Cheetah 36XL Product Manual, Rev. E | 11 |
enabled and disabled independently via the Mode Select command. However, in actual operation the prefetch feature overlaps cache operation somewhat as is noted in Section 4.5.1 and 4.5.2.
All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Tables 7 and 8.
4.5Cache operation
In general, 4096 kbytes of the physical buffer space in the drive can be used as storage space for cache oper- ations. The buffer can be divided into logical segments (Mode Select Page 08h, byte 13) from which data is read and to which data is written. The drive maintains a table of logical block disk medium addresses of the data stored in each segment of the buffer. If cache operation is enabled (RCD bit = 0 in Mode Page 08h, byte 2, bit 0. See SCSI Interface Product Manual, part number 75789509), data requested by the host with a Read command is retrieved from the buffer (if it is there), before any disc access is initiated. If cache operation is not enabled, the buffer (still segmented with required number of segments) is still used, but only as circular buffer segments during disc medium read operations (disregarding Prefetch operation for the moment). That is, the drive does not check in the buffer segments for the requested read data, but goes directly to the medium to retrieve it. The retrieved data merely passes through some buffer segment on the way to the host. On a cache miss, all data transfers to the host are in accordance with
The following is a simplified description of a read operation with cache operation enabled:
Case A - A Read command is received and the first logical block (LB) is already in cache:
1.Drive transfers to the initiator the first LB requested plus all subsequent contiguous LBs that are already in the cache. This data may be in multiple segments.
2.When the requested LB is reached that is not in any cache segment, the drive fetches it and any remaining requested LBs from the disc and puts them in a segment of the cache. The drive transfers the remaining requested LBs from the cache to the host in accordance with the disconnect/reconnect specification men- tioned above.
3.If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
Case B - A Read command requests data, the first LB of which is not in any segment of the cache:
1.The drive fetches the requested LBs from the disc and transfers them into a segment, and from there to the host in accordance with the disconnect/reconnect specification referred to in case A.
2.If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.
Each buffer segment is actually a
4.5.1Caching write data
Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to be written to the medium is stored in one or more segments while the drive performs the write command.
If read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made avail- able for future read cache hits. The same buffer space and segmentation is used as set up for read functions. The buffer segmentation scheme is set up or changed independently, having nothing to do with the state of