By Kelly Cash
Introduction
Even if you've managed to avoid all of the storage trade publications
for the past decade, you've probably heard of solid-state disks.
Chances are though, you haven't given them much more than a passing
thought. Sure, they have a high "cool" factor, being that
they're made of chips instead of spinning platters. They've always
been so expensive though- everyone's got a budget, and these days
it's probably a lot less than it used to be. It just wouldn't make
sense to spend a big part of it on whiz-bang disks, or would it?
Naturally, to justify the purchase a legitimate need for solid-state
disks must be shown. To begin, let's first recap the changes in
computing and storage over the years.
Background
Long ago, CPUs were slow, and disks were even slower. To get around
this performance disparity, striping (later to be called RAID-0
) was introduced. This allowed us to add spindles to a volume to
increase the number of simultaneous I/O operations the storage system
could perform. As CPU speeds got faster, more storage tricks were
used- bigger stripes, different types of RAID1, big front-end
caches on the arrays and even on disks themselves.
All the while, solid-state disks were available. Sure, they were
only used for very specific purposes. Their capacity was low, and
their cost was astronomical. Because of this they've always been
considered the "last resort" for solving performance problems.
I remember selling a 4MB solid-state disk in 1988 that cost $125,000!
Unless you were running something like a nuclear simulation there
was little chance you could swing THAT kind of purchase price.
However, times have changed due to shifts in both memory and CPU
technology. First, the price of memory chips has plummeted beyond
anyone's expectations. Additionally, the capacity has gone way up
while form factor has gotten smaller. This makes solid-state disks
much more affordable than they used to be.
Second, technology advances have caused an explosion in CPU performance.
While it's true that CPUs have always been faster than disks, this
performance gap has dramatically widened in the past couple of years.
CPUs have moved from 15 to 100, to 500MHz, and now past 3GHz! No
matter how you look at it, this is a phenomenal performance progression.
Although disk technology has progressed, it's been mostly in terms
of capacity and form factor instead of performance. The 5MB 5¼"
disk grew to 512MB, then to gigabyte capacities in a smaller package,
and now we see disks over 150 and 200GB in a 3½" form
factor. It's true that rotational speed has increased, and so has
seek performance. However, the performance improvements haven't
been dramatic. We've seen disks with an average access time of 5-10
milliseconds for many years now. Yes, some disks are now advertising
a 3.2ms access time, but that assumes optimal conditions are maintained.
On a heavily utilized disk, it's common to see service times2
of 100-200 milliseconds and up! Even if a disk could sustain its
advertised access time, it's hardly the "multiple order of
magnitude" jump in performance we've seen with CPUs.
_____________
1 RAID is an acronym for 'Redundant
Array of Inexpensive Disks.' RAID-0 is a means of storing data evenly
across all disks in the 'array stripe.
2 A disk's service time is the time required for
the CPU to get the data to or from the disk. It is a combination
of the time the I/O request has spent in the disk queue, access
time of the disk, and the data transfer time of the disk and bus.
A solid-state disk is so fast that the I/O requests are handled
before a long disk queue can build, thus keeping the service times
tremendously small.
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