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Re: Effects of setting linux block device readahead size

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Mark Wong <markwkm(at)gmail(dot)com>
Cc: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>, Gabrielle Roth <gorthx(at)gmail(dot)com>, Selena Deckelmann <selenamarie(at)gmail(dot)com>
Subject: Re: Effects of setting linux block device readahead size
Date: 2008-09-10 14:49:06
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Tue, 9 Sep 2008, Mark Wong wrote:

> I've started to display the effects of changing the Linux block device
> readahead buffer to the sequential read performance using fio.

Ah ha, told you that was your missing tunable.  I'd really like to see the 
whole table of one disk numbers re-run when you get a chance.  The 
reversed ratio there on ext2 (59MB read/92MB write) was what tipped me off 
that something wasn't quite right initially, and until that's fixed it's 
hard to analyze the rest.

Based on your initial data, I'd say that the two useful read-ahead 
settings for this system are 1024KB (conservative but a big improvement) 
and 8192KB (point of diminishing returns).  The one-disk table you've got 
(labeled with what the default read-ahead is) and new tables at those two 
values would really flesh out what each disk is capable of.

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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