From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, Pierre C <lists(at)peufeu(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, Dave Crooke <dcrooke(at)gmail(dot)com>, Paul McGarry <paul(at)paulmcgarry(dot)com>, pgsql-performance(at)postgresql(dot)org |
Subject: | Re: shared_buffers advice |
Date: | 2010-03-16 21:58:34 |
Message-ID: | 15417.1268776714@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Tom Lane escribi:
>> Reorder to what, though? You still have the problem that we don't know
>> much about the physical layout on-disk.
> Well, to block numbers as a first step.
fsync is a file-based operation, and we know exactly zip about the
relative positions of different files on the disk.
> However, this reminds me that sometimes we take the block-at-a-time
> extension policy too seriously.
Yeah, that's a huge performance penalty in some circumstances.
> We had a customer that had a
> performance problem because they were inserting lots of data to TOAST
> tables, causing very frequent extensions. I kept wondering whether an
> allocation policy that allocated several new blocks at a time could be
> useful (but I didn't try it). This would also alleviate fragmentation,
> thus helping the physical layout be more similar to logical block
> numbers.
That's not going to do anything towards reducing the actual I/O volume.
Although I suppose it might be useful if it just cuts the number of
seeks.
regards, tom lane
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