| From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)2ndquadrant(dot)com> |
| Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Bruce Momjian <bruce(at)momjian(dot)us>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Sawada Masahiko <sawada(dot)mshk(at)gmail(dot)com>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Samrat Revagade <revagade(dot)samrat(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Patch for fail-back without fresh backup |
| Date: | 2014-01-16 17:25:51 |
| Message-ID: | CAMkU=1zNE0DzFgKi9J+czJCcium2vLESP_KFy5y_Jr37onQPgQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Nov 21, 2013 at 2:43 PM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:
> On 2013-11-21 14:40:36 -0800, Jeff Janes wrote:
> > But if the transaction would not have otherwise generated WAL (i.e. a
> > select that did not have to do any HOT pruning, or an update with zero
> rows
> > matching the where condition), doesn't it now have to flush and wait when
> > it would otherwise not?
>
> We short circuit that if there's no xid assigned. Check
> RecordTransactionCommit().
>
It looks like that only short-circuits the flush if both there is no xid
assigned, and !wrote_xlog. (line 1054 of xact.c)
I do see stalls on fdatasync on flush from select statements which had no
xid, but did generate xlog due to HOT pruning, I don't see why WAL logging
hint bits would be different.
Cheers,
Jeff
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