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Monitoring disk usage helps to balance the load of servers. When monitoring disk performance, log the performance data to another disk or computer to prevent it from skewing the data for the disk you are testing.
Disk counters are permanently enabled in Windows Small Business Server and Microsoft Windows XP. To enable disk counters for Microsoft Windows 2000 or Windows NT, use the Diskperf command. (Type diskperf -? at a command prompt for additional information.)
The following list shows the minimum recommended counters for monitoring the server’s disk performance for possible bottlenecks:
PhysicalDisk\ Current Disk Queue Length (all instances) Monitors the number of system requests that are waiting for disk access. This number should remain steady at no more than 1.5 to 2 times the number of spindles that make up the physical disk. Most disks have one spindle. The exception is Redundant Array of Independent Disks (RAID) devices, which usually have more than one spindle. You need to observe this value over several intervals because it is an instantaneous counter.
PhysicalDisk\ % Disk Time Indicates how busy your server’s disk drives are by displaying the percentage of time that a drive is active. If the value of this counter rises to more than 90 percent or if you are using a RAID device, check the PhysicalDisk\ Current Disk Queue Length (all instances) counter to see how many disk requests are queued for disk access. RAID devices can cause the PhysicalDisk\ % Disk Time value to exceed 100 percent and thus give an incorrect reading.
PhysicalDisk\ Avg. Disk Sec/Transfer Monitors the amount of time a disk takes to fulfill a request. A high value might indicate that the disk controller is continually trying to access the disk as a result of failures. For most systems, a value of 0.3 seconds or higher indicates a high average disk transfer time.
To monitor possible usage problems, use these counters:
PhysicalDisk\ Avg. Disk Bytes/Transfer Monitors the average number of bytes that are transferred from or to a disk during read or write operations. A value less than 20 KB indicates that an application is accessing the disk drive inefficiently.
PhysicalDisk\ Disk Reads/Sec and Disk Writes/Sec If these counters show that actual usage is near the specified transfer rate of the physical disks, consider reducing the server’s workload or upgrading to a RAID setup.
Tuning and Upgrading Tips for Disks
If you are experiencing problems with disk performance, try the following solutions:
Verify that you have installed the latest driver software for your host adapters.
Install additional disks, or upgrade your hard disk to a faster disk. Update the bus and the disk controller at the same time.
On servers, create striped volumes on several physical disks to increase throughput.
Distribute applications among your servers to help balance the workload.
Optimize disk space by running Disk Defragmenter.
To help in balancing the server workload, isolate tasks that use disk I/O heavily to separate disk controllers or physical disks.
See Chapter 7, “Disk Management,” for more information about disk management and optimization.
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