Performance counters - Disk

Disk Throughput

The Disk subsystem could be a major system performance bottleneck, because of this we decided to focus on many disk performance counters and present the information by either graphing the total combined counters from all the Disks in the System or to graph the individual disk counters to allow you to pinpoint issues.

\PhysicalDisk(*)\Disk Read Bytes/sec

The Disk Read Bytes/sec shows the Disk Throughput of the Read Operation of the Disk. This is normally displayed in bytes, but we do the calculation when graphing the counter to display it in the standard MB/sec.

Microsoft® Description - Disk Read Bytes/sec is the rate at which bytes are transferred from the disk during read operations.

\PhysicalDisk(*)\Disk Write Bytes/sec

The Disk Write Bytes/sec shows the Disk Throughput of the Write Operation of the Disk. This is normally displayed in bytes, but we do the calculation when graphing the counter to display it in the standard MB/sec.

Microsoft® Description - Disk Write Bytes/sec is rate at which bytes are transferred to the disk during write operations.

Disk Queue Length

There are two main counters for the Disk Queue Length, the Average Disk Queue Length and the Current Disk Queue Length. There are arguments for only using either counter, so to stay out of the debate on which to use, we simply show values for both counters.

\PhysicalDisk(*)\Avg. Disk Queue Length

The Avg. Disk Queue Length counter is the “estimated” average number of requests that are either in process or waiting to be processed by the Disk.
Sustained counters above 5 per Disk could indicate a disk subsystem bottleneck. Remember to also keep an eye on the Pages/sec counter for low memory issues since low memory could cause Disk performance issues if the disk subsystem has to continuously process paging operations.

Microsoft® Description - Avg. Disk Queue Length is the average number of both read and write requests that were queued for the selected disk during the sample interval.

\PhysicalDisk(*)\Current Disk Queue Length

The Current Disk Queue Length counter is the actual number of requests that are either in process or waiting to be processed by the Disk at the time the counter is captured.
Sustained counters above 5 per Disk could indicate a disk subsystem bottleneck. Remember to also keep an eye on the Pages/sec counter for low memory issues since low memory could cause Disk performance issues if the disk subsystem has to continuously process paging operations.

Microsoft® Description - Current Disk Queue Length is the number of requests outstanding on the disk at the time the performance data is collected. It also includes requests in service at the time of the collection. This is a instantaneous snapshot, not an average over the time interval. Multi-spindle disk devices can have multiple requests that are active at one time, but other concurrent requests are awaiting service. This counter might reflect a transitory high or low queue length, but if there is a sustained load on the disk drive, it is likely that this will be consistently high. Requests experience delays proportional to the length of this queue minus the number of spindles on the disks. For good performance, this difference should average less than two.

Disk Latency and Fragmentation

\PhysicalDisk(*)\Avg. Disk sec/Read

The Avg. Disk sec/Read displays how long in milliseconds it takes for a read operation from the Disk. Watch this counter during times of heavy disk utilization to see if your system would benefit from a more robust Disk Subsystem (such as advanced RAID or newer flash drive technologies).

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

\PhysicalDisk(*)\Avg. Disk sec/Write

The Avg. Disk sec/Write displays how long in milliseconds it takes for a write operation to the Disk. Watch this counter during times of heavy disk utilization to see if your system would benefit from a more robust Disk Subsystem (such as advanced RAID or newer flash drive technologies).

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

\PhysicalDisk(*)\Split IO/sec

The Split IO/sec counter displays the physical disk requests that are split into multiple requests. This counter is a primary indicator if a disk is fragmented and needs to be optimized.