Re: Avoiding unnecessary reads in recovery

Tom Lane wrote:
> "Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
>> As regards the zero_damaged_pages question, I raised that some time ago
>> but we didn't arrive at an explicit answer. All I would say is we can't
>> allow invalid pages in the buffer manager at any time, whatever options
>> we have requested, otherwise other code will fail almost immediately.
>
> Yeah --- the proposed new bufmgr routine should probably explicitly zero
> the content of the buffer. It doesn't really matter in the context of
> WAL recovery, since there can't be any concurrent access to the buffer,
> but it'd make it safe to use in non-WAL contexts (I think there are
> other places where we know we are going to init the page and so a
> physical read is a waste of time).

Is there? I can't think of any. Extending a relation doesn't count.

Zeroing the buffer explicitly might be a good idea anyway. I agree it
feels a bit dangerous to have a moment when there's a garbled page in
buffer cache, even if only in recovery.

> Also, this would let the patch be
>
> + if (alloc_only)
> + MemSet...
> + else
> smgrread...
>
> and you don't need to hack out the PageHeaderIsValid test, since it will
> allow zeroed pages.

Well, I'd still put the PageHeaderIsValid test in the else-branch. Not
like the few cycles spent on the test matter, but I don't see a reason
not to.

> Possibly ReadZeroedBuffer would be a better name?

Yeah, sounds good. "Read" is a bit misleading, since it doesn't actually
read in the buffer, but it's good that the name is closely related to
ReadBuffer.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com