On Aug 4, 2009, at 7:01 AM, Ross Walker wrote:
On Aug 4, 2009, at 7:26 AM, Joachim Sandvik <no-
re...@opensolaris.org> wrote:
does anybody have some numbers on speed on sata vs 15k sas? Is it
really a big difference?
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.
From a disk vendor's perspective, the mechanics can be the same
with different electronics. It is the mechanics which drive performance
for HDDs, which is why if you can eliminate the mechanics you can
go really fast. To look at this another way, a SATA SSD will run circles
around a FC or SAS HDD.
-- richard
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