Conner CFP2105 series, Product Manual, Page: 34 / 90

Chapter 3 Drive Operations
Page 26 Filepro CFP2105 Series
Tagged Command Queuing
The drive, operating using the Tagged Command Queuing code, can queue up to
32 commands. Commands in the queue which involve seeks are re-ordered
using a Scan (elevator seek methodology). Read and Write commands are
coalesced (combined into a single operation to the background processor) to
minimize inter processor communication overhead and reduce mechanical
motion.
Seek Re-ordering
Seeks are re-ordered using the Scan or what is sometimes referred to as the
elevator seek method. Seeks already in the queue are re-ordered so they can be
executed sequentially on a sweep toward the inner diameter (ID) or the outer
diameter (OD) of the disk. Any new seeks entering the queue ahead of the
sweep is ordered for execution during the current sweep. Any new seeks
entering the queue behind the current sweep are held for re-ordering during the
reverse sweep. When there are no more commands in the queue ahead of the
current sweep, the direction of the sweep is reversed.
Seek re-ordering is controlled by Message Code (Simple, Head of Queue or
Ordered Queue) during the Message Phase. Re-ordering of seeks issued with a
Simple Queue Tag message may also be restricted using the Queue Algorithm
Modifier bits in MODE SELECT page 0A
H
. Refer to the Eleventh Generation
SCSI Interface Manual for additional information on Messages and MODE
SELECT page 0A
H
.
Buffer Management
The 512K byte buffer is treated by the queuing code as two 240KB buffers (F0
H
sectors) to maximize coalescing. Look ahead reads are performed by the drive
when there are no commands in the queue awaiting execution. Look ahead
reads are not performed when there are commands in the queue since another
command will be waiting for execution as soon as the current command
completes and because the queue affords pre-knowledge of subsequent
commands instead of having to anticipate them.
Sequential read or write commands are coalesced into single commands to the
background processor. On a read operation, the data associated with each queue
tag is transmitted to the host as the buffer is filled by the background processor.
On write operations, the drive will connect to the initiator(s), transmit the data
into the buffer and disconnect; coalescing sequential data in the buffer before
initiating the write to the background processor. The drive will reconnect with
each of the writes, using the queue tags, completing the command after the data
has been written to the disk. This operation provides the performance of write
caching without the exposure of completing the command prior to writing the
data to the disk.