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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Daniel Farina <daniel@heroku.com>
Cc: Craig Ringer <ringerc@ringerc.id.au>
Cc: Harold A. Giménez <harold.gimenez@gmail.com>
Subject: Re: [PERFORM] DELETE vs TRUNCATE explanation
Date: Sun, 15 Jul 2012 18:37:12 -0400
Message-ID: <24158.1342391832@sss.pgh.pa.us> (raw)
In-Reply-To: <19949.1342376967@sss.pgh.pa.us>
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	<19949.1342376967@sss.pgh.pa.us>

... btw, in the penny wise and pound foolish department, I observe that
smgrdounlink calls mdunlink separately for each possibly existing fork
of a relation to be dropped.  That means we are queuing a separate fsync
queue entry for each fork, and could immediately save a factor of four
in FORGET_RELATION_FSYNC traffic if we were to redefine those queue
entries as applying to all forks.  The only reason to have a per-fork
variant, AFAICS, is for smgrdounlinkfork(), which is used nowhere and
exists only because I was too chicken to remove the functionality
outright in commit ece01aae479227d9836294b287d872c5a6146a11.  But given
that we know the fsync queue can be a bottleneck, my vote is to refactor
mdunlink to apply to all forks and send only one message.

I am also wondering whether it's really necessary to send fsync request
messages for backend-local relations.  If rnode.backend says it's local,
can't we skip sending the fsync request?  All local relations are
flush-on-crash anyway, no?

			regards, tom lane



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