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Re: Proposal: Snapshot cloning

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Hannu Krosing" <hannu(at)skype(dot)net>
Cc: "Jan Wieck" <JanWieck(at)Yahoo(dot)com>,"PostgreSQL Development" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal: Snapshot cloning
Date: 2007-01-26 14:36:11
Message-ID: 1169822171.3772.353.camel@silverbirch.site (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, 2007-01-26 at 16:09 +0200, Hannu Krosing wrote:
> Ühel kenal päeval, R, 2007-01-26 kell 12:25, kirjutas Simon Riggs:

> > Great idea. It can also be used by pg_dump to publish its snapshot so
> > that we can make VACUUM continue to process effectively while it pg_dump
> > is running.
> 
> Do you mean we that vacuum would clean up tuples still visible to
> pgdump ?

No, that would break MVCC. But we may have done lots of updates/deletes
that are *not* visible to any Snapshot, yet are not yet removable
because they are higher than OldestXmin but we don't know that because
previously the Snapshot details were not available. ISTM that this
proposal is a way of making the Snapshot limits publicly available so
that they can be used by VACUUM. Sure it isn't every backend, but the
details may be useful. So this is an additional benefit to this
proposal. (There's a hole in the above idea, so don't jump on my back to
explain it - I see it and am trying to work out a way around it...)

-- 
  Simon Riggs             
  EnterpriseDB   http://www.enterprisedb.com



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Subject: Re: autovacuum process handling
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