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Note – Never use two compression methods on the same data. The use of a second
compression method rarely compresses the data further; double compressed data
can actually expand in size.
C.4.2
Software Data Compression
Data compression can be done via software. The compress and restore commands
are used for writing and reading, respectively. The Solstice Backup package
incorporates the capability to do software-based data compression. Software data
compression results in a better compression ratio than the drive's internal data
compression but takes significantly longer.
Note – Never use two compression methods on the same data. The use of a second
compression method rarely compresses the data further; double compressed data
can actually expand in size.
C.5
Choosing a Blocking Factor or a Block
Size
You must use the proper block size in the data transfers to the tape drive. The block
size determines the amount of data sent to the drive in one command and, more
importantly, determines the amount of data on each logical block on the drive.
You get the best performance and throughput when you use the largest block size
(blocking factor) supported. Typically, this is 63k bytes which is specified as a
blocking factor of 126. The b parameter and a numerical parameter are specified in
the command to the drive. A block size must be specified on both the write and the
read operation. If one is not selected, a default is used that may not be the largest or
best choice. Be aware that the read operation must always specify an equal or larger
block size than the write operation. There is no penalty for choosing a larger block
size on read. Thus, you should always choose the largest block size for any read
operation.
Appendix C
8 mm Tape Drive Information
C-3
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