Sizing Your NT RAID Array

How do you know where to start when you need to add storage capacity to your existing Windows NT solution? You can't simply add more disk space and expect to improve performance. As you increase your storage capacity in an enterprise environment, you need to be able to detect bottlenecks in your RAID subsystems, know which RAID levels to consider, and know how to size your RAID arrays according to current and future performance requirements.

If you're new to RAID or just need to brush up on your technology, see Table 1, page 186, for a comparison of RAID levels and definitions or go to the RAID Advisory Board Web site (http://www.raid-advisory.com) for an extensive RAID review. The disk subsystem is one of the most flexible resources you can configure in NT. How well you design your disk subsystem can drastically influence NT's overall performance.

Getting the Big Picture
Before you can detect a disk subsystem bottleneck, you need to determine whether your system is suffering from other bottlenecks associated with the CPU, memory, disk, network, applications, clients, and NT resources. (For information about tuning NT to improve performance, see "The Beginner's Guide to Optimizing Windows NT Server," part 1 and part 2, June and August 1997.) If you add resources to an area of NT that isn't throttling your system's performance, you won't improve NT's overall performance. Tuning a resource or purchasing additional hardware only to find that your efforts were in vain can be frustrating. Assuming your disk subsystem is causing the only bottleneck on your NT system, you can take several steps to detect and correct the bottleneck and improve your system's disk performance.

Detecting Single-Disk Bottlenecks
Detecting a bottleneck in your disk subsystem is an important first step in helping you determine how much additional disk space and disk performance capacity you need. On NT systems with one hard disk, the disk becomes a bottleneck that throttles the system when the disk can't keep up with the requested workload. As a result, the disk's response time for processing application requests becomes unacceptable. This delay forces applications to wait on disk service.

NT's Performance Monitor is an excellent tool for detecting disk bottlenecks (for an explanation of Performance Monitor, see John Savill, "Troubleshooting NT Performance Moni-
toring," April 1998). To collect disk subsystem statistics for use with Performance Monitor, you must type

diskperf -ye

at the NT command prompt and reboot the server; otherwise, the performance counters will all report zero. The -y option tells NT to start the disk counters when you restart NT, and the -e option enables the disk counters you need to measure the performance of physical disks in striped disk sets. (You might not have a striped disk set now, but turning on these counters will save you from having to reboot later.)

Selecting Disk Counters
The number of disk-related counters that Performance Monitor provides can be overwhelming. A good counter to watch is %Disk Time, which is available under Performance Monitor's LogicalDisk object. The %Disk Time counter reports the percentage of elapsed time that the selected disk is busy servicing read or write requests. If %Disk Time averages 60 percent to 80 percent, the disk is not causing a bottleneck. However, this level of performance warrants taking a closer look at the disk in question. When %Disk Time exceeds 80 percent, the disk is getting busy. At this level of performance, the time the disk requires to service each request increases, and you need to closely monitor several other disk-related counters that are also available under Performance Monitor's LogicalDisk object.

The first of these additional counters is Avg. Disk Queue Length. This counter measures the average number of read and write requests that NT queued for the selected disk during a sample interval. A hard disk becomes a serious bottleneck when the Avg. Disk Queue Length exceeds 2 for a sustained period. When this delay occurs, applications are waiting to access the disk.