Caching Write Data; Synchronized Spindle Operation - Seagate ST12400N Product Manual

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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 segmentation is
used as set up for read functions. The buffer segmentation scheme is set up or changed independently, 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 2 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 can increase the
system capacity and transfer rate in a cost effective manner.
For Hawk 2 models the interface consists of a twisted pair cable that connects the drives in the synchronized
system in a daisy-chain configuration as shown in Figure 5.7-1. For models "NC", the reference index signal
(SYNC) signal is on pin J1-37 of the 80-pin I/O connector. Master/Slave operation is autoarbitrated by each drive
after head load. Alternatively, each drive in the system can be configured by the host (using the Mode Select
command Rigid Disc Drive Geometry page*) to operate in either the master or slave mode.
After loading heads, a drive autoarbitrates for occurrences of the Reference Index Signal (REFIND+). If the drive
detects no REFIND+ signal on the line, it assumes master status and begins generating the REFIND+ signal.
This signal will not be visible on the line until the last drive has powered up, since any unpowered drive holds the
line low.
Using Mode Select command, drives can be re-configured by the host to be master or slave any time after the last
drive has powered up. The master provides the reference signal to which all other drives phase lock, including the
master. The master can be a drive or the host computer. All the drives may be configured as slaves, allowing the
host to provide the REFIND+ signal. All drives default to the synchronized mode when power up, unless J6 pins
5 and 6 are jumpered.
Each drive can be configured (jumpered for the non-synchronized mode in which it ignores any reference signal
that might be present. The connection of the synchronized reference signal to the host is required only if the host
is to provide the reference signal. If the host does not provide the reference signal, the host should not be
connected.
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*SCSI Interface Product Manual P/N 77738479.
Product Manual - Hawk 2 Family SCSI-2 (Volume 1), Rev. D