Simon Riggs wrote:
On Tue, 2008-12-23 at 18:23 +0200, Heikki Linnakangas wrote:
(later) OldestXmin <xid>
When a hot standby server is running read-only queries,
indicates the
current OldestXmin on the standby. The primary can refrain from
vacuuming tuples still required by the slave using this value, if so
configured.
This is all reading like you are relaying someone else's thoughts, or
that of a committee.
No, I can assure you all the confusing words are from my head only :-).
The above is the exact opposite of your position on 11 Sep, where you
said having a matching xmin between primary and standby "makes an awful
solution for high availability" which Richard, Greg, Robert at least
agreed explicitly with.
It does, for high availability. There's other use cases where it might
be desired (spreading load of read-only queries across servers). And a
softer version where the master only respects the slaves OldestXmin up
to a point is a good compromise for high availability setups too.
I haven't seen any one-size-fits-all solution to this issue, so we have
to cater for many. Note that I proposed this exact scheme, where the
slave sends its OldestXmin to the master, at the bottom of that same email.
That will ensure that the standby doesn't need to stall WAL
application because of read-only queries.
It doesn't need to. That is already optional.
Oh right. I should've added, "without having to kill queries".
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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