Seagate ST3500071FC Start/stop time, Prefetch/multi-segmented cache control, Cache operation

4.3Start/stop time

If the Motor Start option is disabled, the drive becomes ready within 25 seconds after DC power is applied. If a recoverable error condition is detected during the start sequence, the drive executes a recovery procedure and the time to become ready may exceed 25 seconds. Stop time is less than 30 seconds (maximum) from removal of DC power.

If the Motor Start option is enabled, the internal controller accepts the commands listed in the Fibre Channel Interface Manual less than 3 seconds after DC power has been applied. After the Motor Start command has been received, the drive becomes ready for normal operations within 25 seconds (excluding the error recovery procedure). The Motor Start command can also be used to command the drive to stop the spindle.

There is no power control switch on the drive.

4.4Prefetch/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 opera- tion, 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 25.

4.5Cache operation

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

Of the 8,192 kbytes physical buffer space in the drive, 6,991 kbytes can be used as a cache. The buffer can be divided into logical segments (using Mode Select Page 08h, byte 13) 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 to the initiator the first logical block requested plus all subsequent contiguous logical blocks that are already in the cache. This data may be in multiple segments.

2.When a requested logical block is reached that is not in any cache segment, the drive fetches it and any remaining requested logical block addresses from the disc and puts them in a segment of the cache. The drive transfers the remaining requested logical blocks from the cache to the host in accordance with the Mode Select Disconnect/Reconnect parameters, page 02h.

3.If the prefetch feature is enabled, refer to section 4.5.2 for operation from this point.

NL35 Series FC Product Manual, Rev. D 13