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).

To implement that correctly, I think we'd need to take the content lock
to clear the buffer if it's already found in the cache. It doesn't seem
right to me for the buffer manager to do that, in the worst case it
could lead to deadlocks if that function was ever used while holding
another buffer locked.

What we could have is the semantics of "Return a buffer, with either
correct contents or completely zeroed out". It would act just like
ReadBuffer if the buffer was already in memory, and zero out the page
otherwise. That's a bit strange semantics to have, but is simple to
implement and works for the use-cases we've been talking about.

Patch implementing that attached. I named the function "ReadOrZeroBuffer".

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