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From Shawn Heisey <s...@elyograg.org>
Subject Re: Hardware Specs Question
Date Fri, 03 Sep 2010 18:14:28 GMT
  On 9/3/2010 3:39 AM, Toke Eskildsen wrote:
> I'll have to extrapolate a lot here (also known as guessing).
> You don't mention what kind of harddrives you're using, so let's say
> 15.000 RPM to err on the high-end side. Compared to the 2 drives @
> 15.000 RPM in RAID 1 we've experimented with, the difference is that the
> striping allows for concurrency when the different reads are on
> different physical drives (sorry if this is basic, I'm just trying to
> establish a common understanding here).
>
> The chance for 2 concurrent reads to be on different drives with 3
> harddrives is 5/6, the chance for 3 concurrent reads is 1/6 and the
> chance for 3 concurrent reads to be on at least 2 drives is 5/6. For the
> sake of argument, let's say that the 3 * striping gives us double the
> concurrency I/O.

I actually didn't know that there were 15,000 RPM SATA drives until just 
now when I googled.  I knew that Western Digital made some 10,000 RPM, 
but most SATA drives are 7200.  Dell doesn't sell any SATA drives faster 
than 7200, and the 500GB drives in my servers are 7200.  I'm using the 
maximum 1MB stripe size to increase the likelihood of concurrent reads.  
Our query rate is quite low (less than 1 per second), so any concurrency 
that's achieved will be limited to possibly allowing all three VMs on 
the server to access the disk at the exact same time.  With three 
stripes and two copies of each of those stripes, the chance of that is 
fair to good.

So with all that, I probably only see around a third (and possibly maybe 
up to half) the performance of SSDs.  Thanks!


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