Справочник Пользователя для Seagate ST973451FC

Savvio 15K.1 FC Product Manual, Rev. B

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The START STOP UNIT command may be used to command the drive to stop the spindle. Stop time is 30 sec-
onds (maximum) from removal of DC power. There is no power control switch on the drive.

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 4.5.1 and 4.5.2. 

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

Of the 16 Mbytes physical buffer space in the drive, approximately

13,000 Kbytes can be used as a cache. The 

buffer 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 buffer. If the 
cache is enabled (see RCD bit in the Fibre Channel Interface Manual ), data requested by the host with a read 
command is retrieved from the buffer, if possible, before any disc access is initiated. If cache operation is not 
enabled, the buffer (still segmented with the 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 the explanation provided with the information about Mode Page 02h (dis-
connect/reconnect control) in the Fibre Channel Interface Manual.

The following is a simplified description of the prefetch/cache operation: 

Case A—read command is received and the first logical block is 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 disc 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 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.

Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to 
be written to the medium is stored while the drive performs the Write command.