Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> |
---|---|
To: | Amit Kapila <amit(dot)kapila(at)huawei(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Behavior for crash recovery when it detects a corrupt WAL record |
Date: | 2012-10-10 14:50:45 |
Message-ID: | 50758B45.9000901@vmware.com |
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Lists: | pgsql-hackers |
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
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