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The system is telling me I am out of space on my compressed disk even though I should still have space left

Explanation: The system said I had a certain amount of space free on my compressed volume but when I tried to copy an amount of files that should have fit, the disk ran out of space before copying all the files.

Diagnosis: It is impossible to know exactly how much free space there is on a compressed volume. The reason is that the amount you can store on the volume depends on how much its contents can be compressed, but the system doesn't know what the right ratio is until after the files have been copied. Therefore, the compression driver only estimates the amount of free space on the drive. If you copy a bunch of files to the volume that don't compress very much, you will use up a disproportionate amount of the free space. This is discussed in much more detail here, including an example.

Recommendation:

• Make sure that any compressed volumes on your system have realistic estimated compression ratios. These dictate how much estimated free space is reported on the volume. If you tell the system to estimate a 2 to 1 compression ratio but the files you are storing only compress at 1.5 to 1, then you are going to run out of space faster than you expect on the drive.
• Don't copy files to a compressed volume that are already compressed or that will not compress very well. Some people think that they can compress a disk and then fill it with ZIP files, doubling the amount of information they can archive. Unfortunately, TANSTAAFL, and with compression, compressing a second time rarely accomplishes much.
• See this section, which describes other situations (unrelated specifically to compression) that can cause free space to appear to be missing on a disk volume. (Note that the cluster size of a compressed volume generally cannot be changed and usually is small enough that it is not a major source of lost space.)

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