Looking for disk in all the wrong places


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The shrinking storage array
Data protection policies and the provisioning process compound the problem. Local politicians complain about sending a dollar to Washington and getting back a quarter--the storage provisioning process seems to follow the same pattern.

Here's how a quantity of storage shrinks at each stage as it's configured and provisioned (

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Chart 1):

Let me explain:

PHYSICAL: This represents raw, unconfigured storage; it's the aggregate amount that someone signing off on a purchasing request sees. A small amount of capacity (the light-colored part of the column) is lost due to overhead--management volumes, hot spares, etc.

LOGICAL: This is available storage after applying data protection policies. Protection policies are RAID levels that reduce the available physical storage. The gap between physical and logical is predictable and driven solely by the organization's protection policies. The difference between RAID 10 and RAID 5, for example, can dramatically impact storage costs.

ALLOCATED: When service requests are made, logical storage is allocated. In most organizations, however, the logical storage pool is rarely 100% depleted because some capacity is set aside for unanticipated needs or emergencies. The size of this buffer is directly affected by the efficiency of the organization's purchasing process.

CLAIMED: Allocated storage must be claimed and used. In large organizations, some allocated volumes remain unclaimed. These "orphan" LUNs were handed off from the storage admin to a system admin, but never put into production. Another source of orphans is storage associated with a retired system or application that's never been reclaimed. This is a process problem and may represent a significant, low-impact opportunity to recover capacity. Ideally, the delta between allocated and claimed should be near zero.

ASSIGNED: Claimed storage is assigned to servers and presented to apps as volumes and file systems. The gap between claimed and assigned is another area for improvement.

WRITTEN: This is the assigned storage that contains data. The efficiency of written to assigned depends on the application and relates back to the accuracy of the initial storage request.

Defining utilization
An executive sees storage as a IT budget line item; a storage admin sees it as frames and LUNs allocated to systems; and a system admin sees storage as volumes and file systems. When we speak of utilization, whose perspective are we taking?

Each view must be considered. When a storage manager says he's targeting 70% utilization, which two levels are being compared? How does a system admin arrive at a utilization metric of 40%? When an executive complains that storage utilization is only 7%, what's his comparison based on? They're all looking at the same environment, but the storage manager is viewing logical vs. allocated storage, the system admin is seeing file-system utilization numbers (assigned vs. written), and the executive is comparing actual data written to physical storage purchased. All of their assessments may be accurate, and each perspective provides an important piece of the consumption puzzle.

This was first published in July 2006

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