Do you really mean a compressed volume or a volume which supports compression of indivual files?
GetVolumeInformation:
Zitat:
The FS_VOL_IS_COMPRESSED flag is the only indicator of volume-based compression. The file system name is not altered to indicate compression, for example, this flag is returned set on a DoubleSpace volume. When compression is volume-based, an entire volume is compressed or not compressed.
The FS_FILE_COMPRESSION flag indicates whether a file system supports file-based compression. When compression is file-based, individual files can be compressed or not compressed.
The FS_FILE_COMPRESSION and FS_VOL_IS_COMPRESSED flags are mutually exclusive. Both bits cannot be returned set.