Controlling cluster sizes to get more disk space in Windows

By Rick Cook

Larger isn't necessarily better when it comes to setting cluster sizes on a hard disk. Nor are the default settings provided by Windows necessarily the best ones for your application.

Windows manages storage in terms of clusters of sectors of 512 bytes each. In the case of the NTFS file system used in Windows NT and 2000, the default size is 8 sectors, or 4096 bytes of space. The minimum amount of storage that can be assigned to any file is one cluster. On the average every file wastes the equivalent of one half a cluster's worth of storage because the last cluster in every file is on the average half empty.

Cluster size represents a trade-off between efficient storage and efficient data handling. Not only is there a limit on the number of clusters available per volume, but larger clusters allow information to be stored and retrieved faster because fewer logical operations are needed to move the same amount of data. That is why the non-NTFS versions of Windows before 3.51 would allow cluster sizes of up to 128 sectors or 64KB.

The Default Allocation Sizes listed by Windows when being formatted by Windows NTFS are:

Disk Size Cluster size Number of Sectors Per Cluster
<=512MB 512 bytes 1
513 MB to 1 GB 1 KB 2
1 to 2 GB 2 KB 4
>2 GB 4 KB 8

The downside of large clusters is more wasted space. If the system has a lot of small files, those unoccupied half-clusters can add up to a substantial amount of storage. If you don't exceed the number of available clusters in a volume, you can reclaim substantial space on a volume by choosing a smaller cluster size when you have many small files. Alternatively, if you can divide the physical disk into multiple smaller volumes you can get more usable space on the same hard drive.

This also means there are 'magic numbers' in setting your volume sizes. Because there is less wasted space, you will probably get more actual storage in a volume of 950 megabytes (1-KB clusters) than in a volume of 1GB (2-KB clusters.)

For example, by reducing the size of the Windows volumes on the machine I use for writing (a job that generates hundreds of small files) from 1 GB to 512MB, I got nearly 20 percent more usable space on the same hard drive. However the partitions on my graphics machine (which generates fewer, much larger, files) are set to 4 GB each, which gives me the best balance between capacity and performance.

For a discussion of cluster sizes in Windows, go to the Microsoft Knowledge Base at: http://search.support.microsoft.com/kb/c.asp and enter "clusters" in the search window. Stay tuned here for more tips on managing cluster size.

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

This was first published in December 2000

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