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52 PCM-3420 User's Manual
1) We have measured propagation delays from various cables
and found an average of 1.7 nanoseconds per foot, which is
roughly 5.25 ns per meter.
2) The turn-around time is the amount of time the SCSI chip takes
to change an output in response to an input. If REQ is an input
then ACK is an output. Or if ACK is an input then REQ is an
output.
3) The asynchronous transfer uses an interlocked handshake
where a device cannot do the next thing until it receives positive
acknowledgment that the other device received the last thing.
T
TAGGED QUEUING
A SCSI-2 feature that increases performance on SCSI disk drives.
With tagged queuing, the host adapter, the host adapter driver,
and the hard disk drive work together to increase performance by
reordering the requests from the host adapter to minimize head
switching and seeking. For example, the host adapter may ask for
the following data in the following order; LBA 0, 1, 101, 102, 5, 6
(LBA = logical block address, or a byte of data)
Without tagged queuing:
If tagged queuing was not enabled, the drive would seek to LBA
0, transfer bytes 0, then 1, then seek to 101, transfer 101 and 102,
then seek back to lba 5, transfer 5, then 6. ===>involves 3
seeks (initial seek to 5, seek to 101, then seek back to 5)
With tagged queuing:
If tagged queuing was enabled, the drive would seek to LBA 0,
transfer bytes 0, then 1, 5 and 6, then seek to 101, transferring 101
and 102. At this point all the data would be transferred.
===>involves 2 seeks (initial seek to 5, then the seek to 101)
Seeking on a disk drive takes a relatively long time, so having
seeks and head switches really speeds up performance.