On most vxfs file systems, the block or character special file for the file system provides access to a raw image of the file system for purposes such as backing up the file system to tape. On a snapshot file system, access to the corresponding block or character special provides little useful information. The fscat utility, however, provides a stream of bytes representing the file system snapshot. The datastream is written by default to standard output, although the -f output_file option can be used to specify another destination. The datastream on standard output can be processed in any of several ways, such as being processed in a pipeline, being written to a tape, and so on. fscat will work when executed on the special device of any vxfs file system.
By default, the output is a stream of bytes that starts at the beginning of the file system and goes to the end. On a snapshot file system, data is read from the file system using special ioctls on the mount point. On other vxfs file systems, data is read from the specified special file. Unless otherwise specified, data is written to standard output.
All numbers entered as options may have 0 prepended to indicate octal, or 0x prepended to indicate hexadecimal. A b or B may be appended to indicate the value is in 512-byte blocks, a k or K to indicate the value is in kilobytes, an m or M to indicate the value is in megabytes or a g or G to indicate the value is in gigabytes. An appended letter may be separated from the number by a space, in which case the letter and number should be enclosed in a set of quotes (for example, "512 b").
All numbers entered as options must be in multiples of 512 bytes. So, for example, a value of 5713 as an offset will be rejected.
A snapshot file system cannot be written to. A snapshot file system exists only as long as it is mounted; once unmounted, the special file no longer contains a snapshot file system.