Controlling Data Compression; Table 2-5: Data Compression Page Descriptor - Tandberg Data SLR7-140 PROGRAMMER Manual

Tandberg Data
Note that in some cases data will actually expand when
subjected
instance be the case when feeding already compressed
data to the drive and the compression option is enabled.
When
adaptively turn compression on and off. When a certain
expansion has been detected the Drive decides that the
data are not compressible and this piece of data will be
written
again
compression ratio), compression is turned back on (if
enabled at all).
When
SLRtape75,
will
compressible. A worst case expansion of 12.5% may occur
on certain data patterns.
When using SLRtape140 to SLRtape7 or SLR32, media any
partition may hold a mix of compressed and uncompressed
data.
When using an SLR5 or DC9250 medium the drive will not
allow a mix of compressed and uncompressed data on the
same medium. Before starting a write operation from BOM,
compression may be turned on or off. At a later stage,
it will not be possible to change the compression mode.
When appending data on a tape, the drive will ensure
that compression mode is preserved (on or off). This
means that if the pre-written data on the medium is not
compressed, the appended data will not be compressed
(even if the DCE bit in the Data Compression Page has
configured compression on). If, however, the pre-written
data on the medium is compressed, the appended data will
be compressed (even if compression has been configured
off).
2.8.2.
Data compression is controlled by fields in the Data
Compression page of the MODE SENSE/SELECT page (page
code
interest are marked with " XX" .
BYTE BIT 7
00 XX
01
02 DCE
03 DDE
04
 Compression Algorithm
07
08
 Decompression Algorithm
11
12 XXXX
13 XXXX
14 XXXX
15 XXXX

Table 2-5: Data Compression Page Descriptor

2-30
to the compression
writing on a
uncompressed.
is presented
writing on
SLRtape100
keep compression

Controlling Data Compression

0Fh). This page
6 5
XX Page Code = 0Fh
XX
DCC XX
XX
Tandberg SLR Product Line SCSI Functional Specifications
process.
SLRtape50 media
When data that
to the drive
SLRtape7, SLRtape40,
and SLRtape140
on even if
is shown below.
4 3 2
XX
About Tape Streamers
This may for
the drive will
is compressible
(with a favorable
SLRtape60,
media the drive
the data is not
Fields of
1 0
no