Err Msg: The File System Does Not Support Compression

When you try to compress an NTFS partition using Windows NT Explorer, the option is unavailable while looking at properties or, while using the Command Line utility Compact.exe, you receive the error message: For example:

The partition is a very large partition, and when the partition was formatted, it was formatted with a cluster size greater than the default size of 4,096 bytes. NTFS compression is not supported for allocation unit sizes above 4k bytes

You can verify what the allocation unit size is by doing a CHKDSK at the command prompt and look at the bytes in each allocation unit.

To resolve this problem, you must backup your data, then reformat the NTFS partition using a cluster size of 4,096 bytes or less.

You can use one of three methods to reformat:

• Start Windows NT Explorer, right-click on the drive icon, select format, and then set the allocation unit size to 4,096 or less.
  
  
• Start Disk Administrator, right-click on the partition, select format, and then set the allocation unit size to 4,096 or less.
  
  
• Use the FORMAT command with the /A:size option and format it for an allocation unit size 4,096.
  
  For example,
  format c: /A:4096

NTFS needs to process an entire compression unit at a time when writing compressed files, where a compression unit consists of 16 clusters.

Windows NT memory management's largest paging write size is 64 KB and Windows NT limits operations to the maximum memory management transfer size.

This results in a maximum cluster size that can be manipulated within the compression code to 4 KB. [64 KB/16 clusters = 4 KB per cluster]

Some defrag software also uses the same compression code and may have the same limitation of 4 KB clusters.