Quantum DLT 2000 Handbook page 66

very effectively, as can big graphics files. Other material, like tables,
for example, doesn't compress much at all. Files that have already
been compressed may even get bigger if you try to compress them.
The bottom line is that compression may or may not be relevant to
your application, depending on the kind of data you deal with. Just
make sure that you compare native-to-native, or compressed-to-
equally-compressed figures. And, if the manufacturer has not been
very clear that the figure you see is native mode, you probably
should assume that the figure is for compressed data. For DLTtape
systems, the stated capacity is in both native mode and in com-
pressed mode, assuming 2:1 compression.
Performance and Compression
Performance (data transfer rate) is typically described in terms of
MB per second. You need to determine if the transfer rate figure is
for native mode, or if it assumes 2:1 data compression, or higher
compression ratios. Be careful to distinguish between terms like
peak transfer rate and sustained transfer rate. Peak transfer or burst
rate usually refers to the maximum transfer rate of the bus. It has
less to do with the performance of the tape drive than with interop-
erability with other equipment you may have in your system. The
performance figure you should pay attention to is the sustained fig-
ure. For the DLTtape system, we list both figures, in native mode.
Another performance-related figure is average access time. That is
the average time it takes to locate a file that is halfway into the
tape. It assumes this is the first access requested from this specific
tape, so the drive is starting to seek the file from the beginning of
the tape. This is a very conservative approach. It is appropriate in
applications in which files being sought are truly randomly distrib-
uted across the tape. However, in many, if not most, applications,
the next file sought is near the previous file accessed, and actual
access time may be shorter.
QUANTUM DLTtape HANDBOOK 6.3