5.5Cache operation
Note. Refer to the Fibre Channel Interface Manual for more detail concerning the cache bits.
Of the 64 Mbytes physical buffer space in the drive, approximately 30,000 kbytes are available as a data cache. The remaining buffer space is reserved for internal drive use.
The drive keeps track of the logical block addresses of the data stored in each segment of the buffer. If the cache is enabled (see RCD bit in the FC Interface Manual), data requested by the host with a read command is retrieved from the buffer, if possible, before any disk access is initiated. If cache operation is not enabled, the buffer is still used, but only as circular buffer segments during disk 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. All data transfers to the host are in accordance with
The following is a simplified description of the prefetch/cache operation:
Case
Case
1.The drive fetches the requested logical blocks from the disk and transfers them into a segment, and then from there to the host in accordance with the Mode Select Disconnect/Reconnect parameters, page 02h.
2.If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.
Each cache segment is actually a
Note. The size of each segment is not reported by Mode Sense command page 08h, bytes 14 and 15. The value 0XFFFF is always reported regardless of the actual size of the segment. Sending a size specification using the Mode Select command (bytes 14 and 15) does not set up a new segment size. If the STRICT bit in Mode page 00h (byte 2, bit 1) is set to one, the drive responds as it does for any attempt to change an unchangeable parameter.
12 | Savvio 10K.5 FC Product Manual, Rev. D |