Start / Stop Time; Prefetch / Multi - Segmented Cache Control; Cache Operation - Seagate ST900MM0006 Product Manual

4.3 S /
TART 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).
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.
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 11.
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.
S 10K.6 SAS P
AVVIO RODUCT
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SEGMENTED CACHE CONTROL
M , R . C
ANUAL EV
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