Elements of a Symmetrix I/O operation
109
Symmetrix DMX-3 Input/Output Operations
Read miss In a read miss operation shown in Figure 24 on page 108, the
requested data is not in global memory and must be retrieved from a
disk drive. While the channel director creates space in the global
memory, the disk director reads the data from the disk drive. The disk
director stores the data in global memory and updates the directory
table. The channel director then reconnects with the host and
transfers the data. If the requested data is in the process of being
prefetched (sequential read ahead), the miss is considered to be a
short miss. If the requested data is not in the process of being read
into global memory, the disk director requests the data from the
drive. This miss is considered to be a long miss. Because the data is
not in global memory, the Symmetrix system must search for the data
on disk, and then transfer it to the channel. This adds seek and
latency times to the operation shown in Figure 26 on page 109.
During the disconnect time, other commands can be executed on
other devices on the bus, or commands can queue to the same device.
Figure 26 Read miss
Connect time
Overhead
Disconnect time
Total service time
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