Performance problem - reads slower than writes

Stan Hoeppner stan at
Sat Feb 4 03:59:08 CST 2012

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?


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