Buffer Thresholds; Figure 2-8: Host Buffer Data Flow During Write Operations; Figure 2-9: Host Buffer Data Flow During Read Operations - Tandberg Data SLR7-140 PROGRAMMER Manual

SCSI - bus Si de
Dat a goi ng i n
Figure 2-8:
SC SI - bus Si de
Dat a goi ng out
Figure 2-9:
Tandberg Data
underrun condition.
Operation and the sections on MODE SELECT and MODE SENSE
commands for further details about Automatic Velocity
Control, tape speeds and Forced Streaming.
2.7.6.

Buffer Thresholds

Both the host buffer and the tape buffer act like large
FIFOs. Data blocks
buffer also control
blocks) are inserted in one end of the FIFO and the same
data is later removed at the other end.
In write mode, when moving data into the host buffer,
data is entered at a certain preferred amount at the
time. If there is not enough free space in the host
buffer for this
entered at all. The room required is called a buffer
threshold. There must usually be at least room for an
amount of data corresponding to the buffer threshold
before new data are transferred in from the SCSI-bus.
This will minimize the number of disconnects/reconnects.
This buffer threshold is configurable.
T hr es hol d Det ect or
Host Buffer Data flow During Write Operations
When transferring data blocks out of the tape buffer and
onto the tape a similar mechanism exists. When the FIFO
for some reason is empty (an underrun condition), the
tape write operation will not be started again until the
number of blocks
tapemarks) ready in the FIFO is equal to or larger than
another buffer threshold. This will have a tendency to
minimize the number of tape start and stops (underrun
conditions). This threshold is fixed and is not under
user control.
In read mode there are also two buffer thresholds; one
controlling the tape read operation and one controlling
the SCSI-bus transfer. These thresholds work in much the
same way as the threshold in write mode. The host buffer
threshold is configurable
threshold is fixed
T hr es hol d Det ect or
Dat a i n Buf f er
Host Buffer Data flow During Read Operations
2-26 Tandberg SLR Product Line SCSI Functional Specifications
See 2.9.
and tapemarks
blocks, filler
amount, no logical
Dat a i n Buf f er
(data, control,
while
About Tape Streamers
Optimizing Streaming
(and for the
blocks and
data blocks
Dat a F or m at t er Si de
Dat a goi ng out
filler, ECC
the tape buffer
Dat a For m at t er Si de
Dat a goi ng i n
tape
ECC
are
or