Prefetch/Multi-Segmented Cache Control - Seagate Elite 9 Product Manual

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5.5

Prefetch/multi-segmented cache control

The drive provides a prefetch/multi-segmented cache algorithm, which in
many cases enhances system performance. To select this feature, the host
sends the Mode Select command with the proper values in the applicable
bytes in page 08h (see the SCSI-2 Interface Product Manual ). Default is
prefetch and cache operation enabled.
Of the 1,024 Kbytes physical buffer space, approximately 960 Kbytes can be
used as a cache. The cache can be 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 cache. If the cache is enabled (see RCD bit, Table 5.2.1-27
in the SCSI-2 Interface Product Manual ), data requested by the host with a
read command is retrieved from the cache before any disc access is initiated.
Data in contiguous logical blocks immediately beyond that requested by the
Read command can be retrieved and stored in the cache for immediate
transfer to the initiator on subsequent read commands. This is referred to as
the prefetch operation. Since data that is prefetched may replace data
already in the cache segment, an initiator can limit the amount of prefetch
data to optimize system performance. The drive never prefetches more
sectors than the number specified in bytes 8 and 9 of Mode page 08h (see
the SCSI-2 Interface Product Manual ). If the cache is not enabled, 960
Kbytes of the buffer are used as a circular buffer for read/writes, with no
prefetch operation and no segmented cache operation.
The following is a simplified description of the prefetch/cache operation:
Case A. A read command is received and the first logical block is already in
the cache.
1. The drive transfers to the initiator the first logical block requested plus all
subsequent contiguous logical blocks that are already in the cache. This
data may be in multiple segments.
2. When a requested logical block is reached that is not in any segment,
the drive fetches it and any remaining requested logical block addresses
from the disc and puts them in a segment of the cache. The drive
transfers the remaining requested logical blocks from the cache to the
initiator in accordance with the "buffer-full" ratio specification given in
Mode Sel ect Disconnect/Reconnect parameters, page 02h (see the
SCSI-2 Interface Product Manual ).
3. The drive prefetches additional logical blocks contiguous to those
transferred in step 2 above and stores them in the segment. The drive
stops filling the segment when the maximum prefetch value has been
transferred (see the SCSI-2 Interface Product Manual ).
Case B. A read command is received and the first logical block address
requested is not in any segment of the cache.
1. The drive fetches the requested logical blocks from the disc and
transfers them into a segment, then from there to the initiator in
accordance with the "buffer-full" ratio specification given in Mode Select
Disconnect/Reconnect parameters, page 02h (see the SCSI-2 Inter-
face Product Manual ).
Elite 9 Product Manual, Rev. C