From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Alfred Perlstein <bright(at)wintelcom(dot)net>, Tatsuo Ishii <t-ishii(at)sra(dot)co(dot)jp>, vmikheev(at)SECTORBASE(dot)COM, peter_e(at)gmx(dot)net, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: RE: [COMMITTERS] pgsql/src/backend/access/transam ( xact.c xlog.c) |
Date: | 2000-11-11 19:13:02 |
Message-ID: | 200011111913.OAA23336@candle.pha.pa.us |
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Lists: | pgsql-hackers |
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> >> Not really, I thought an ack on a commit would mean that the data
> >> is actually in stable storage, breaking that would be pretty bad
> >> no?
>
> > The default is to sync on commit, but we need to give people options of
> > several seconds delay for performance reasons. Inforimx calls it
> > buffered logging, and it is used by most of the sites I know because it
> > has much better performance that sync on commit.
>
> I have to agree with Alfred here: this does not sound like a feature,
> it sounds like a horrid hack. You're giving up *all* consistency
> guarantees for a performance gain that is really going to be pretty
> minimal in the WAL context.
It does not give up consistency. The db is still consistent, it is just
consistent from a few seconds ago, rather than commit time. This is
standard Informix practice at most law firms I work with.
>
> Earlier, Vadim was talking about arranging to share fsyncs of the WAL
> log file across transactions (after writing your commit record to the
> log, sleep a few milliseconds to see if anyone else fsyncs before you
> do; if not, issue the fsync yourself). That would offer less-than-
> one-fsync-per-transaction performance without giving up any guarantees.
> If people feel a compulsion to have a tunable parameter, let 'em tune
> the length of the pre-fsync sleep ...
That would work.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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