User Manual

SCSI Commands Reference Manual, Rev. J 392
www.seagate.com Parameters for direct access devices
CAP (Caching Analysis Permitted) bit
1 The Caching Analysis Permitted (CAP) bit, when set to one, enables caching analysis.
0 A zero indicates caching analysis is disabled. Caching analysis results are placed in the SCSI logging information table.
See individual drive’s Product Manual, Volume 1, SCSI Bus Conditions and Miscellaneous Features Supported table.
DISC (Discontinuity) bit
1 The Discontinuity (DISC) bit, when set to one, requests that the SCSI device continue the PREFETCH across time discontinuities, such as
across cylinders or tracks up to the limits of the buffer, or segment, space available for PREFETCH.
0 When set to zero, the DISC requests that prefetches be truncated at time discontinuities.
SIZE (Size Enable) bit
1 The Size Enable (SIZE) bit, when set to one, indicates that the Cache Segment Size is to be used to control caching segmentation.
0 When SIZE equals zero, the Initiator requests that the Number of Cache Segments is to be used to control caching segmentation. For
Seagate drives covered by this manual, SIZE is always zero.
WCE (Write Cache Enable) bit
0 SCSI WRITE commands may not return status and completion message bytes until all data has been written to the media.
1 SCSI WRITE commands may return status and completion message bytes as soon as all data has been received from the host.
MF (Multiplication Factor) bit
0 The Minimum PREFETCH and Maximum PREFETCH fields are interpreted as a number of logical blocks.
1 Specifies that the target shall interpret 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 logical blocks for each
of the respective types of PREFETCH.
RCD (READ Cache Disable) bit
0 SCSI READ commands may access the cache or the media.
1 SCSI READ commands must access the media. Data cannot come from the cache.
DEMAND READ RETENTION PRIORITY field
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 0.
WRITE RETENTION PRIORITY field
WRITE RETENTION PRIORITY. The cache replacement algorithm does distinguish between retention in the cache of host-requested data and
PREFETCH data. Therefore, this half byte is always 0.
DISABLE PREFETCH TRANSFER LENGTH field
DISABLE PREFETCH TRANSFER LENGTH. PREFETCH is disabled for any SCSI READ command whose requested transfer length exceeds this value.
MINIMUM PREFETCH field
The MINIMUM PREFETCH specifies the minimum number sectors to prefetch, regardless of the delay it may cause to other commands.
MAXIMUM PREFETCH field
The MAXIMUM PREFETCH specifies the maximum number of logical blocks that may be prefetched. The PREFETCH operation may be aborted
before the MAXIMUM PREFETCH value is reached, but only if the MINIMUM PREFETCH value has been satisfied.
MAXIMUM PREFETCH CEILING field
The MAXIMUM PREFETCH Ceiling specifies an upper limit on the number of logical blocks computed as the maximum prefetch. If the MAXI-
MUM PREFETCH value is greater than the MAXIMUM PREFETCH CEILING, the value is truncated to the MAXIMUM PREFETCH CEILING value.