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Active-active doublespeak
Active-active describes a configuration where two controllers have the ability to provide high availability and uninterrupted access to their back-end disk drives. However, EMC (and other storage vendors selling midrange arrays) uses the term "dual active" in its data sheets to describe its Clariion CX3 UltraScale Series architecture: "The CX3 UltraScale architecture utilizes 'dual active' storage processors that are each able to access every drive in the array," according to the firm.
However, dual active isn't the same as active-active. A midrange array operating in dual-active mode lets both controllers accept I/O all of the time, although only one controller at a time can control and access specific LUNs assigned to that controller.
Most low-end enterprise arrays, such as Hitachi Data Systems' TagmaStore Network Storage Controller model NSC55, operate in an active-active configuration. This means that either controller can accept I/O from any of a server's host bus adapters (HBAs) and load balance the server's I/Os across the HBAs to the same back-end array LUN. With dual-active configurations, servers may only access the LUN through one controller. An active-active configuration also improves performance during firmware upgrades or when a failed part needs to be replaced.
Justin Cirelli, senior Unix administrator at Fiberlink Communications Corp., Blue Bell, PA, uses the high end of HDS' midrang...
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e class, the HDS TagmaStore Adaptable Modular Storage model AMS1000, which operates in dual-active mode. He finds this configuration adds administrative time and complexity to his environment because it forces him to manually balance the LUNs between the AMS1000's two controllers.
On the host side, Cirelli must ensure that the LUNs used by the server app are evenly balanced across the two controllers. It's common for an admin to assign odd-numbered LUNs to one controller and even-numbered LUNs to the second controller before presenting them to the server. However, this type of LUN assignment adds an additional administrative burden because Cirelli must keep track of how many odd- and even-numbered LUNs are assigned to a specific app. If an app is assigned too many odd- or even-numbered LUNs to store its data, one storage array controller may end up handling the majority of that app's I/O and performance will suffer.
To prevent this, Cirelli's first choice was the HDS NSC55--an enterprise-class array targeted at midsized companies--because of its active-active configuration. The NSC55's controllers offer equal, uninhibited access to all back-end LUNs, while the server's multipath software balances I/O across the server's HBAs regardless of which LUN is assigned to the app. But Cirelli ended up buying the AMS1000 instead of the NSC55 for financial, not technical, reasons: "A year ago the AMS1000 cost 35% to 40% less than the NSC55," he says.
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