Compression and wasted quota space in Windows NTFS

If you use the disk compression feature in Windows NTFS, you probably assume that it increases the disk space available to users under storage quotas.

But, it turns out that using the feature accomplishes exactly the opposite: It decreases the storage capacity under quotas. Turning the feature on may send some users over their quotas.

This happens because of the way NTFS compresses files and performs its internal bookkeeping. The net result is that the file size shown on the My Computer listing can be substantially more than the space actually used by compressed files -- and substantially less than the space charged against the user's quota. In fact, file size can mean several different things in Windows, and three of those numbers come into play when dealing with quotas and compressed files.

The term file size usually refers to the actual file size -- the number of bytes in the uncompressed file. However, since operating systems assign storage space by the cluster, the allocated space -- the amount of space the file takes up on the disk -- is rounded up. That means the allocated space for a file is almost always larger than the file size.

Check it out for yourself by right clicking on a file and then selecting Properties. You will see both the actual size of the file and the "size on disk," which is the allocated space.

NTFS calculates disk quotas on the basis of allocated space. That is even true for compressed files because the files

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have to be decompressed to be used.

The conflict arises because Windows NTFS doesn't compress files by clusters. Instead, it uses compression units of 16 clusters and calculates file space on the basis of the number of compression units needed. This wastes an average of 8 clusters of additional space for each file. In a typical user's account with hundreds or thousands of files, that space adds up. And when it is charged against users' quotas, they run out of quota space even though the file size on their screen shows that they still have plenty of space left in their quotas.

Microsoft suggests two ways to handle the situation -- by either not using compression or by increasing users' quotas to accommodate the wasted space. It discusses the situation in a Knowledge Base article, which you can find at http://support.microsoft.com/default.aspx?scid=kb;en-us;320686.

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Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.

This was first published in September 2005

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