From: | David Kerr <dmk(at)mr-paradox(dot)net> |
---|---|
To: | Andomar <andomar(at)aule(dot)net> |
Cc: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Why does splitting $PGDATA and xlog yield a performance benefit? |
Date: | 2015-08-25 17:54:09 |
Message-ID: | 20150825175409.GB2656@mr-paradox.net |
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Lists: | pgsql-general |
On Tue, Aug 25, 2015 at 10:16:37AM PDT, Andomar wrote:
> >However, I know from experience that's not entirely true, (although it's not always easy to measure all aspects of your I/O bandwith).
> >
> >Am I missing something?
> >
> Two things I can think of:
>
> Transaction writes are entirely sequential. If you have disks
> assigned for just this purpose, then the heads will always be in the
> right spot, and the writes go through more quickly.
>
> A database server process waits until the transaction logs are
> written and then returns control to the client. The data writes can
> be done in the background while the client goes on to do other
> things. Splitting up data and logs mean that there is less chance
> the disk controller will cause data writes to interfere with log
> files.
>
> Kind regards,
> Andomar
>
hmm, yeah those are both what I'd lump into "I/O bandwith".
If your disk subsystem is fast enough, or you're on a RAIDd SAN
or EBS you'd either overcome that, or not neccssarily be able to.
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