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Seagate Enterprise Capacity 2.5 HDD v3 SAS Product Manual, Rev. D 14
4.3 Start/stop time
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 command
with the START bit equal to 1, the drive waits 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).
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. However, power can be cycled on the
drive by utilizing SAS Power Disable feature defined by T10 (i.e. drive Pin 3 high).
4.4 Prefetch/multi-segmented cache control
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 Section
11.3.2.
4.5 Cache operation
Of the 128MB physical buffer space in the drive, approximately 60,000KB 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.
Note
Refer to the SAS Interface Manual for more detail concerning the cache bits.
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.