| From: | Hannu Krosing <hannu(at)2ndQuadrant(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org, Merlin Moncure <mmoncure(at)gmail(dot)com>, Gregory Stark <stark(at)enterprisedb(dot)com>, Csaba Nagy <nagy(at)ecircle-ag(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Chris Browne <cbbrowne(at)acm(dot)org>, Jochem van Dieten <jochemd(at)gmail(dot)com>, Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Richard Huxton <dev(at)archonet(dot)com> |
| Subject: | Re: Transaction Snapshots and Hot Standby |
| Date: | 2008-09-12 10:53:05 |
| Message-ID: | 1221216785.7026.18.camel@huvostro |
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| Lists: | pgsql-hackers |
On Fri, 2008-09-12 at 11:19 +0100, Simon Riggs wrote:
> On Thu, 2008-09-11 at 01:07 +0100, Simon Riggs wrote:
> > Transaction snapshots is probably the most difficult problem for Hot
> > Standby to resolve.
>
> In summary of thread so far:
>
> When queries on standby run for significantly longer than longest
> queries on primary, some problems can occur. Various people have argued
> for these responses to the problems:
>
> 1. Master uses Standby's OldestXmin
...
> 2. Master ignores Standby's OldestXmin
...
> 3. Ignore problem
...
> Summary OK for everyone?
Maybe we should at least mention option 4.
4. Slave keeps copies of removed pages or rows when WAL apply removes
old versions .
Possible ways to do this
* inside Slave - have some backup store tied to OldestXmin intervals
* variant 1 - have one global store, accessed through shared mem
* variant 2 - present removed pages to interested backends and
let them (decide to) keep them
* outside Slave - having file system keep old snapshots as long as
needed, still must tie to OldestXmin intervals, but most of work done by
storage layer (SAN or overlay file system).
-------------
Hannu
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