The downside to thin provisioning

With most storage array vendors now offering thin provisioning (TP), the question is not who has it and who doesn't, but whether all implementations are alike, which apps aren't a good fit and how to avoid some problems associated with TP.

All TP applications do the same thing, but vary slightly in the way they deliver certain features. TP creates allocated storage for virtual volumes spread among several disks. A nice benefit is that you pay only for the capacity used, rather than for the entire allocated amount.

To alleviate the problem of running out of storage, thresholds are set to show when more capacity is required; the TP program then sends alerts when capacity consumption nears the allocated limit. But TP isn't a total win-win feature; it opens the door to problems such as possible performance degradation, more complicated data replications and, worst of all, unexpectedly running out of allocated disk space.

According to Marc Staimer, president of Dragon Slayer Consulting, TP is most useful when two factors are present: the applications are "TP-friendly," meaning that assigned volumes aren't filled up with data immediately; and many applications on the same array share access to the common disk pool. An example of an unfriendly TP app is a user-accessible file system that might become the unanticipated repository of a large number of personal MP3 files, which when deleted don't free up space for use by another virtual volume.

There are other potential problems to consider. There may be a performance hit because thin-provisioned LUNs/volumes aren't automatically striped over a large number of disk spindles. With TP, fewer disk drives support the data and I/O. It's also important to check whether remote replication services are TP-aware; if not, disaster recovery implementations will be very expensive, says Staimer.

Finally, consider how difficult it will be to add more capacity to the thin-provisioned volumes. For example, you may need to manually preconfigure the new capacity to appropriate RAID groups that are then assigned to individual pools--a tedious, labor-intensive process. Also consider whether the product offers a way to turn a thinly provisioned volume back into a normal volume.

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