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ARCHIVE PYTHON DDS-DC OAT DRIVES---PRODUCT DESCRIPTION MANUAL
When a drive is positioned at the beginning of an entity and the host issued a READ command in fIxed-block
mode, a nondecompressing drive returns either the number of bytes in one SCSI block or the total number of
bytes in the entity, whichever is smaller. If the host issues a READ command in variable-block mode, a
nondecompressing drive returns either the number of bytes requested in the Command Descriptor Block
(CDB) or the number of bytes in the entity, whichever is smaller. The drive then positions to the end of the
entity.
If the RED bit is 0, or if the RED bit is 1 and this entity is the fIrst one encountered since the last READ
command was issued, the drive reports a Check Condition.
The additional Sense code is set to
Decompression Exception.
If the Supress Incorrect Length Indicator (SILl) bit is not set (0), or if the SILl bit is set (1) but the number of
bytes in the entity is greater than the number of bytes actually transferred, an Illegal Length condition occurs,
and the ILl bit in the Sense Data is set (1).
NOTE: If an Illegal Length condition exists, and the setting of the RED bit is such tlUlt a
Decompression Exception is not reported, a Check Condition is reported. The ILl bit
will be set (1) and the Sense Key will be set to NO SENSE.
A strategy for reading compressed entities would be to set the RED bit to 1 and issue READ commands in
variable-length mode with a maximum transfer size and the SILl bit set. A Check Condition would only be
reported if the entity is larger than the transfer size or for the first entity encountered. The maximum transfer
size allowed by SCSI is FFFFFF hex (16 MB). The host should allocate as much internal buffer space as
possible to allow a large transfer size.
C.S.2 SCSI Command Operation for Noncompressing Modes
An entity must always be regarded as one or more contiguous blocks rather then one large variable-length
block. In order to guarantee accurate tape positioning, this method of handling entities must be maintained
whether or not the drive supports compression. This method of handling, however, limits drives that are
unable to decompress entities. Such drives are unable to position to a point within an entity. This limitation
affects the READ, SPACE Blocks, LOCATE, and SEEK commands.
For example, assume a tape contains a mix of compressed and uncompressed blocks. This tape is inserted
into a drive that does not support compression, and the drive is sending the data to a host that mayor may not
have the ability to decompress the data. Figure C-1 illustrates this situation and the Logical Block Addresses
(LBAs) on the tape.
I
LBA =
48
I
LBA
=
50
I
LBA = 65
I
LBA=67
Figure
C-l.
LBAs of Tape In Example
C-8
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