On Tue, 2009-02-03 at 15:55 +0100, Andres Freund wrote:
> Hi,
>
> On 02/03/2009 02:26 PM, Hannu Krosing wrote:
> >> I don't see any way around the fact that when a tuple is removed, it's
> >> gone and can't be accessed by queries. Either you don't remove it, or
> >> you kill the query.
> > Actually we came up with a solution to this - use filesystem level
> > snapshots (like LVM2+XFS or ZFS), and redirect backends with
> > long-running queries to use fs snapshot mounted to a different
> > mountpoint.
> Isn't that really, really expensive?
>
> A single write on the master logical volume yields writes of PE size
> for _every_ single snapshot (the first time the block is touched) -
> considering that there could quite many such snapshots I don't think
> that this is really feasible - io quite possible might be saturated.
If we did that we would provide an option to select the MVCC snapshot
that went with the filesystem snapshot. There need not be one per user.
-- Simon Riggs www.2ndQuadrant.comPostgreSQL Training, Services and Support