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Tuning the Allocation Unit Size

NTFS uses clusters as the fundamental unit of disk allocation. A cluster consists of a fixed number of disk sectors. When you use the Format command or NT's Disk Administrator, clusters are known as the allocation units. In NTFS, the default allocation unit size depends on the volume size. Using the Format command from the command line to format your NTFS volume, you can specify a variety of allocation unit sizes for a specific NT disk volume. For example, to format a volume using an 8KB allocation unit size, go to the command prompt and type the following:

Format k: /f:NTFS /A:8192

For information about the Format command, go to the command prompt and type the following:

Format /? | more

The default allocation unit size that NT provides is a great place to start if you're unaware of the disk workload characteristics. Before you set up a RAID array or new standalone disks, you need to determine the size of the average disk transfer on your disk subsystem, and set the allocation unit size to match it as closely as possible. By matching the allocation unit size with the amount of data that you typically transfer to and from the disk, you'll incur lower disk subsystem overhead and gain better overall performance. To determine the size of your average disk transfer, use Performance Monitor to review two counters (Avg. Disk Bytes/Read and Avg. Disk Bytes/Write) under the LogicalDisk object. The Avg. Disk Bytes/Read counter measures the average number of bytes transferred from the disk during read operations, and the Avg. Disk Bytes/Write counter measures the average number of bytes transferred to the disk during write operations.

After you measure the number of bytes written to and read from your disk subsystem, you can set the allocation unit size so that you can achieve maximum performance. Of course, if you want to change the allocation unit size for a file system on the disk device, you have to back up the data from the disk device, use the new allocation unit size to format the disk device, and restore your data. Just as you need to use multiple RAID levels or standalone disks to accommodate different disk workloads, you will want to use this same approach when formatting multiple RAID arrays and standalone disks. Customize each disk device with the allocation unit size appropriate for the projected workload characteristics. If you can't determine the disk workload characteristics, use a smaller allocation unit size on devices that you expect to be random or read-intensive, and use a larger allocation unit size for disk devices that will experience sequential and write-intensive workloads.

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