Does geom_uzip act as a variable size storage device?

[Loren M. Lang]

How does the uzip geom work as a block device for a file system.  My
understanding of it is that it operates like a block device that trys to
compress all data written to it using the same algorithm as used by
pkzip and gzip.  The problem I see with that is that not all data
compresses by the same amount, some compresses more and other less, so a
disk that acts as a compressor can hold different amounts of data
depending on what's written to it.  Filesystems like msdos and ufs2
don't support running on block devices of variable sizes as far as I know
so how can I possibly format a uzip disk with a regular filesystem.  I
know that ufs supports being resized, but that's not the same as a block
device that appears to be constantly changing size as data is being
written to it so how does uzip work?  Does it appear as some fixed size
that may have wasted space if the data was able to compress really well,
and when the data doesn't compress well enough, well, I don't know what
would happen then.

Am I just missing something here or can the uzip geom be dangerous
depending on how it's used and what fs it's formatted as.

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