Quantum 10K II - Manual (Page 304)

Feature Descriptions
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives 6-3
6.4 ZERO LATENCY READ/WRITE
An average of half a revolution of latency is saved by starting to read or write as
soon as a seek settles on the designated track. If reading, the initial data read is
stored in a buffer (cache) until the actual starting address data block is reached and
read. The starting address data is then transferred to the bus as the rest of the data
is transferring from media to the buffer. When the initial data address read is
reached again, that data in the buffer is transferred to the bus. This ensures that the
track data goes out in media order and eliminates the rotational latency associated
with waiting for the starting address to rotate to the head.
Writing works essentially the same way only in reverse. The data is first transferred
to the drive’s buffer. After seeking to the desired track, a write pointer is set to the
current location of the head on the track and writing
is commenced from the buffer,
starting at the corresponding place in the data.
6.5 DISCONNECT-RECONNECT
System throughput can be improved by disconnecting the drive from the initiator
during physical positioning operations, thereby freeing up the SCSI bus for other I/O
processes. After the drive has determined that there will be a delay, it disconnects
itself from the SCSI bus by sending a DISCONNECT message and enters the BUS
FREE phase.
When the drive is ready to resume data transfer, it arbitrates for the SCSI bus and,
after winning arbitration, reconnects to the initiator and sends an IDENTIFY
message via the MESSAGE IN phase. This revives the I_T_L nexus (initiator-target-
logical unit connection) so the initiator can retrieve the correct set of data pointers
for the I/O process. The initiator restores the active pointers to their most recent
saved values, and the drive continues to finish the original I/O process.
Disconnect-Reconnect is controlled with the Mode Select Command (15h) for the
Disconnect/Reconnect (02h) mode page, as described in Chapter 6.
6.6 TRACK AND CYLINDER SKEWING
The disk drive improves data throughput by skewing track and cylinder addresses.
When the drive switches heads or tracks, or both, to access sequential data, the
rotation of the disk media allows one or more physical blocks to pass by the
read/write head before the head is ready to continue the transfer. Skewing adjusts
the block addresses so that the head switch and settle times and the media rotation
coincide to bring the head exactly to the next logical block. This minimizes
rotational latency (and increases throughput) when data is accessed sequentially.
6.7 AVERAGE ACCESS TIME
A 10,000 rpm rotation speed yields an average latency of 3.00 ms.