Re: shared_buffers advice

Alvaro Herrera <alvherre@commandprompt.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