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Re: cheaper snapshots

From: Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Hannu Krosing <hannu(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: cheaper snapshots
Date: 2011-07-29 00:12:07
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, Jul 29, 2011 at 2:20 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Well, again, there are three levels:
> (A) synchronous_commit=off.  No waiting!
> (B) synchronous_commit=local transactions, and synchronous_commit=on
> transactions when sync rep is not in use.  Wait for xlog flush.
> (C) synchronous_commit=on transactions when sync rep IS in use.  Wait
> for xlog flush and replication.
> So basically, you can't be more asynchronous than the guy in front of
> you.

(A) still gives a guarantee - transactions that begin after the commit
returns see
the commited transaction. A weaker variant would say that if the commit
returns, and the server doesn't crash in the meantime, the commit would at
some point become visible. Maybe even that transactions that begin after the
commit returns become visible after that commit.

Ants Aasma

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