On Aug 4, 2009, at 2:11 PM, Richard Elling <richard.ell...@gmail.com>
wrote:
On Aug 4, 2009, at 8:01 AM, Ross Walker wrote:
On Tue, Aug 4, 2009 at 10:33 AM, Richard Elling<richard.ell...@gmail.com
> wrote:
On Aug 4, 2009, at 7:01 AM, Ross Walker wrote:
For random io the number of IOPS is 1000/(mean access + avg
rotational
latency) (in ms)
Avg rotational latency is 1/2 the rotational latency which is 1/
(rotations
per second), which for 15K = 250 and for 7200 = 120.
This means for 15K drives the arl = 2ms and for 7200 the arl =
4.150ms.
So for each random io take the mean access time add the arl and
divide
1000 by that number.
SAS disks tend to have faster access times, so say a top SAS disk
has an
access time of 4ms + ARL = 6ms, a top SATA disk has an access
time of 8ms +
ARL = 12ms
Unfair! You are comparing 2.5" drives to 3.5" drives. The seek
times
depend on the size (diameter) of the disk. Smaller diameter ==
faster
seek. Where you have to be careful is that laptop drives tend to
be 2.5"
but they also run slower, 5,400 rpm. 1 TB 2.5" drives are
beginning to
appear, but it looks like they will be slower -- targeted at
laptops, not
servers.
You are fooling yourself though if you think the size makes a
difference in the seek times. Take a look at the technical
specifications. Average Latency = Average Rotational Latency, Average
Seek Time = Mean Access Time, when looking at the specs.
I've never found a spec where a 3.5" disk had faster seek time than a
2.5" disk.
In straight apples-to-apples comparison (type, speed, etc) no, they
are about equal.
But you were alluding to 2.5" being a faster technology, which it
really isn't, but they do use less power to spin the smaller disks and
you can fit more per u then 3.5" disks.
From what I have read online my example is very optimistic.
Actually, it is about right. I SWAG 80 iops for a 3.5" 7,200 rpm disk
and 170 for 2.5" 15k rpm disk. For planning purposes, it seems to
work well. Of course, some of the new SSDs they are bragging about
do 40,000 iops... game over.
Well we still need to see how SSDs fair long term.
Not that I wouldn't love to get rid of my spinning platters, I just
need their replacement to be as, if not more reliable.
-Ross
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