JSI Tip 0862. Windows NT default allocation unit.

Jerold Schulman

November 23, 1998

1 Min Read
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Regardless of the file system, Windows NT uses cluster size (allocation unit) when organizing your hard disk. An Allocation unit represents the smallest amount of disk space used for a file. If allocation unit was 8192 bytes, on average each file would waste 4096 bytes. The average amount of wasted space on a partition is (Allocation Unit / 2) * (Number of files).

When formatting a disk, without the /A: (Allocation Unit) switch, the following default Allocation Unit sizes are used (assuming a standard 512 byte physical sector size):

NTFS

Partition Size (MB)(low)

Partition Size (MB)(high)

Allocation Unit

Sectors

 1 

 512 

 512 bytes 

 1 

 513 

 1024 

 1024 bytes 

 2 

 1025 

 2048 

 2048 bytes 

 4 

 2049 

 2**64 

 4096 bytes 

 8 

FAT-16

Partition Size (MB)(low)

Partition Size (MB)(high)

Allocation Unit

Sectors

 1 

 15 

 4KB (FAT-12) 

 8 

 16 

 127 

 2K 

 4 

 128 

 255 

 4KB 

 8 

 256 

 511 

 8KB 

 16 

 512 

 1023 

 16KB 

 32 

 1024 

 2047 

 32KB 

 64 

 2048 

 4095 

 64KB 

 128 

 4096 

 8191 

 128KB 

 256 

 8192 

 16384 

 256KB 

 512 

On NTFS partitions, a MFT (Master File Table) entry takes 1024 bytes, so using 512 bytes Allocation Units practically guarantees fragmentation of the MFT. Unfortuneatly, CONVERT, uses 512 bytes Allocation Units. A CONVERT is also used when you install and choose NTFS, unless the partition is preformated as NTFS.

Preformatting with NTFS requires one of the following:

1. Multiple disk drives.
2. Multiple partitions.
3. Another Windows NT system that can mount your drive.

How to install with multiple drives (or partitions).

1. Install a trash copy of Windows NT on drive 1 as FAT. I partition it as ~500MB so I can do the install.
2. Format the 2nd drive.
3. Install a maintenance (emergency) copy of Windows NT on the 2nd drive into NTmaint.
4. From the maintenance install, use Disk Administrator to repartition drive 1 as 1 partition.
5. Format the 1st drive.
6. Install your primary Windows NT on the first drive.
7. Edit C:boot.ini and add the maintenance install. See tip 055 or tip 286.
8. Create a boot floppy.

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