What seems to be the problem? One of the benefits of a storage area network (SAN) over direct-attached storage (DAS) is better provisioning of storage -- at least theoretically. With DAS you can easily underestimate the requirements for storage for one server and dramatically overestimate the requirements of storage for a second server. Thus you could find yourself in the unpleasant position of having to add storage to one server while having more than enough storage on another server. Now the promise of a SAN was that you could share a pool of storage among many servers (and applications). However, you still had to allocate logical unit numbers (LUNs) to a particular server. This hard allocation creates partitions in the storage pool that are not easy to change. So you can still run into the problem of mis-sizing of storage. So although SANs have a number of benefits over DAS, you can still have a problem of the inefficient use of storage assets.

What do you need to know? The lure of thin provisioning is that it promises to solve the storage under-provisioning and over-provisioning problem. Thin provisioning does this by virtually allocating each application the storage that it is likely to need in the foreseeable future. The amount of storage that is virtually allocated to all applications as a total may exceed the actual physical space available.

That means if an application requires more space than would have otherwise estimated it can get that space without the need to interrupt the application for the reallocation of physical LUNs. And if an application is using less space than necessary, it is not tying up physical LUNs that it does not need.

Of course, one of the potential downsides is that you have all of your storage eggs in one physical basket and it would be very unwise to run out of real physical storage. So you must monitor carefully to prevent an out-of-storage condition.

However, thin provisioning gets you out of the guessing game as to how much storage a server or application will demand. And that guessing game is unlikely to succeed as estimating storage growth demands is problematic at best.

Thin provisioning leads to better storage utilization as you can use a larger percentage of the storage that is already available. That meant that the requirement to purchase additional storage can be delayed until it is really needed. So management should be happy.

Storage operations and end users should also be happy as there is no need for administrator downtime activity and consequently no service level impacts on end users for application disruptions due to reallocation of storage.

But thin provisioning must be applied carefully. Check the fine print. Some applications, files systems, and databases may try to gobble up unused disk space and thus not be good candidates for thin provisioning. Thin provisioning may impose seemingly unrelated limitations, such as no RAID 6 or a limited number of snapshots. So check carefully.

What can you do about it? Thin provisioning may be an answer to storage mis-allocation problems, but it is not a panacea. You must look very carefully into what it will do for you and whether or not it is suitable for your storage environment. You must investigate carefully how you would go about preventing out-of-space conditions on physical storage. All in all, however, thin provisioning is likely to become more widely used and popular as time goes on. You just have to know when making the move would be best for you and that may or may not be right now.

» posted by jnaze

Mesabi Group

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