Thin Provisioning: Helping Storage Administrators Write Bad Checks

If you are a storage administrator in a large company, here is what your life is probably like:

All day long people come to you with urgent requests for storage. "I need one TB LUN for a critical new application, and I need it now. In fact, this app is going to be such a success, that you better give me two TB, because it'll be a pain to expand later on."

But a year later, if you go back and investigate that 2 TB LUN, nine times out of ten you discover that the application only used half a terabyte.

This scenario, repeated over and over, leads to low storage utilization. From the storage administrator's perspective, he may have handed out all of the storage on a given system, which is 100% utilization, but at the application level, there may only be 25% utilization.

I like analogies:

A Sun employee, an EMC employee and a NetApp employee are all at the funeral of a mutual friend. The Sun employee says, "We have a tradition at Sun. If someone dies, we put ten dollars into their coffin," and he drops in a ten dollar bill.

The EMC employee says, "At EMC our tradition is to put $100 in their coffin," and he pulls a hundred dollar bill out of his wallet.

The NetApp employee says, "A friend like this deserves $1000." He takes out his checkbook, writes a check for $1,110, puts it into the coffin, and grabs the ten and the hundred.

The point is, if you are going to write a check that probably won't be cashed, no need to add money to your bank account. If you are a storage administrator provisioning LUNs that probably won't be fully used, no need to add disks to your storage system.

If you think of the total capacity in your storage system as your bank account, and each thinly provisioned LUN as a check written against that account, then you should have some immediate intuition about how to manage thin provisioning. Thin provisioning works best if you give lots of small LUNs to lots of different people. That way you've got statistics on your side. You need enough spare capacity to handle normal utilization, and there's always a chance that a handful of users could suddenly use their whole LUN, so you'd better keep enough excess capacity to handle that. Keep an eye on your spare capacity to make sure it doesn't get too low. Having good sense of usage patterns is important. For a given class of user, how much additional storage are they likely to use over a week or a month? Using thin provisioning for brand new applications could be dangerous, but thin provisioning is perfect if experience shows that a particular group of users or a particular application always requests more storage than it really uses.

Thin provisioning isn't the right solution for every situation, but where appropriate it can really improve utilization. NetApp isn't the only storage vendor to support thin provisioning. This article talks about thin provisioning from NetApp, 3PAR and DataCore. Here are links to more details for each vendor: DataCore, 3PAR, NetApp.