20-1
Tandberg SLR Product Line SCSI Functional Specifications
20.
Read Position
20.1. Command Description
The READ POSITION command requests the Drive to return a
special position key that identifies the current
position in the data stream on the medium. While writing
(or reading), the READ POSITION command is typically
executed every time the medium is at a position that the
host system might want to go back to at a later time.
The returned position key can then be stored and used as
an input to the LOCATE command later. The LOCATE command
will then bring the medium back to the same position as
it was when the READ POSITION command was executed.
The READ POSITION command requests the Drive to return
the block address associated with the current position.
The value indicates the block address of the next data
block to be transferred between the initiator and the
target if a READ or WRITE command is issued.
The READ POSITION command may request this block address
to be the logical position or the physical position.
For SLRtape140 to SLRtape7 or SLR32 media the READ
POSITION command supports logical tape positions only.
For other medium types both physical and logical
positions are supported.
If the READ POSITION command is requested to return
block positions as physical tape block addresses, it may
give different physical position keys for reading,
spacing or writing to the same position on the tape. The
READ POSITION physical command can therefore not be used
to determine whether a certain position is reached. It
will also not be generally possible to calculate
(mathematically) the block address of any other position
relative to the current position.
If the READ POSITION command returns block positions as
physical tape block addresses, two logical blocks might
have the same physical address on SLR5 and DC9250 media
and the current block size is 512 bytes. The reason is
that the READ POSITION command will - if necessary - pad
the last half of a QIC-2GB-DC or QIC-4GB-DC (1024 bytes
long) physical tape block during WRITE and move the tape
position to the start of the next physical tape block
before returning any block positions in order to be able
to assign unique addresses to all logical blocks.
Note that when using the READ POSITION command to return
physical block positions on tapes with compressed data,
multiple positions may also yield the same physical
block address.
When using a SLRtape140 to SLRtape7 or SLR32 medium
tapemarks on the tape count as one logical block each.
When using other medium types, tapemarks are not counted
as logical blocks.
The Drive will disconnect when executing the READ
POSITION command.
