How to Identify and Resolve Disk Bottlenecks

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Introduction Disks store programs and the data that programs process. While waiting for a computer to respond, it is frequently the disk that is the bottleneck. In this case, the disk subsystem can be the most important aspect of I/O performance. However, problems can be hidden by other factors, such as the lack of memory.

Performance disk counters are available with both the LogicalDisk and PhysicalDisk performance objects. LogicalDisk monitors logical partitions of physical drives. It is useful to determine which partition is causing the disk activity, which may indicate the application or service that is generating the requests. PhysicalDisk monitors individual hard disk drives and is useful for monitoring disk drives as a whole.

Important Both LogicalDisk and PhysicalDisk objects are automatically enabled on demand. Therefore, you do not have to enable them manually with the diskperf- y command.

When analyzing disk subsystem performance and capacity, monitor the following Performance disk subsystem counters for bottlenecks:

■ % Disk Time. Indicates the amount of time that the disk drive is busy servicing read and write requests. If this is consistently close to 100 percent, the disk is being used very heavily. Monitoring individual processes helps determine which process or processes are making the majority of the disk requests.

Counters used to determine whether the disk is a bottleneck

■ Current Disk Queue Length. Indicates the number of pending disk I/O requests for the disk drive. If this value is consistently over two, it indicates congestion.

■ Avg. Disk Bytes/Transfer. The average number of bytes transferred to or from the disk during write or read operations. The larger the transfer size, the more efficient the system is running.

■ Disk Bytes/sec. This is the rate at which bytes are transferred to or from the disk during write or read operations. The higher the average, the more efficient the system is running.

■ LogicalDisk\% Free Space. This is the amount of disk space available.

Actions to resolve disk If you determine that the disk subsystem is a system bottleneck, a number of bott|eneck solutions are possible, including the following:

■ Defragment the disk by using Disk Defragmenter.

■ Rule out a memory shortage. When memory is scarce, the Virtual Memory Manager writes more pages to disk, resulting in increased disk activity. Before you add hardware, make sure that memory shortage is not the source of the problem because low memory is a common cause of bottlenecks.

■ Add a faster controller, such as Fast SCSI-2, or an on-board caching controller.

■ Add more disk drives in a RAID environment. This solution spreads the data across multiple physical disks and improves performance, especially during read operations.

■ Offload processing to another system on the network, such as users, applications, or services.

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