Re: Data write speed

Josh Berkus wrote:
> Folks,
>
> I have a new system with an Adaptec 2200S RAID controller. I've been
> testing some massive data transformations on it, and data write speed
> seems to top out at 3mb/second .... which seems slow to me for this
> kind of hardware. As it is, I have RAM and CPU sitting idle while they
> wait for the disks to finish.
>
> Thoughts, anyone?

That does seem low. What rate do you get with software RAID (of the
same type, of course) to the same disks (might have to be through a
standard SCSI controller to be meaningful) with roughly the same
disks/channel distribution?

My experience with hardware RAID (at least with the hardware available
a few years ago) versus software RAID is that software RAID is almost
always going to be faster because RAID speed seems to be very
dependent on the speed of the RAID controller's CPU. And computer
systems usually have a processor that's significantly faster than the
processor on a hardware RAID controller. It's rare that an
application will be as CPU intensive as it is I/O intensive (in
particular, there are relatively few applications that will be burning
CPU at the same time they're waiting for I/O to complete), so the
faster you can get your I/O completed, the higher the overall
throughput will be even if you have to use some CPU to do the I/O.

That may have changed some since CPUs now are much faster than they
used to be, even on hardware RAID controllers, but to me that just
means that you can build a larger RAID system before saturating the
CPU.

The Adaptec 2200S has a 100MHz CPU. That's pretty weak. The typical
modern desktop system has a factor of 20 more CPU power than that. A
software RAID setup would have no trouble blowing the 2200S out of the
water, especially if the OS is able to make use of features such as
tagged queueing.

Since the 2200S has a JBOD mode, you might consider testing a software
RAID setup across that, just to see how much of a difference doing the
RAID calculations on the host system makes.

--
Kevin Brown kevin(at)sysexperts(dot)com