IOPS / Disks

It has been a while so I decided I'd post on something I've been having to work with a lot lately.

I/O can be a key bottle neck in disk intensive systems...such as Exchange.  

First let's establish some disks and their relative IO load capacity:

Fiber Channel 15k: 150-180 IOPS

SAS 15k: 150-180 IOPS (notice about the same, but less price than FC..)

Small Form Factor (SFF): Faster..usually ~230

SATA 7.2k: Claim 60-80, some will argue as low as 40.

Let's assume for our test case that we look at a customer with 1000 mailboxes on Exchange, and average-heavy users (spanning from light to very heavy).  Let us also assume they have a BES, an Archive server, AV, and DPM.

.32 IOPS per user is about a heavy user.  To gauge the others, view the follow:

DPM: Add 20%

Archiving:  Add 50% if performing compliance, 25% for compression/storage

AV: 50%

BES: Debatable.. I've heard any where from 2x to 7x!?!  I'd believe any of them..  Let's say 2x for the sake of argument. You're welcome RIM.

That .32 I/O mailbox has now turned into a 1.2 I/O mailbox (rough estimation that is usually on the safe side given involvement from outside entities).

1000 x 1.2 = 1200 / Disks = IOPS required per disk.

For our example let's say the company has a Stovevault S500.  This particular unit comes with a shelf of 14, 7200 RPM disks.  Remember to not include parity disks!   14 - 2 = 12

1000 x 1.2= 1200 / (14 - 2) = 100 IOPS/disk

Now let's compare.. 100 IOPS/disk requirement.  SATA 7200  RPM disks can pull 60-80 advertised (40 according to many in practice).

Looks like we have a problem.  Couple easy, though potentially costly ways around this.  The fact is we need more IOPS.  This can mean more speed, or it could mean more spindles..or both.

How about another SAN with, say, fourteen 15k SAS disks?  (150-180) > 100 -- Looks much better.

How about adding another shelf to the S500?

1000 x 1.2= 1200 / (28 - 4) = 50 IOPS/disk -- Looks better than advertised, but a bit above what some research has turned up.  This may work in production, but it would be ill advised to implement a system in which you run it so close it's breaking point right from the get-go.

So what good are SATA disks then?  SATA disks still have their uses.  Archiving storage, file storage, home drives, or any other storage that isn't being bombarded with read/writes will suffice with this type of storage, and a fraction of the cost.

If you want additional specifics, check out:

http://msexchangeteam.com/archive/2007/01/15/432207.aspx

There is an updated link off of it too.

**EDIT**


SATA disks can be great for Exchange 2010 depending on the deployment!