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Chapter 11. Performance considerations 225
Figure 11-3 High availability and increased bandwidth connecting both DA to two logical loops
These two switched point-to-point loops to each drive, plus connecting both DAs to each
switch, accounts for the following:
There is no arbitration competition and interference between one drive and all the other
drives because there is no hardware in common for all the drives in the FC-AL loop. This
leads to an increased bandwidth utilizing the full speed of a Fibre Channel for each
individual drive. Note that the external transfer rate of a Fibre Channel DDM is 200
MB/sec.
Doubles the bandwidth over conventional FC-AL implementations due to two
simultaneous operations from each DA to allow for two concurrent read operations and
two concurrent write operations at the same time.
Despite the superior performance, don’t forget the improved RAS over conventional
FC-AL. The failure of a drive is detected and reported by the switch. The switch ports
distinguish between intermittent failures and permanent failures. The ports understand
intermittent failures which are recoverable and collect data for predictive failure statistics.
If one of the switches itself fails, a disk enclosure service processor detects the failing
switch and reports the failure using the other loop. All drives can still connect through the
remaining switch.
Processor
Adapter
Adapter
Memory
Memory
Processor
Adapter
Adapter
To host servers
Storage server
ooo
Fibre Channel switch
Fibre Channel switch
16 DDM
To next
switch