6.3ZERO 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.
6.4DISCONNECT-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
6.5TRACK 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.
6.6AVERAGE ACCESS TIME
A 10,000 rpm rotation speed yields an average latency of 3 ms.
6.7EMBEDDED SERVO SYSTEM
Embedded servo information is written in a spoke configuration on every track, on every disk surface. The spokes (or headers) consist of quadrature analog patterns and
Maxtor Atlas 10K V