Re: Hot Backup with rsync fails at pg_clog if under load

On Oct27, 2011, at 15:51 , Simon Riggs wrote:
> On Thu, Oct 27, 2011 at 12:29 AM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
>> Here's what I image CreateCheckPoint() should look like:
>>
>> 1) LogStandbySnapshot() and fill out oldestActiveXid
>> 2) Fill out REDO
>> 3) Wait for concurrent commits
>> 4) Fill out nextXid and the other fields
>> 5) CheckPointGuts()
>> 6) Rest
>>
>> It's then no longer necessary for LogStandbySnapshot() do modify
>> the nextXid, since we fill out nextXid after LogStandbySnapshot() and
>> will thus derive a higher value than LogStandbySnapshot() would have.
>>
>> We could then also fold GetOldestActiveTransactionId() back into
>> your proposed LogStandbySnapshot() and thus don't need two ProcArray
>> traversals.
>
> I think you make a good case for doing this.
>
> However, I'm concerned that moving LogStandbySnapshot() in a backpatch
> seems more risky than it's worth. We could easily introduce a new bug
> into what we would all agree is a complex piece of code. Minimal
> change seems best in this case.

OTOH, we currently compute oldestActiveXid within LogStandbySnapshot().
Your proposed patch changes that, which also carries a risk since something
could depend on these values being in sync. Especially since both the logged
snapshot and oldestActiveXid influence the snapshot tracking on the slave.

But since you wrote most of that code, your judgement about the relative
risks of these two approaches obviously out-weights mine.

best regards,
Florian Pflug