DISK DRIVE OPERATION
Thus Read Caching can provide substantial time savings during at least half of all disk requests. For example, Read Caching could save most of the disk transaction time by eliminating the seek and rotational latency delays that prominently dominate the typical disk transaction.
Read Caching operates by continuing to fill its cache memory with adjacent data after transferring data requested by the host. Unlike a
The cache memory consists of a 512KB sync DRAM buffer allocated to hold the data. It can be directly accessed by the host by means of read and write commands. The unit of data stored is the logical block, or a multiple of the
•IDENTIFY DRIVE (ECh)
•FORMAT TRACK (50h)
•EXECUTE DRIVE DIAGNOSTIC (90h)
•READ LONG (23h)
•WRITE VERIFY (3Ch)
•INITIALIZE DEVICE PARAMETER (91h)
•SLEEP (99h, E6h)
•STANDBY IMMEDIATELY (94h,E0h)
•READ BUFFER (E4h)
•WRITE BUFFER (E8h)
•WRITE SAME (E9h)
5.5.2Write Caching
Write caching improves both single and
The data is then written collectively to the drive thereby minimizing the disk seeking operation. Data is held in cache no longer than the maximum seek time plus rotational latency. Host retries must be enabled for Write Caching to be active.
If the data request is random, the data of the previous command is written to the disk before COMMAND COMPLETE is posted for the current command. Read commands work similarly. The previous write is allowed to finish before the read operation starts.
If a defective sector is found during a write, the sector is automatically relocated before the write occurs. This ensures that cached data that already has been reported as written successfully gets written, even if an error should occur.
If the sector is not automatically relocated, the drive drops out of write caching and reports the error as an ID Not Found. If the write command is still active on the AT interface, the error is reported during that command. Otherwise, it is reported on the next command.
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