Choosing the Right Type of Disks
When researching SAN
and NAS devices, you discover that you have several types of disks
available to you. These disks vary by architecture (SCSI versus SATA
versus SAS versus Fibre Channel) and by size. Current disks are
available in sizes ranging from 72GB to 1.5TB.
In terms of size, your decisions will be based on three factors:
Generally
speaking, the larger the disk, the more you pay for it. Capacity refers
to the total amount of space you plan to deploy. If, for example, you
need to deploy 2TB of space, you can use eight 250GB disks or 32 72GB
disks. Why would you pick one configuration over the other?
If you opt to use 8
250GB disks, you use less capacity on your SAN or NAS device. If you
expect to expand capacity in the future, you can expand further before
needing to purchase additional disk shelves or chassis. The potential
downside to this approach is that 8 250GB disks might be more expensive
than 32 72GB disks. The other more noticeable impact is in the area of
I/O performance. Assuming the spindle speeds were the same for both
disks, you would get four times more I/O out of the 32 72GB disks than
you would from the 8 250GB disks. Depending on whether your application
needed the additional I/O, this might be a deciding factor.
Tip
If random access disk
I/O performance is a concern, pay close attention to the spindle speed
of the disks. Traditionally, the largest disks available to SAN or NAS
applications operate at a lower revolutions per minute (rpm) than
smaller disks. Typical random access I/O per second ratings of hard
drives is roughly rpm/100. For example, a 15,000-rpm hard drive offers
150 random access disk I/O per second.
Useful to note is that
with sufficient memory in an Exchange Server 2010 server, disk I/O
requirements are roughly one-fourth what they were in an Exchange 2003
server with the same number of users. This behavior was specifically
engineered into Exchange Server 2010 to take advantage of the
ever-increasing capacity of hard disks. Hard disk capacity is increasing
drastically every year with nearly no improvements in I/O performance.
According to Seagate, although disk capacity increased 15,000 times from
1987 to 2004, the random I/O performance increased only 11 times during
the same period.
In addition to
choosing the size of the disks you deploy, you also have a choice in
terms of the disk architecture. Your most common choices are as follows:
SATA is generally the
least-expensive option. SATA disks provide excellent throughput, nearly
equal to SCSI, at a much better price. High-capacity disks are usually
available as SATA first because it is a more common market for disks.
Newer implementations of SATA include high-performance functions such as
command queuing, which give them performance that approaches that of
SCSI.
SCSI disks have been
around for decades. It’s a well-proven technology and is known for
having high performance and high reliability. SCSI disks are less
expensive than Fibre Channel
disks but offer lower throughput through the bus. This results in
needing more controllers to manage the disks themselves and lower
performance than Fibre Channel disks.
SAS disks are growing in
popularity due to their reduced form factor (typically 2.5″) and their
high performance. By virtue of their form factor, they actually achieve a
higher level of I/O than a 3.5″ disk of a similar rotational speed.
This is because the read/write heads don’t need to travel as far to get
from one side of the disk to the other. They do suffer a bit in the area
of overall throughput because the liner speed at the outer portions of
the platter are lower than that would be on a 3.5″ disk at the same
rotational speed.
Fibre Channel disks are
the highest-performance drives available today. They are also the most
expensive and generally trail a full generation behind other formats in
terms of capacity. If performance is your number-one concern, the Fibre
Channel disk can’t be beat.
Tip
Don’t be afraid to mix
and match disk types for different applications. A typical SAN or NAS
supports multiple disk shelves of different types. Consider something
such as Fibre Channel disks for the databases, SCSI drives for the logs,
and Serial ATA disks for archive storage. A similar concept can be
applied to disk sizes to maximize capacity where I/O loads are
relatively low.