Compression Not Available With Allocation Units > 4K


W

Will

Why is it that when you create an NTFS volume, Microsoft disables
the option to compress the file system when the allocation units
are over 4K in size?
 
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J

John Wunderlich

Will said:
Why is it that when you create an NTFS volume, Microsoft disables
the option to compress the file system when the allocation units
are over 4K in size?

Quoting from the TechNet article "How NTFS Works"

<http://www.microsoft.com/technet/pr...Ref/8cc5891d-bf8e-4164-862d-dac5418c5948.mspx>

Before you choose a cluster size other than the default, note the
following important limitations:
[...]
• Because file compression is not supported on cluster sizes greater
than 4 KB, the default NTFS cluster size for Windows Server 2003
never exceeds 4 KB.

[...]

The compression algorithms in NTFS support cluster sizes of up to 4
KB. When the cluster size is greater than 4 KB on an NTFS volume,
none of the NTFS compression features are available.

And from "Optimizing NTFS" found at:
<http://www.microsoft.com/technet/archive/winntas/tips/winntmag/optntfs.mspx>

....however, using cluster sizes greater than 4KB has several
potentially negative side effects. For example, when you use cluster
sizes larger than 4KB, disk-defragmentation utilities can't
defragment the volume, you can't use NTFS file compression on the
volume, and the amount of wasted disk space increases because user
data files stored on the volume don't end evenly on cluster
boundaries

HTH,
John
 

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