Seagate ST91000641SS ユーザーズマニュアル

Constellation.2 SAS Product Manual, Rev. H

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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 opera-
tion, the prefetch feature overlaps cache operation somewhat as described 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 Table 10.

Refer to the SAS Interface Manual for more detail concerning the cache bits.

Of the 64MB physical buffer space in the drive, approximately 30,000 kbytes are available as a data cache. 
The remaining buffer space is reserved for internal drive use. 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: 

—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.

—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 5.5.2 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 capa-
bility 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 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 parameter.