Performance Counters for SQL Server Related to iSCSI Disk Access
I agree with Mr. Denny.  

When analyzing disk performance issues I would look into this counters. 

Avg. Disk sec/Read - is the average time, in seconds, of a read of data from the disk.

Avg. Disk sec/Write - is the average time, in seconds, of a write of data to the disk.

Ideally you would be looking less than 15ms latency but in reality I have seen it as high as 30ms in Tier II storage.  

Another counter is Avg. Disk Queue Length and you want see that number lower if possible.