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!
