IIS 7.0 : Performance and Tuning - Hard Disks

As faster and faster disks evolve, chances of the disk being the issue become less likely. Redundant Array of Inexpensive Disk (RAID) and striping technologies are not really IIS-related performance tricks, but they can help increase your server’s overall performance. The real effect on hard disks depends on how much RAM your machine has.

What Causes Hard Disk Pressure?

Typically, pressure is a matter of the amount of disk reads and writes. A Web server that has a high ratio of reads to writes tends to perform better. The Reliability and Performance Monitor is a great tool for analyzing the total number of reads and writes a Web server generates. If your application requires a lot of writes, you should have faster disks and a RAID implementation to support them. Disk bottlenecks can be improved by using Kernel-mode caching, which greatly eliminates the number of direct reads from the disk.

Hard Disk Counters to Monitor

See Table 1 for a list of common hard disk counters that help identify which processes and how much of the server resources are being used when your IIS 7.0 server is experiencing hard disk-related issues.

Table 1. Disk Counters to Measure

Counter Name	Description
PhysicalDisk(_Total)\% Disk Time	The percentage of elapsed time that the selected disk drive was busy servicing read or write requests.
PhysicalDisk(_Total)\% Disk Read Time	The percentage of elapsed time that the selected disk drive was busy servicing read requests.
PhysicalDisk(_Total)\% Disk Write Time	The percentage of elapsed time that the selected disk drive was busy servicing write requests.
PhysicalDisk(_Total)\Current Disk Queue Length	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 an instantaneous snapshot, not an average over the time interval. Multispindle 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.
PhysicalDisk\Disk Reads/sec	The rate of read operations on the disk.
PhysicalDisk\Disk Writes/sec	The rate of write operations on the disk.
PhysicalDisk\Avg. Disk Bytes/Read	The average number of bytes transferred from the disk during read operations.
PhysicalDisk\Avg. Disk Bytes/Write	The average number of bytes transferred to the disk during write operations.

Impact of Constraints

Your application’s performance and throughput will be affected if your IIS 7.0 server does not have enough RAM. When a server has to start writing and retrieving information from disk, there will be latency to your application’s performance. Disk paging can be an expensive task, and using disk performance counters can help you measure how your server is performing.