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Re: cheaper snapshots

From: Jeff Davis <pgsql(at)j-davis(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: cheaper snapshots
Date: 2011-07-28 21:25:06
Message-ID: 1311888306.13638.24.camel@jdavis-ux.asterdata.local (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Thu, 2011-07-28 at 14:27 -0400, Robert Haas wrote:
> > Right, but if the visibility order were *defined* as the order in which
> > commit records appear in WAL, that problem neatly goes away.  It's only
> > because we have the implementation artifact that "set my xid to 0 in the
> > ProcArray" is decoupled from inserting the commit record that there's
> > any difference.
> Hmm, interesting idea.  However, consider the scenario where some
> transactions are using synchronous_commit or synchronous replication,
> and others are not.  If a transaction that needs to wait (either just
> for WAL flush, or for WAL flush and synchronous replication) inserts
> its commit record, and then another transaction with
> synchronous_commit=off comes along and inserts its commit record, the
> second transaction will have to block until the first transaction is
> done waiting.  We can't make either transaction visible without making
> both visible, and we certainly can't acknowledge the second
> transaction to the client until we've made it visible.  I'm not going
> to say that's so horrible we shouldn't even consider it, but it
> doesn't seem great, either.

I'm trying to follow along here.

Wouldn't the same issue exist if one transaction is waiting for sync rep
(synchronous_commit=on), and another is waiting for just a WAL flush
(synchronous_commit=local)? I don't think that a synchronous_commit=off
is required.

	Jeff Davis

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