Hitachi HDS725050KLA360 - Command Overhead; Table 4. Command Overhead

Deskstar 7K500 and Deskstar E7K500 Hard Disk Drive Specification
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- Latency
• Data transfer speed
• Buffering operation (Look ahead/Write cache)
All the above parameters contribute to drive performance. There are other parameters that contribute to the performance of the
actual system. This specification tries to define the bare drive characteristics, not system throughput, which depends on the
system and the application.
4.4.1 Command overhead
Command overhead is defined as the time required from the time the command is written into the command register by a host
to the assertion of DRQ for the first data byte of a READ command when the requested data is not in the buffer excluding
Physical seek time and Latency.
The table below gives average command overhead.
Table 4: Command overhead
Command type (Drive is in quiescent state) Time (typical)
(ms)
Time (typical) for
queued command
(ms)
Read (cache not hit) (from Command Write to Seek Start) 0.5 0.5
Read (cache hit) (from Command Write to DRQ) 0.1 0.2
Write (from Command Write to DRQ) 0.015 0.2
Seek (from Command Write to Seek Start) 0.5 not applicable