Seagate ST31200N, Руководство по эксплуатации продукта

Страница: 20 / 78

12 Product Manual - Hawk 1LP Family SCSI-2 (Volume 1), Rev. A
_______________________________________________________________________________________
5.5 Prefetch/multi segmented cache control
The drive provides prefetch (read look-ahead) and multi-segmented cache control algorithms that in many
cases can enhance system performance. “Cache” as used herein refers to the drive buffer storage space when
it is used in “cache” operations. To select prefetch and cache features the host sends the Mode Select
command with the proper values in the applicable bytes in Mode Page 08h*. Prefetch and cache operation are
independent features from the standpoint that each is enabled and disabled independently via the Mode Select
command. However, in actual operation the prefetch feature overlaps cache operation somewhat as is noted
in sections 5.5.1 and 5.5.2.
All default cache and prefetch Mode parameter values (Mode Page 08h) for standard OEM versions of this drive
family are given in Tables 11.3.2-1 through 11.3.2-4.
5.5.1 Cache operation
In general, 240 Kbytes of the 256 Kbytes of physical buffer space in the drive can be used as storage space
for cache operations (984 Kbytes of 1 MB for drives having the optional 1 MB buffer). 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*), 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. All data transfers to the host are in accordance with “buffer-full”
ratio rules. See explanations associated with Mod page 02h (disconnect/reconnect control) in the SCSI
Interface Product Manual*.
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 LB’s 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 LB’s from the disc and puts them in a segment of the cache. The drive transfers the remaining
requested LB’s from the cache to the host in accordance with the disconnect/reconnect specification
mentioned above.
3. If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.
__________
*SCSI Interface Product Manual, 77738479.