Re: Dell Hardware Recommendations

On Thu, 9 Aug 2007, Joe Uhl wrote:

> The MD1000 holds 15 disks, so 14 disks + a hot spare is the max. With
> 12 250GB SATA drives to cover the 1.5TB we would be able add another
> 250GB of usable space for future growth before needing to get a bigger
> set of disks. 500GB drives would leave alot more room and could allow
> us to run the MD1000 in split mode and use its remaining disks for other
> purposes in the mean time. I would greatly appreciate any feedback with
> respect to drive count vs. drive size and SATA vs. SCSI/SAS. The price
> difference makes SATA awfully appealing.

The SATA II drives in the MD1000 all run at 7200 RPM, and are around
0.8/GB (just grabbed a random quote from the configurator on their site
for all these) for each of the 250GB, 500GB, and 750GB capacities. If you
couldn't afford to fill the whole array with 500GB models, than it might
make sense to get the 250GB ones instead just to spread the load out over
more spindles; if you're filling it regardless, surely the reduction in
stress over capacity issues of the 500GB models makes more sense. Also,
using the 500 GB models would make it much easier to only ever use 12
active drives and have 3 hot spares, with less pressure to convert spares
into active storage; drives die in surprisingly correlated batches far too
often to only have 1 spare IMHO.

The two SAS options that you could use are both 300GB, and you can have
10K RPM for $2.3/GB or 15K RPM for $3.0/GB. So relative to the SATA
optoins, you're paying about 3X as much to get a 40% faster spin rate, or
around 4X as much to get over a 100% faster spin. There's certainly other
things that factor into performance than just that, but just staring at
the RPM gives you a gross idea how much higher of a raw transaction rate
the drives can support.

The question you have to ask yourself is how much actual I/O are you
dealing with. The tiny 256MB cache on the PERC 5/E isn't going to help
much with buffering writes in particular, so the raw disk performance may
be critical for your update intensive workload. If the combination of
transaction rate and total bandwidth are low enough that the 7200 RPM
drives can keep up with your load, by all means save yourself a lot of
cash and get the SATA drives.

In your situation, I'd be spending a lot of my time measuring the
transaction and I/O bandwidth rates on the active system very carefully to
figure out which way to go here. You're in a better position than most
people buying new hardware to estimate what you need with the existing
system in place, take advantage of that by drilling into the exact numbers
for what you're pushing through your disks now. Every dollar spent on
work to quantify that early will easily pay for itself in helping guide
your purchase and future plans; that's what I'd be bringing in people in
right now to do if I were you, if that's not something you're already
familiar with measuring.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD