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Tandberg Data
2.8.
Data Compression
The Tandberg SLR Product Line Tape Drives support the
Adaptive Lossless Data Compression (ALDC) algorithm. The
ALDC is a variant of the LZ 1 (Lempel-Ziv 1) class of
compression algorithms. For further details about the
compression algorithm please refer to the description
[8].
The
actual
performed in hardware. For further details about the
drive electronics, refer to the Tandberg SLR Product
Line Reference Manual [1].
2.8.1.
Background
Data
compression
formats. When data compression is active, all data sent
to the drive will be compressed (see section 2.8.2.2 for
an exception). When compressed data is read the data is
automatically decompressed. The Initiator may then issue
a MODE SENSE command and check the Data Compression
Parameter Page to investigate if the data on the tape is
compressed or not.
When compression is enabled, the data will be compressed
into groups of data called Compression Block Groups.
Each of these groups will consist of an uncompressed
Compression Header and compressed data. The data part is
the result of compression of up to 64 KBytes (SLRtape24,
SLR32
and
SLRtape50
SLRtape40,
SLRtape60,
SLRtape140 media) of original data [9] - [17].
Original logical block with uncompressed length of 200 Kbytes, SLRtape24, SLRtape32
or SLRtape50
Compression Block Group
64 Kbytes
Compressed data (uncompressed length given)
Compression Header
Figure 2-10:
Packing Data in Compression Block Groups
The data compression ratio will depend on both the data
pattern
to
compress
commands is issued to the Drive.
command with a block size different from the block size
in
the
previous
Compression Block Group to be written, so a lot of WRITE
commands
transferring
result in a less than optimal compression ratio. Since
the length of the Compression Header is 18 to 24 bytes
large, the best performance is achieved by issuing WRITE
commands writing large blocks or a large number of equal
size blocks. Typical compression ratios are 2:1 to 3:1.
2-29
Tandberg SLR Product Line SCSI Functional Specifications
compression
and
can
be
used
on
media)
or
256
SLRtape75,
64 Kbytes
64 Kbytes
and
how
the
Each start of a WRITE
WRITE
command
blocks
of
different
About Tape Streamers
decompression
all
supported
tape
KBytes
(SLRtape7,
SLRtape100
8 Kbytes
sequence
of
WRITE
will
force
a
sizes
will
are
and
new
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