Second, storage systems are no longer responsible for servicing the requests of a single server. In shared environments the storage controllers can be responding to dozens if not hundreds of attached servers and workstations. To compound the issue, many environments, thanks to server virtualization, will have multiple server instances making multiple storage I/O requests from a single host. This leads to extremely random and unpredictable workloads in the environment which can saturate controllers.
Today's storage controllers are responding to requests from more performance hungry servers that are attached to them. In web applications for example the storage may be responding to thousands of simultaneous users all accessing the same database or randomly each accessing different photos. Again, in virtualized server environments, these demands can come from 1,000 VMs spread out across a few hundred physical systems.
The classic dual controller architecture itself is misleading. In many cases both controllers are not equally handling the storage I/O for a given RAID volume. Usually one is assigned as the primary and the other as the secondary. The secondary is only used if the primary fails. The result is a single controller, handling all the above highly random conditions, can more realistically get saturated.
The classic response has been to overbuy storage compute muscle. The problem with this is that we all know that today's processors will be replaced by tomorrow's for less money. When you overbuy processing you are paying extra for compute muscle that you may not need until next year or later.
This has lead to some of the major manufacturers following the lead of a few storage innovators to start delivering clustered storage systems. Although how these clusters are assembled and the services they offer vary, they address the storage controller performance bottleneck that we detail in our article "What's Causing the Storage I/O Bottleneck?".
These systems allow continuous scale of capacity, I/O bandwidth as well as storage compute performance. Not every data center is going to encounter storage controller performance issues, and for those data centers standard systems are more than acceptable, but if your data center can cause any of the above conditions then storage controllers are something you need to pay attention to.
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George Crump is lead analyst of Storage Switzerland, an IT analyst firm focused on the storage and virtualization segments. Find Storage Switzerland's disclosure statement here.