Re: Transaction Snapshots and Hot Standby

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
backendsand                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).

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Hannu