Re: Avoiding unnecessary reads in recovery

I was actually thinking that we could slip this in 8.3. It's a simple,
well-understood patch, which fixes a little data integrity quirk as well
as gives a nice recovery speed up.

Bruce Momjian wrote:
> I assume this is 8.4 material.
>
> ---------------------------------------------------------------------------
>
> Heikki Linnakangas wrote:
>> 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
>
>
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--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com