Caching Write Data - Seagate ST39102FC Product Manual

Fibre channel disc drives

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Table of Contents

Cheetah 9LP FC Product Manual, Rev. C

Of the 1,024 Kbytes physical buffer space in the drive, 967.5 Kbytes can be used as a cache. The cache can

be divided into logical segments 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 cache. 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 cache, if possible, before any disc access is initiated. Data in contiguous logical

blocks immediately beyond that requested by the Read command can be retrieved and stored in the cache for

immediate transfer to the initiator on subsequent read commands. This is referred to as the prefetch operation.

Since data that is prefetched may replace data already in the cache segment, an initiator can limit the amount

of prefetch data to optimize system performance. The drive never prefetches more sectors than the number

specified in bytes 8 and 9 of Mode page 08h. If the cache is not enabled, 967.5 Kbytes of the buffer are used as

a circular buffer for read/writes, with no prefetch operation and no segmented cache operation.

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 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 segment, the drive fetches it and any remain-

ing 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 initiator in accordance with the

"buffer-full" ratio specification given in Mode Select Disconnect/Reconnect parameters, page 02h.

3. The drive prefetches additional logical blocks contiguous to those transferred in step 2 above and stores

them in the segment. The drive stops filling the segment when the maximum prefetch value has been

transferred.

Case B—read command is received and the first logical block address requested is not in any segment of the

cache.

1. The drive fetches the requested logical blocks from the disc and transfers them into a segment, and then

from there to the initiator in accordance with the "buffer-full" ratio specification given in Mode Select Dis-

connect/Reconnect parameters, page 02h.

2. The drive prefetches additional logical blocks contiguous to those transferred in Case A, step 2 above and

stores them in the segment. The drive stops filling the segment when the maximum prefetch value has

been transferred.

During a prefetch, the drive crosses a cylinder boundary to fetch data only if the Discontinuity (DISC) bit is set

to 1 in bit 4 of byte 2 of the Mode Select parameters page 08h. Default is zero for bit 4.

Each cache segment is actually a self-contained circular buffer whose length is an integer number of logical

blocks. The wrap-around capability of the individual segments greatly enhances the cache's overall perfor-

mance, allowing a wide range of user-selectable configurations. The drive supports operation of any integer

number of segments from 1 to 16. Divide the 967.5 Kbytes in the buffer by the number of segments to get the

segment size. Default is 3 segments.

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 specifi-

cation 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.

4.5.1