Seagate Fibre Channel Interface User Manual
Page 245
Fibre Channel Interface Manual, Rev. D
231
Demand Read Retention Priority
The cache replacement algorithm does not distinguish between retention in the cache of host-requested data and prefetch
data. Therefore, this half byte is always zero (0).
Disable Prefetch Transfer Length
Prefetch is disabled for any SCSI Read command whose requested transfer length exceeds this value.
DISC (Discontinuity)
1
The SCSI device continues the prefetch across time discontinuities, such as across cylinders or tracks up to the lim-
its of the buffer, or segment, space available for prefetch.
0
Prefetches are truncated at time discontinuities.
DRA (Disable Read-Ahead)
1
The target will not read any logical blocks into the buffer beyond the addressed logical block(s).
0
The target may continue to read logical blocks into the buffer beyond the addressed logical block(s).
FSW (Force Sequential Write)
1
Multiple block writes are to be transferred over the interface and written to the media in an ascending, sequential,
logical block order.
0
The target is allowed to reorder the sequence of writing addressed logical blocks in order to achieve a faster com-
mand completion.
IC (Initiator Control)
1
Adaptive read look-ahead (ARLA) is disabled.
0
Adaptive read look-ahead (ARLA) is enabled.
Since Seagate drives covered by this manual never organize the cache according to size of segment, but rather by number
of segments, this bit is used to enable or disable adaptive RLA.
Note.
ARLA cannot be disabled in some Seagate drives using the ASA II code. See individual drive’s Product Manual,
Volume 1.
LBCSS (Logical Block Cache Segment Size)
Not used at this time.
Maximum Prefetch
The maximum number of logical blocks that may be prefetched. The prefetch operation may be aborted before the maxi-
mum prefetch value is reached, but only if the minimum prefetch value has been satisfied.
Maximum Prefetch Ceiling
The upper limit on the number of logical blocks computed as the maximum prefetch. If the Maximum Prefetch value is
greater than the Maximum Prefetch Ceiling value, the value is truncated to the Maximum Prefetch Ceiling value.
MF (Multiplication Factor)
0
The Minimum Prefetch and Maximum Prefetch fields are interpreted as a number of logical blocks.
1
The target interprets the minimum and maximum prefetch fields to be specified in terms of a scalar number which,
when multiplied by the number of logical blocks to be transferred for the current command, yields the number of log-
ical blocks for each of the respective types of prefetch.
Minimum Prefetch
The minimum number of sectors to prefetch, regardless of the delay it may cause to other commands.
Non-Cache Segment Size
If the Non-Cache Buffer Size field (Bytes 17–19) is greater than zero (0), this field specifies to the target the number of
bytes the initiator requests that the target allocate for a buffer function when all other cache segments are occupied by data
to be retained. If the number is at least 1, caching functions in the other segments need not be impacted by cache misses
to perform the SCSI buffer function.