ContentsPrevious ArticleNext ArticleHome
  Missing Disk Space! (The Great Cluster Robbery)

I have a 1 gigabyte hard drive with 123 megabytes showing as free space. When I go into Windows Explorer and highlight everything on the drive and check the properties, the used space is shown as 643MB, which means that I am missing about 226MB of drive space that I sorely need. Am I reading the information right?

Windows 95, with the exception of the "B" release, uses the FAT16 file system. Space on the hard drive is divided up into storage allocation units, or "clusters." Each file on the disk takes up one or more of these clusters. Since most files do not fit evenly into a certain number of clusters, there is usually some slack. For example, a 20,428-byte file stored on a disk using an 8KB (8192 byte) cluster size actually occupies 3 clusters (24,576 bytes) -- 4148 bytes are "wasted." In other words, the total amount of space used by your files is less than the amount of disk space occupied by those files.
     Worse yet, FAT16 allows a maximum of 65,536 clusters. As the disk, or disk partition, gets bigger, so must the cluster size. And larger clusters result in more average slack space per file. For big disks, the waste is quite significant. The following table shows what happens to cluster size as disk (partition) size increases.

Partition Size   Cluster Size
  0 to 16MB   4KB
>16 to 128MB   2KB
>128 to 256MB   4KB
>256 to 512MB   8KB
>512 to 1024MB   16KB
>1024 to 2048MB   32KB
1MB = 1024x1024 = 1,048,576 bytes
1KB = 1024 bytes

The solution? Subdivide large drives into two or more smaller partitions. This reduces cluster size and increases storage efficiency. Unfortunately, this also means that you will lose the entire contents of your disk in the process. If you don't wish to reload the operating system and all your applications and files, you will need to buy a third-party utility such as PartitionMagic, which can create or re-size partitions on the fly.

Note: When you are partitioning a disk, it is safest to stay 1MB under the maximum partition size to ensure that "round-up" issues do not bump you to the next largest cluster size. The most popular partition size is 511MB as it is a reasonable trade-off between capacity and storage efficiency.

If you want a better idea of how cluster size affects storage efficiency, there is an addendum to this article describing a rough formula for calculating cluster-dependent waste space.
w950510


Maintained by William K. Walker
Copyright 1997 by William K. Walker
Last update: 22 Jan 97