[SunRescue] Question about SSAs

[David Passmore]

On Sat, May 05, 2001 at 10:38:15PM -0500, Patrick Giagnocavo wrote:

> According to one guy I know, large arrays like EMC etc. NEVER use more than
> half the disk, since performance begins to degrade quite a bit - not only do
> you use the slower ID part of the disk, but the head has to travel more,
> resulting in slower seeks.  Some makers use the "middle" part of the disk in
> an effort to further reduce seeks.

It doesn't really matter what part of the disk you are seeking across, as
long as you are seeking across the fewest cylinders possible. The secret to
this, and really the secret to EMC, is to throw a large number of spindles
at the problem and use very narrow, very wide stripes at the outer edge of
the disks. I generally work with an application that has an RDBMS component
which is extremely I/O intensive but uses little space; this is what goes on
the outer edge. There is an accompanying 'object store' which needs a lot
more raw disk space but is still somewhat demanding; this takes up the
remainder of the disk.

Now, normally you would expect a lot of head contention and thrashing
between the two stripe elements on the disk. I set this up on both the EMC
and the A5200 and purposely ran both workloads at the same time to test for
this contention.

To accomplish this on the Symm I used 2 splits on each disk striped in two
seperate volumes on the host with VxVM, and on the A5200 I just used 2
seperate VxVM subdisks on each disk. On the A5200s, you saw exactly what you
would expect-- high service times from head thrashing. On the EMC... well
this is where you get the magic of the Symmetrix-- it has such a HUGE cache
(in this case 8GB) it can afford to delay writes for a long time so it can
re-order them to reduce or eliminate contention between elements on the
disk. It's beautiful, really.

So in short it's easy to use the entire disk on an array like the Symmetrix,
but a bit trickier on JBOD like A5x00. Or the T3s or A3500s for that
matter... neither of them are as smart as the Symmetrix.

The thing about the A5x00 series is that since they're 'dumb', they don't
suffer from the reliability problems that you see with Sun's attempts to do
hardware RAID... and they're so (relatively) cheap and have such great rack
density you can literally just throw tons of 9GB or 18GB spindles at
something.

David