From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Heikki Linnakangas <heikki(at)enterprisedb(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Avoiding unnecessary reads in recovery |
Date: | 2007-04-27 14:37:29 |
Message-ID: | 200704271437.l3REbTH26623@momjian.us |
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Thread: | |
Lists: | pgsql-hackers |
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
>
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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