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Re: Proposal of PITR performance improvement for 8.4.

From: Gregory Stark <stark(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Koichi Suzuki <koichi(dot)szk(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Proposal of PITR performance improvement for 8.4.
Date: 2008-10-28 12:34:40
Message-ID: 87zlkoc2zj.fsf@oxford.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:

> I'm happy with the idea of a readahead process. I thought we were
> implementing a BackgroundReader process for other uses. Is that dead
> now?

You and Bruce seem to keep resurrecting that idea. I've never liked it -- I
always hated that in Oracle and thought it was a terrible kludge.

I think the inter-process communication would be way too heavy-weight, by the
time the other process is schedule the process which needed the blocks would
have probably have done many of them already anyways. Worse, you would need a
large number of reading processes and would start to run into locking
contention on the work-queue as well.

In any case it would be a lot of code to do what posix_fadvise does for us
with a simple syscall anyways.

Am I misjudging this? Is this a popular idea? We could always implement it as
a fall-back implementation for mdprefetch() where posix_fadvise (and libaio
assuming we implement that as well) don't work. It has the advantage of
working with any system at all even if it predates 1003.1.

But it seems like an awful lot of code to implement a solution that I fear
won't work very well. And are people running raid arrays not likely to be
running modern OSes anyways?

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning

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