Hitachi 7K400 Specifications - Page 28

4.5 Performance characteristics Drive performance is characterized by the following parameters: • Command overhead • Mechanical head positioning - Seek time - 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. 4.5.1 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 5: 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) Read (cache hit) (from Command Write to DRQ) Write (from Command Write to DRQ) Seek (from Command Write to Seek Start) 0.5 0.1 0.015 0.5 0.5 0.1 0.05 not applicable Deskstar 7K400 Hard Disk Drive Specification 14