Disk Counters

There are two sets of disk counters: LogicalDisk and PhysicalDisk. LogicalDisk is the most useful counter because it simplifies tracking drive usage. You must enable the disk counters. By default, they are turned off because they have a small impact on performance.

   To enable Performance Monitor disk counters

• At the command prompt, type diskperf -y. The counters begin functioning when the server is next rebooted.

The following are the important disk counters for Microsoft Exchange Server.

Object	Counter
LogicalDisk	Disk Bytes Written/sec
	Disk Bytes Read/sec
	Disk Reads/sec
	Disk Writes/sec
	Avg. Disk Queue Length
	% Disk Time (general indicator only; not a reliable indicator of disk saturation).

Compare the disk operations per second that are recorded with the specifications for sustained operations provided by your vendor. If your disk operations per second are getting close to the vendor's specifications, you are nearing capacity. Note that the % Disk Time counter is not a fair indication of disk saturation. A disk that is busy 100 percent of the time might actually be capable of doing much more work as a result of using smart disk controllers and scheduling methods, such as elevator algorithms.

Summary of Recommendations

If the disk subsystem is your bottleneck, consider taking the following actions:

• Use the fastest possible disks for the information store log and database.
• Use dedicated disks for the information store log.
• Use stripe sets on the information store database disks.
• Mirror the information store log; use RAID 5 on the information store database.
• Use high-performance controllers, and make sure that there are enough of them for your disks.
• Use a separate disk for MTA queues, or place MTA queues in the stripe set with the information store database.
• Use a separate disk for Internet Mail Service queues, or place Internet Mail Service queues in the stripe set with the information store database.