Re: Hot Backup with rsync fails at pg_clog if under load

On 27.10.2011 09:57, Heikki Linnakangas wrote:
>> My suggestion is to fix the CLOG problem in that same way that you fixed
>> the SUBTRANS problem, i.e. by moving LogStandbySnapshot() to before
>> CheckPointGuts().
>>
>> Here's what I image CreateCheckPoint() should look like:
>>
>> 1) LogStandbySnapshot() and fill out oldestActiveXid
>> 2) Fill out REDO
>> 3) Wait for concurrent commits
>> 4) Fill out nextXid and the other fields
>> 5) CheckPointGuts()
>> 6) Rest
>>
>> It's then no longer necessary for LogStandbySnapshot() do modify
>> the nextXid, since we fill out nextXid after LogStandbySnapshot() and
>> will thus derive a higher value than LogStandbySnapshot() would have.
>
> Hmm, I don't think that fully fixes the problem. Even if you're certain
> that CheckPointGuts() has fsync'd the clog page to disk, VACUUM might
> decide to truncate it away again while the checkpoint is running.

Oh, scratch that. During recovery, we merrily treat missing slru files
as they were filled with zeros.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com