Prefetch operation. Seagate HAWK 2 ST12400W/WD/WC

Product Manual - Hawk 2 Family (Wide bus) SCSI-2 (Volume 1), Rev. A 13

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Case B - A Read command requests data, the first LB of which is not in any segment of the cache:

1.

The drive fetches the requested LB’s from the disc and transfers them into a segment, and from there to the host in accordance with the disconnect/reconnect specification referred to in case A.

2.

If the prefetch feature is enabled, refer to section 5.5.2 for operation from this point.

Each buffer segment is actually a self-contained circular storage (wrap-around occurs), the length of which is an integer number of disc medium sectors. The wrap-around capability of the individual segments greatly enhances the buffer’s overall performance as a cache storage, allowing a wide range of user selectable configurations, which includes their use in the prefetch operation (if enabled), even when cache operation is disabled (see section 5.5.2). For the ST12400 wide family the number of segments may be selected using the Mode Select command, but the size can not be directly selected. Size is selected only as a by-product of selecting the segment number specification. The size in Kbytes of each segment is reported by the Mode

Sense command page 08h, bytes 14 and 15. If a size specification is sent by the host in a Mode Select command (bytes 14 and 15) no new segment size is set up by the drive, and 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 unchangeable parameters (see SCSI I/O Product Manual*). The ST12400 wide family of drives supports operation of any integer number of segments from 1 to 16. Divide the 245,760 bytes (or 1,007,616 bytes for optional 1 MB buffer) in the buffer by the number of segments to get the size (in bytes) of each segment.

5.5.2

Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that which was requested by a Read command can be retrieved and stored in the buffer for immediate transfer from the buffer to the host on subsequent Read commands that request those logical blocks (this is true even if “cache” operation is disabled). Though the prefetch operation uses the buffer as a “cache”, finding the requested data in the buffer is a prefetch “hit”, not a “cache” operation “hit”. Prefetch is enabled using Mode

Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch. Since data that is prefetched replaces data already in some buffer segment(s), the host can limit the amount of prefetch data to optimize system performance. The drive never prefetches more logical blocks than the number specified in the prefetch logical blocks ceiling bytes (8 and 9) of Mode page 08h.

During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if the Discontinuity

(DISC) bit is set to one in bit 4 of byte 2 of Mode parameters page 08h.

The operation of the prefetch feature (when enabled by DRA = 0) can be modified to an adaptable prefetch mode, wherein prefetch is disabled as long as two sequential read operations are not for contiguous blocks of data. If two sequential Read operations request contiguous logical blocks of data, the drive automatically enables the prefetch feature for the end of the second Read operation. As long as sequential Read operations request contiguous LB’s of data, prefetch operation remains enabled.

If the host uses software interleave, there will not likely ever be two sequential Read operations for contiguous LB’s, so the adaptive prefetch feature keeps prefetch disabled, even if the DRA bit enables it. Adaptive prefetch is enabled when the IC (Initiator Control) bit (Mode page 08h, byte 2, bit 7) is set to 0 and when DRA.

= 0 (prefetch enabled).

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*SCSI-2/SCSI-3 Interface Product Manual, P/N 77738479.