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EMC Symmetrix DMX-3 Product Guide
Symmetrix DMX-3 Input/Output Operations
I/O response time:
Open systems
environment
In the open systems environment, I/O response time can be divided
into a host queuing time, a command connect time, a disconnect time,
and a data connect time, as shown in Figure 22 on page 106.
Figure 22 I/O response time (open systems environment)
The Host queuing time is the time the request is in the host queue
before it is dispatched on the Fibre Channel path.
The Command connect time is the length of time the channel is
transferring a Fibre Channel command.
The Disconnect time is the length of time involving device seek and
latency. During this time, the Fibre Channel path can be used by other
devices.
Note: In case of a memory hit in an I/O request, the Disconnect time
requirement is eliminated.
The Data connect time is the length of time the channel is transferring
data.
Symmetrix I/O
operations
There are four basic types of Symmetrix I/O operations Figure 23 on
page 107:
â—† Read Hit
â—† Read Miss
â—† Fast Write
â—† Delayed Fast Write
The Symmetrix system performs read operations from global
memory and always processes write operations in global memory.
This global memory operation is transparent to the host operating
system. A read operation causes the channel director to scan the
global memory directory for the requested data. If the requested data
is in global memory, the channel director transfers this data
immediately to the channel with a channel end and device end (or a
SCSI good ending status).
Host
queuing
time
Command
connect
time
Disconnect
time
Data
connect
time
Device service time
I/O response time
SYM-000453
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