| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| Cc: | pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Deriving Recovery Snapshots |
| Date: | 2008-10-15 19:58:16 |
| Message-ID: | 1224100696.6254.16.camel@dell.linuxdev.us.dell.com |
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| Lists: | pgsql-hackers |
On Tue, 2008-10-14 at 18:50 +0100, Simon Riggs wrote:
> I've worked out what I think is a workable, efficient process for
> deriving snapshots during recovery. I will be posting a patch to show
> how this works tomorrow [Wed 15 Oct], just doing cleanup now.
How will this interact with an idea like this?:
http://archives.postgresql.org/pgsql-hackers/2008-01/msg00400.php
> I've had to change the way XidInMVCCSnapshot() works. We search the
> snapshot even if it has overflowed. This is actually a performance win
> in cases where only a few xids have overflowed but most haven't. This is
> essential because if we were forced to check in subtrans *and*
> unobservedxids existed then the snapshot would be invalid. (I could have
> made it this way *just* in recovery, but the change seems better both
> ways).
I don't entirely understand this. Can you explain the situation that
would result in an invalid snapshot?
Regards,
Jeff Davis
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