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Re: Dead Space Map version 2

From: "Jim C(dot) Nasby" <jim(at)nasby(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: Dead Space Map version 2
Date: 2007-02-27 16:58:26
Message-ID: 20070227165825.GJ29041@nasby.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patchespgsql-performance
On Tue, Feb 27, 2007 at 12:55:21AM -0500, Tom Lane wrote:
> "Jim C. Nasby" <jim(at)nasby(dot)net> writes:
> > Yes, DSM would make FSM recovery more important, but I thought it was
> > recoverable now? Or is that only on a clean shutdown?
> 
> Currently we throw away FSM during any non-clean restart.  This is
> probably overkill but I'm quite unclear what would be a safe
> alternative.

My thought would be to revert to a FSM that has pages marked as free
that no longer are. Could be done by writing the FSM out every time we
add pages to it. After an unclean restart backends would be getting
pages from the FSM that didn't have free space, in which case they'd
need to yank that page out of the FSM and request a new one. Granted,
this means extra IO until the FSM gets back to a realistic state, but I
suspect that's better than bloating tables out until the next vacuum.
And it's ultimately less IO than re-vacuuming every table to rebuild the
FSM.
-- 
Jim Nasby                                            jim(at)nasby(dot)net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)

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