| From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
|---|---|
| To: | Florian Pflug <fgp(at)phlo(dot)org> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org, Bruce Momjian <bruce(at)momjian(dot)us>, Greg Smith <greg(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com> |
| Subject: | Re: max_standby_delay considered harmful |
| Date: | 2010-05-10 12:00:45 |
| Message-ID: | 4BE7F56D.1020305@enterprisedb.com |
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| Lists: | pgsql-hackers |
Florian Pflug wrote:
> On May 10, 2010, at 11:43 , Heikki Linnakangas wrote:
>> If you're not going to apply any more WAL records before shutdown, you
>> could also just release all the AccessExclusiveLocks held by the startup
>> process. Whatever the transaction was doing with the locked relation, if
>> we're not going to replay any more WAL records before shutdown, we will
>> not see the transaction committing or doing anything else with the
>> relation, so we should be safe. Whatever state the data on disk is in,
>> it must be valid, or we would have a problem with crash recovery
>> recovering up to this WAL record and then starting up too.
>
> Sounds plausible. But wouldn't this imply that HS could *always* postpone the acquisition of an AccessExclusiveLocks until right before the corresponding commit record is replayed? If fail to see a case where this would fail, yet recovery in case of an intermediate crash would be correct.
I guess it could in some situations, but for example the
AccessExclusiveLock taken at the end of lazy vacuum to truncate the
relation must be held during the truncation, or concurrent readers will
get upset.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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