The inefficiencies crop up as raw storage (all available unformatted physical storage) is formatted with a RAID set. Not all of the raw storage is formatted, configured or assigned to a LUN. Some is saved for spares, emergencies, or unanticipated demand which reduces raw capacity utilization.
Two more serious inefficiencies occur when sizable chunks of the allocated storage (configured capacity mapped to servers or virtual servers) is not claimed by those servers and not all of the claimed storage is presented as file systems and/or volumes to the applications and users. This wasted unused storage capacity is often referred to as "dark storage". It will typically go undetected by storage administrators because of the firewall between the storage and server administrators.
And one final trouble spot crops up when a server/virtual server or application is over-provisioned. Over provisioning occurs when the application administrator requires far more capacity then they will actually use.
Capacity management tools will to varying degrees (depending on the vendor and the product) identify unformatted raw storage, unidentified unused dark storage, and over-provisioned storage. Some capacity management tools will also have sophisticated algorithms that predict future storage requirements based on historical trends of consumption and growth rates, while including policy based thresholds that notify admins when more storage must be acquired or allocated. Whereas others are not that sophisticated.
But no tool is all-knowing and all-seeing and any tool is only as good as the administrator operating it.
Capacity management tools cannot predict the unexpected. No matter how sophisticated a forecast algorithm, it is based on past performance. Stock prospectuses always state that past performance is not a guarantee of future performance. That applies to storage consumption as well. If there is a spike in users or demand such as occurs for Web-based applications, no capacity management tool is going to foresee that situation. All software tools utilize assumptions made by the software architects and writers. These assumptions are unlikely to apply to every situation.
Capacity management tools cannot prevent the fire drill that occurs when an application runs out of storage. If an application, server, or storage admin ignores or misses the alerts, alarms, and warnings generated by the capacity management tools (when thresholds are reached), there is not much the tool can do. The probability of increased server/virtual server and application downtime goes up as utilization goes up. In other words, capacity management tools are not a substitute for process and discipline; however, they make those processes and disciplines more effective.
Ultimately, capacity management tools are a delicate balancing act between increased storage utilization and increased risk of and actual downtime. In the end it doesn't matter where the storage administrator leans in this balance (more towards higher savings and higher risk or more towards lower savings and lower risk), a capacity management tool is an extremely valuable and important asset.
Marc Staimer is founder and analyst with Dragon Slayer Consulting
This was first published in January 2009