Read-Ahead; Underrun/Overrun - Tandberg Data SLR7-140 PROGRAMMER Manual

Tandberg Data
2.7.4.

Read-Ahead

When operating in read mode the Drive always tries to
fill up the buffer with read-ahead data. This means that
when a read type command terminates the Drive continues
to read data off the tape. The read-ahead only stops
when the data buffer is full, if a non-read command is
issued or if there is no more data to read. The read-
ahead also stops when an uncorrectable block has been
detected.
Read-ahead minimizes tape start and stops because when a
read type command (including SPACE and VERIFY
another read type command the wanted data may already be
ready in the data buffer.
When an error has been detected by the tape system the
Drive does not report the error until all data up to the
point where the error occurred has been read out of the
data buffer. This means that if an unrecovered read
error has occurred this error is not reported until an
Initiator requests the data block in error.
2.7.5.

Underrun/Overrun

When the Drive is in write mode data is moved from the
data buffer on to the tape. The tape write operation can
only continue as long as there is any data left to write
in the data buffer. If the buffer for any reason becomes
empty the write operation must be stopped. This can
happen if the Initiator is too slow transferring data or
if the Drive is configured in non-buffered mode. When
the tape is stopped in this way an underrun condition
has occurred. Note,
stopped immediately. The Drive is able to delay the
stopping for some limited amount of time. This may be
controlled by the Initiator by manipulating the Forced
Streaming Count
(see the MODE SELECT command). When the data buffer
later becomes non-empty the tape write operation can
continue (see also the next section)
When the Drive is in read mode data is moved from the
tape and into
becomes full the tape read operation must be stopped
immediately. This can happen if the Initiator is too
slow transferring data or if the Initiator has stopped
issuing READ, SPACE or VERIFY
tape read-ahead). When the tape is stopped in this way
an overrun condition has occurred. When the data buffer
later is emptied the tape read operation may continue
(see also the next section).
The number of underrun/overrun conditions encountered
may be reduced
Control (AVC) and/or Forced Streaming. If the Automatic
Velocity Control is enabled, the drive will select the
speed which, based
optimize streaming
repositioning (only if more than one speed is available
for the active tape format). If Forced Streaming is
enabled, the drive will keep on rewriting old data a
configurable number
1
VERIFY is not supported by SLR7 and SLR140
2-25 Tandberg SLR Product Line SCSI Functional Specifications
however, that
in the Miscellaneous
the data buffer.
1
by utilizing the
on the data
activity
of times
About Tape Streamers
1
) follows
the tape is
Parameters
If the data
commands (remember the
Automatic Velocity
transfer rate,
and minimize
before going into
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Page
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