You want to look at the RAID, but also the storage system it's being implemented on. For example, what is involved...
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in migrating a LUN or a volume from one RAID level to another, and can it be done by the controller while data is actually being read or written. Many vendors tell me, yes the data can be accessed while it's being modified, but mumble and answer when pressed about the 'read and written' part.
Check out the entire RAID FAQ guide.