Windows Server Troubleshooting - Counters

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When analyzing disk performance, consider that normally there are many more read operations that write operations. Write operations involve more overhead and normally require a verification read operation. Random requests cause longer queues and lower utilization than sequential access.

Disk performance is problematic and requires further analysis when you observe the following;

  • Percentage disk time > 67%
  • Current disk queue length > 2

Performance Objects and Counters for Disk Monitoring

Counters are available for both

  • Physical disks - counters for each physical disk
  •   Logical disks - counters for each partition, eg. C:, D:, ...
Counter Description
LogicalDisk\% Free Space The percentage of unallocated disk space to the total usable space on the logical volume.
LogicalDisk|PhysicalDisk
\Avg. Disk Bytes/Transfer
Measures the size of I/O input/output  operations. The disk is efficient if it transfers large amounts of data relatively quickly.
LogicalDisk|PhysicalDisk
\Avg. Disk sec/Transfer
Indicates how fast data is being moved in seconds. Measures the average time of each data transfer, regardless of the number of bytes read or written. A high value for this counter might mean that the system is retrying requests due to lengthy queuing or possibly disk failures.
LogicalDisk|PhysicalDisk
\Avg. Disk Queue Length
Tracks the number of requests that are queued and waiting for a disk. If more than two requests are continuously waiting on a single-disk system, the disk might be a bottleneck.
LogicalDisk|PhysicalDisk
\Current Disk Queue Length
Indicates the number of disk requests that are currently waiting as well as requests currently being serviced.
LogicalDisk|PhysicalDisk
\Disk Bytes/sec
Indicates the rate at which bytes are transferred and is the primary measure of disk throughput.
LogicalDisk|PhysicalDisk
\Disk Transfers/sec
Indicates the number of read and writes completed per second, regardless of the amount of data.
LogicalDisk \Free Megabytes Reports the amount of bytes on the disk that are not allocated.
LogicalDisk|PhysicalDisk
\Split IO/sec
Reports the rate at which the operating system divides I/O requests to the disk into multiple requests. On single-disk systems, a high rate for this counter tends to indicate disk fragmentation.
LogicalDisk|PhysicalDisk
\% Disk Time
Reports the percentage of time that the selected disk drive is busy servicing read or write requests.
LogicalDisk|PhysicalDisk
\% Disk Write Time
Reports the percentage of time that the selected disk drive is busy servicing write requests.
LogicalDisk|PhysicalDisk
\% Disk Read Time
Reports the percentage of time that the selected disk drive is busy servicing read requests.
LogicalDisk|PhysicalDisk
\% Idle Time
Reports the percentage of time that the disk system was not processing requests and no work was queued. Notice that this counter, when added to % Disk Time, might not equal 100 percent, because % Disk Time can exaggerate disk utilization.