Message-ID: <43946EA1.8090908@tgharold.com>
Date: Mon, 05 Dec 2005 11:45:21 -0500
From: Thomas Harold <tgh@tgharold.com>
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CC: pgsql-performance@postgresql.org
Subject: Re: two disks - best way to use them?
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David Lang wrote:

> the application can' tell the difference, but the reason for seperating 
> them isn't for the application, it's so that different pieces of 
> hardware can work on different things without having to bounce back and 
> forth between them.
> 
> useing the same drives with LVM doesn't achieve this goal.
> 
> the problem is that the WAL is doing a LOT of writes, and postgres waits 
> until each write is completed before going on to the next thing (for 
> safety), if a disk is dedicated to the WAL then the head doesn't move 
> much. if the disk is used for other things as well then the heads have 
> to move across the disk surface between the WAL and where the data is. 
> this drasticly slows down the number of items that can go into the WAL, 
> and therefor slows down the entire system.
> 
> this slowdown isn't even something as simple as cutting your speed in 
> half (half the time spent working on the WAL, half spent on the data 
> itself), it's more like 10% spent on the WAL, 10% spent on the data, and 
> 80% moveing back and forth between them (I am probably wrong on the 
> exact numbers, but it is something similarly drastic)

Yeah, I don't think I was clear about the config.  It's (4) disks setup 
as a pair of RAID1 sets.  My original config was pgsql on the first RAID 
set (data and WAL).  I'm now experimenting with putting the data/pg_xlog 
folder on the 2nd set of disks.

Under the old setup (everything on the original RAID1 set, in a 
dedicated 32GB LVM volume), I was seeing 80-90% wait percentages in 
"top".  My understanding is that this is an indicator of an overloaded / 
bottlenecked disk system.  This was while doing massive inserts into a 
test table (millions of narrow rows).  I'm waiting to see what happens 
once I have data/pg_xlog on the 2nd disk set.

Thanks for the input.