Product manual

Barracuda 18FC Product Manual, Rev. B 11
4.5 Cache operation
Note. Refer to the
Fibre Channel Interface Manual for more detail concerning the cache bits.
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 Caching write data
Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to
be written to the medium is stored while the drive performs the Write command.
Write caching is enabled independently of read caching. Write caching is enabled by default. To disable the
write cache, use the Write Caching Enable (WCE) bit.