Seagate ST450MP0004 - 512N ユーザーズマニュアル

S

EAGATE 

E

NTERPRISE 

P

ERFORMANCE 

15K HDD 

AND 

E

NTERPRISE 

T

URBO 

SSHD SAS P

RODUCT 

M

ANUAL

, R

EV

. B

 

 10

  

The drive accepts the commands listed in the SAS Interface Manual less than 3 seconds after DC power has been applied.

If the drive receives a NOTIFY (ENABLE SPINUP) primitive through either port and has not received a START STOP UNIT command with 

the START bit equal to 0, the drive becomes ready for normal operations within 20 seconds (excluding the error recovery procedure).

If the drive receives a START STOP UNIT command with the START bit equal to 0 before receiving a NOTIFY (ENABLE SPINUP) 

primitive, the drive waits for a START STOP UNIT command with the START bit equal to 1. After receiving a START STOP UNIT comma

nd 

with the START bit equal to 1, the drive wait s for a NOTIFY (ENABLE SPINUP) primitive.  After receiving a NOTIFY (ENABLE SPINUP)  

primitive through either port, the drive becomes ready for normal operations within 20 seconds (excluding the error recovery procedure).

If the drive receives a START STOP UNIT command with the START bit and IMMED bit equal to 1 and does not receive a NOTIFY 

(ENABLE SPINUP) primitive within 5 seconds, the drive fails the START STOP UNIT command.

The START STOP UNIT command may be used to command the drive to stop the spindle. Stop time is 20 seconds (maximum) from 

removal of DC power. SCSI stop time is 20 seconds. 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 sys tem 

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 

All default cache and prefetch mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Table 

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

Of the 128MB physical buffer space in th e drive, approximately 60,000KB are available as  a data cache. The remaining buffer spa ce 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 retr ieved 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 d isk medium read 

operations (disregarding Prefetch operation for the moment). That  is, the drive does not check  in the buffer segments for the r equested 

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 ho

st 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.
Note.

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.