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Re: Performance problem - reads slower than writes

To: Brian Candler <B.Candler@xxxxxxxxx>
Subject: Re: Performance problem - reads slower than writes
From: Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx>
Date: Sat, 04 Feb 2012 03:59:08 -0600
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120203221015.GA2675@xxxxxxxx>
References: <20120130220019.GA45782@xxxxxxxx> <20120131020508.GF9090@dastard> <20120131103126.GA46170@xxxxxxxx> <20120131145205.GA6607@xxxxxxxxxxxxx> <20120203115434.GA649@xxxxxxxx> <4F2C38BE.2010002@xxxxxxxxxxxxxxxxx> <20120203221015.GA2675@xxxxxxxx>
Reply-to: stan@xxxxxxxxxxxxxxxxx
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:9.0) Gecko/20111222 Thunderbird/9.0.1
On 2/3/2012 4:10 PM, Brian Candler wrote:
> 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.

Wow, you're right.  How did I miss so many zeros?  Got in hurry I guess.

> 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.

SSDs would fulfill criteria 1/2 but obviously not 3/4.

> 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 :-)

Will you be using mdraid or hardware RAID across those 24 spindles?

-- 
Stan

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