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CHEETAH 15K.7 SAS PRODUCT MANUAL, REV. F 10
4.5 CACHE OPERATION
Note. Refer to the SAS Interface Manual for more detail concerning the cache bits.
Of the 16 Mbytes physical buffer space in the drive, approximately 13,000 kbytes can be used as a cache. The buffer is
divided into logical segments from which data is read and to which data is written.
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 SAS 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 buffer-full ratio rules. See the explanation provided with the information about Mode Page 02h (disconnect/
reconnect control) in the SAS Interface Manual.
The following is a simplified description of the prefetch/cache operation:
Case A—read command is received and all of the requested logical blocks are already in the cache:
1. Drive transfers the requested logical blocks to the initiator.
Case B—A Read command requests data, and at least one requested logical block is not in any segment of the cache:
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 4.5.2 for operation from this point.
Each cache segment is actually a self-contained circular buffer whose length is an integer number of logical blocks. The
drive dynamically creates and removes segments based on the workload. The wrap-around capability of the individual
segments greatly enhances the cache’s overall performance.
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 param-
eter.