Question: What perfmon counters can I trust when using SAN disks?

Answer: It's true that some of the counters can be trusted and some can't be.  Knowing which counters can be trusted to give you valid information and which counters can't be trusted is very important.  The following is a list of counters which you can trust:

(While these numbers are accurate, remember that there is cache at the storage array which may be making some of these numbers look better than they would if the spindles were being queried directly.)

• Avg. Disk Bytes/Read
• Avg. Disk Bytes/Transfer
• Avg. Disk Bytes/Write
• Avg. Disk sec/Read
• Avg. Disk sec/Transfer
• Avg. Disk sec/Write
• Disk Bytes/sec
• Disk Read Bytes/sec
• Disk Reads/sec
• Disk Transfers/sec
• Disk Write Bytes/sec
• Disk Writes/sec
• Split IO/Sec

The following is a list of counters which can sort of be trusted:

(These counters show what Windows sees, which may or may not be accurate as the disk sub system may see different numbers.)

•  Avg. Disk Queue Length
• Avg. Disk Read Queue Length
• Avg. Disk Write Queue Length
• Current Disk Queue Length

The following is a list of counters which can not be trusted:

(As Windows doesn't have any idea what the actual busy and free times are these numbers are basically useless.)

• % Disk Read Time
• % Disk Time
• % Disk Write Time
• % Idle Time

Whenever looking at storage performance numbers I always double check those numbers against the performance numbers that the storage array is seeing.  I also make sure to look at the performance numbers with and without the cache being taken into account.  Depending on the storage array you may or may not be able to pull these specific numbers.  Your storage vendor can give you more information about the monitoring tools available on their platform.

Denny