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JSI Tip 0862. Windows NT default allocation unit.

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):


 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 

 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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