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Re: Performance problem - reads slower than writes

To: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Subject: Re: Performance problem - reads slower than writes
From: Brian Candler <B.Candler@xxxxxxxxx>
Date: Fri, 3 Feb 2012 22:10:15 +0000
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
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References: <20120130220019.GA45782@xxxxxxxx> <20120131020508.GF9090@dastard> <20120131103126.GA46170@xxxxxxxx> <20120131145205.GA6607@xxxxxxxxxxxxx> <20120203115434.GA649@xxxxxxxx> <4F2C38BE.2010002@xxxxxxxxxxxxxxxxx>
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On Fri, Feb 03, 2012 at 01:42:54PM -0600, Stan Hoeppner wrote:
> You've hit the peak read rate of these Hitachi drives.  As others
> pointed out, if you need more read performance than the dozen of these
> you plan to RAID stripe, then you'll need to swap them for units with a
> faster spindle:
> 7.2k   1.21x
>  10k   1.68x
>  15k   2.53x
> or with SSDs, which will yield an order of magnitude increase.  Your
> stated need is 20M 500-800KB files, or 20GB if my math is correct.

Thanks for your suggestion, but unfortunately your maths isn't correct: 20M
x 0.65MB = 13TB.  And that's just one of many possible datasets like this.

I'm aware that I'm working with low-performance drives. This is intentional:
we need low power consumption so we can get lots in a rack, and large
capacity at low cost.

Fortunately our workload will also parallelise easily, and throwing it
across 24 spindles will be fine.  But obviously I want to squeeze the most
performance out of each spindle we have first.  I'm very happy to have found
the bottleneck that was troubling me :-)



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