Prefetch/Multi-Segmented Cache Control; Cache Operation - Seagate ST39173N Product Manual

Barracuda 9LP Product Manual, Rev. C
provide information indicating the status of the spin up, such as the drive is still in process of coming ready or
the drive requires intervention (see SCSI Interface Product Manual , part number 77738479). Stop time is less
than 20 seconds from removal of DC power.
If the Motor Start Option is enabled, the internal controller accepts the commands listed in the SCSI Interface
Product Manual less than 3 seconds after DC power has been applied. After the Motor Start Command has
been received the drive becomes ready for normal operations within 13 seconds typically (excluding an error
recovery procedure). The Motor Start Command can also be used to command the drive to stop the spindle
(see SCSI Interface Product Manual , part number 77738479).
There is no power control switch on the drive.
4.4

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 com-
mand with the proper values in the applicable bytes in Mode Page 08h (see SCSI Interface Product Manual,
part number 77738479). 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 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 9a and 9b.
4.5

Cache operation

In general, all but 100 Kbytes of the physical buffer space in the drive can be used as storage space for cache
operations. 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 77738479), 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 "buffer-full" ratio rules. On a cache "hit" the drive
ignores the "buffer-full" ratio rules. See explanations associated with Mode 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 a 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 LB's 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 self-contained circular storage (wrap-around occurs), the length of which is
an integer number of disc medium sectors. The wrap-around capability of the individual segments greatly
enhances the buffer's overall performance as a cache storage, allowing a wide range of user selectable config-
urations, which includes their use in the prefetch operation (if enabled), even when cache operation is disabled
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