Elements of a Symmetrix I/O operation
111
Symmetrix DMX-3 Input/Output Operations
Because Symmetrix systems write the data directly to global memory
and not to disk, there are no mechanical delays due to seek, latency,
and RPS miss depicted in Figure 28 on page 111.
Figure 28 Fast write
Delayed fast write A delayed fast write occurs only when the fast write threshold has
been exceeded. That is, the percentage of global memory containing
modified data is higher than the fast write threshold. If this situation
occurs, the Symmetrix system disconnects the channel directors from
the channels.
The disk directors then destage the Least Recently Used (LRU) data
to disk. When sufficient global memory space is available, the
channel directors reconnect to their channels and process the host
I/O request as a fast write (Figure 29 on page 111). The Symmetrix
system continues to process read operations during delayed fast
writes. With sufficient global memory present, this type of global
memory operation rarely occurs.
Figure 29 Delayed fast write
Connect time
Global memory
Total service time
Connect time
Overhead
Disconnect time
Total service time
Delay Normal
fast write
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