USER RESPONSIBLE F O R V E R I F Y I N G VERSION A N D COMPLETENESS
O E M F U N C T I O N A L SPECIFICATION ULTRASTAR X P (DFHC) SSA M O D E L S 1.12/2.25 GB - 1.0" H I G H
for the command can start immediately. This effectively avoids latency time for read operations sequen-
tial on a previous read.
If the data requested by a read operation is not in the read-ahead buffers, there is an increase in the
command overhead time due to the time spent searching the buffers. This time depends on the number
of buffer segments selected by the Mode Select command.
If read-ahead is still in progress when the next command is received and the data requested is not
sequential, the drive aborts read-ahead and starts the command.
The time to perform this abort
increases the Command Execution Overhead by .23ms.
3.4.6 When No Seek is Required
For a Read command, the additional Command Execution Overhead when no seek is required is approxi-
mately .50ms. For a Write, it is approximately .70ms.
3.4.7 For Queued Commands
If commands are sent to the drive when it is busy performing a previous command, they can be queued. In
this case, some of the command processing is performed during the previous command and the overhead for
the queued command is reduced by approximately .20 milliseconds.
3.4.7.1 Reordered Commands
If the Queue Algorithm Modifier Mode Parameter field is set to allow it, commands in the device command
queue may be executed in a different order than they were received. Commands are reordered so that the
seek portion of Command Execution time is minimized.
The amount of reduction is a function of the
location of the 1st requested block per command and the rate at which the commands are sent to the drive.
A Queue Algorithm Modifier Mode Parameter value of 9 enables an algorithm that gives the using system
the ability to place new commands into the drive command queue execution order relative to the out-
standing commands in the queue. For example, if a request is sent to the drive that the using system prior-
itizes such that it's completion time is more important than one or more of the outstanding commands, the
using system can increase the likelihood that command is executed before those others by using a tag value
greater than those outstanding commands.
3.4.7.2 Back-To-Back Commands
If consecutive read/write commands access contiguous data, they can be serviced without incurring disk
latency between commands.
Note:
There is a minimum transfer length for a given environment where continuous access to the disk can
not be maintained without missing a motor revolution. For Write commands with Write Caching enabled
the likelihood is increased that shorter transfers can fulfill the requirements needed to maintain continuous
writing to the disk.
Back-to-back Read is only enabled if Read-ahead is disabled.
3.4.8 Out of Order Transfers
Two bits in the SCSI Command message control out of order transfers. O O T M applies to transfers to/from
the media and OOTI applies to transfers to/from the interface (SSA Link).
The benefit from setting O O T M increases as the transfer length approaches one disk revolution. This affects
both reads and writes and is due to the reduction in latency.
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