# Sizing Your NT RAID ARRAY

• Print
From the author of

## Detecting RAID Bottlenecks

RAID technology lets you group multiple hard disks and present them to NT as one logical disk device. To detect a RAID bottleneck, you use the single-disk bottleneck detection techniques that I just described, but with a twist. The %Disk Time counter uncovers problems that are brewing in any RAID device. When you're attempting to detect a RAID bottleneck, RAID 0, disk striped sets, is the easiest RAID level to work with. RAID 0 takes advantage of all the disks in the array equally. Thus, a three-disk RAID 0 array can support three times as much workload (disk requests) and three times as many outstanding disk requests (Avg. Disk Queue Length) as a one-disk configuration before becoming a bottleneck.

In a RAID 1 mirror with two disks, the array uses both disks for all write activities. To determine the workload that a RAID 1 mirror can support (the number of transfers per second), use the following equation:

(Disk reads/sec + [2 * Disk writes/sec]) / (Number of disks in the RAID array)

Today's RAID 1 mirrors use a two-disk configuration. Despite greater availability, RAID 1 arrays support a slightly lower workload in a write-intensive environment than systems with one hard disk. However, if your Avg. Disk Queue Length divided by the number of disks in the array exceeds 2, you have a serious bottleneck in a RAID 1 mirror.

The RAID 5 disk stripe with parity environment is similar to a RAID 0 stripe set for read-intensive environments. A RAID 5 array with five disks supports almost five times as much workload and up to five times as many outstanding disk requests as a one-disk system before becoming a bottleneck. To calculate how many disk requests a RAID 5 array can support, use the following formula:

(Disk reads/sec + [4 * disk writes/sec]) / (Number of disks in the RAID array)

A RAID 5 array's performance is different than a RAID 0 stripe set's performance because of additional disk activity associated with parity generation. In a RAID 5 array, parity information is spread across all the disks in the array for fault tolerance. To calculate this parity information, each RAID 5 write operation reads the data block, reads the parity block, logically exclusive OR's (XORs) the data, writes the data block, writes the parity block, and so on for each single write operation. Thus, each write request in a RAID 5 array incurs four disk operations. This parity generation slows write operations in RAID 5 environments compared with RAID 0. However, this parity information lets you continue operations if one of the disks in the RAID 5 array fails. You can replace the failed disk and reconstruct the failed disk's data on the new disk using parity information from the other disks in the array.

You can use hardware-based RAID solutions to avoid the performance pitfall associated with generating this parity information. Hardware-based RAID controllers generate parity information using their own CPU, not the system's CPU. As a result, a system using a hardware-based RAID solution can handle more disk I/O operations than a software-based solution. An additional benefit of offloading parity generation to a hardware-based RAID solution is that you can recover and use processing power elsewhere on your system that might otherwise be wasted on disk I/O parity.