Caching Write Data; Synchronized Spindle Operation - Seagate ST11200N Product Manual

Product Manual - Hawk 1 Family SCSI-2 (Volume 1), Rev. D
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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 opera-
tions 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 contigu-
ous 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).
5.6

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 in one or more segments while the drive performs the write command.
Write caching is enabled along with read caching. For write caching, the same buffer space and segmenta-
tion is used as set up for read functions. The buffer segmentation scheme is set up or changed indepen-
dently, having nothing to do with whether or not read and write caching is enabled or disabled. When a write
command is issued, the cache is first checked to see if any logical blocks that are to be written are already
stored in the cache from a previous read or write command. If there are, the respective cache segments are
cleared. The new data is cached for subsequent Read commands.
If the number of write data logical blocks exceeds the size of the segment being written into when the end of
the segment is reached, the data is written into the beginning of the same cache segment, overwriting the
data that was written there at the beginning of the operation. However, the drive does not overwrite data that
has not yet been written to the medium.
Tables 11.3.2-1 through 11.3.2-4 show Mode default settings for the Hawk 1 family of drives.
5.7

Synchronized spindle operation

The synchronized spindles operation allows several drives operating from the same host to operate their
spindles at the same synchronized rotational rate. Drives operating in a system in synchronized mode
increase the system capacity and transfer rate in a cost effective manner.
The interface consists of a twisted pair cable that connects the drives in the synchronized system in a
daisychain configuration as shown in Figure 5.7-1. Master/slave operation is automatically set by the drives
in the system at power up time, or each drive in the system can be configured by the host (using a Mode
Select command*) to operate in either the master or slave mode. While loading heads after power up, a
drive checks for occurrences of the reference index signal (REFIND+). If there are no reference index
pulses occurring, the drive takes over as master and begins sending the reference index signal. Drives can
be re-configured by the host any time after power up to be master or slave by use of the Mode Select
command Rigid Disc Drive Geometry page.* The master provides the reference signal to which all other
drives phase lock, including the master. There is only one master per system, and that can be a drive or the
host computer. All drives may be configured as slaves allowing the host to provide the reference signal. All
drives default to the synchronized mode when powered up.
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*SCSI Interface Product Manual P/N 77738479.
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