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Determining the size and number of LUNs

We're moving some applications to virtual machines. How should we change the way you estimate the size and number of LUNs you need, and how do we create a process for allocating them that makes sense in a virtual environment where virtual servers with different owners reside on the same host and connect to the same storage?

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This is a common challenge, one that we cover in our virtualization assessment, as there are many variables to consider which are not all easily explained within this page. With the portability of the virtual machine (clustering, high availability, etc), you almost need the flexibility of having all storage accessible from all hosts. With replication pools on networked storage, backup and other necessary storage functions, this is no small feat.

The currently available software for monitoring and collecting metrics for your environment typically just wants to know storage allocated versus used, to know how much networked you need to assign. But, important pieces of data are missing: the disk I/O profile of the server, disk queue length, read versus writes, paging, swapping and a variety of other data that can help you make storage architecture decisions. All applications have a performance profile that designates them as either CPU intensive, memory intensive, disk intensive or network intensive, or some combination of all of the above.

If you have a storage intensive application (VM), you will want to consider not grouping those VMs together, just as you would not want to group memory intensive or CPU intensive applications. Another thought to consider: if you group applications or VMs together that have the same recovery requirements, they can be grouped together for more easily replicated storage. RAID configuration impacts performance. Large LUNs are more difficult to backup, replicate and restore. Small LUNs have more complexity, less flexibility and less portability.

This was first published in December 2007

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