File System Alignment

"Allocation Unit Size" is also known as cluster size, allocation unit size is the smallest amount of hard disk space that windows uses to hold a file. Generally, smaller allocation unit sizes result in more efficient use of hard disk space. You can specify the allocation unit size when you format a hard disk drive. If you don't specify a size, windows defaults to a size based on the size of the volume. Use format /A: at the command prompt to override the default allocation unit size, where can be 512, 1024, 2048, 4096, 8192, 16K, 32K, and 64K for FAT, FAT32, and NTFS. FAT and FAT32 also support 128K and 256K for sector sizes greater than 512 bytes. You can't use NTFS compression for allocation unit sizes larger than 4096 bytes.

To improve the performance of your disk drive subsystem, consider matching the file system Allocation Unit Size to the block size of the application you are using. For example, suppose SQL Server is using a 4KB block size. When you format a file system on a new disk drive, launch Disk Administrator, create the partition, commit the partition changes, select Format, and then set the Allocation Unit Size to 4096 bytes. Matching the file system block sizes can improve the efficiency of the disk transfers when you use the application.

For example, if you have four 4KB blocks of data to write to the disk and the disk is slightly fragmented, you might end up with eight separate 2KB writes to disk on a file system created with a 2KB Allocation Unit Size. When reading this file, the disk heads subsequently have to move to eight random locations. If you use a 4KB Allocation Unit Size, NT Server has to write to disk four times, and thus the disk heads have to move four separate times to complete a read of the data. Test your particular Allocation Unit Size configuration to determine your optimum file system layout, because each application and disk subsystem environment is a little different.