Prefetch / Multi - Segmented Cache Control; Cache Operation - Seagate ST9900805SS Product Manual

4.4 P /
REFETCH MULTI
The drive provides a prefetch (read look-ahead) and multi-segmented cache control algorithms that in many cases can
enhance system performance. Cache refers to the drive buffer storage space when it is used in cache operations. To select
this feature, the host sends the Mode Select command with the proper values in the applicable bytes in page 08h. Prefetch
and cache operations are independent features from the standpoint that each is enabled and disabled independently using
the Mode Select command; however, in actual operation, the prefetch feature overlaps cache operation somewhat as
described in sections 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 Table 12.
4.5 C
ACHE OPERATION
Refer to the SAS Interface Manual for more detail concerning the cache bits.
Note.
Of the 64MB physical buffer space in the drive, approximately 30,000KB 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 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.
The size of each segment is not reported by Mode Sense command page 08h, bytes 14 and 15. The value
Note.
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.
S 10K.5 SAS P
AVVIO RODUCT
-
SEGMENTED CACHE CONTROL
M , R . J
ANUAL EV
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