A third option presented by the Hitachi Dynamic Load Balancing Controller offers symmetric active/active
architecture (see Figure 2). Instead of having an active and a passive controller, both controllers are active,
able to work simultaneously, and able to dynamically and automatically assign the access paths from the back
of the controller to the LUN. All LUNs are accessible regardless of the physical port or the server from which
the access is requested. Utilization rates of each controller are monitored and can be automatically brought into
balance whenever necessary. This dramatically simplifies management and offers a number of administrative
benefits.
Figure 2. Active/Active Symmetric Features
4GB
Cache
Controller 0 Controller 1
QE4
4GB
Cache
QE4
Xeon
2GHz
1GB RAM
MCH
+ICH
“Cross” “Cross”“Direct” “Direct”
“Mixed Mode”
MCH
+ICH
1GB RAM
8GB/sec8GB/sec
RAID Processor
(DCTL-H)
RAID Processor
(DCTL-H)
Xeon
2GHz
2GB/sec
In a VMware environment, the active/active symmetric controller’s automatic load balancing reduces setup
costs and eliminates performance bottlenecks. While VMware offers both fixed and MRU (Most Recently Used)
multipathing policies, MRU policies can result in nodes using non-owner controllers with no mechanism to fail
back to the owner path.
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Fixed multipathing, on the other hand, uses the configured path to a LUN in
preference to any other. In the event of a path error, VMware will fail to the next available path. However,
VMware continues to test the failed path, and when the preferred path recovers VMware will route all activity
back through the preferred path for that LUN. In this way, using fixed multipathing with active/active symmetric
load balancing allows a configuration to dynamically repair itself.
An active/active symmetric controller does not rely on third-party host software for failover and load balancing;
instead, those functions are native to the operating system. The removal of third-party host software from the
equation makes it easier to optimize SAN performance and perform maintenance. For example, storage
controller microcode updates can be installed quickly and easily with little impact to operations because only
the individual microprocessor is rebooted instead of the entire controller. This allows the application to continue
running smoothly, even if the server has only one access or path (host bus adapter) to the storage controller.
5
“Recommendations for VMware Multipathing with Hitachi Modular Storage Systems,” Hitachi Data Systems, March 2008.
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