Traditional storage management introduces a lot of waste. Most users leave anywhere from 30% to 50% of their allocated space unused. This is hardly trivial -- for every 10 TB purchased, 3 to 5 TB are left unused because of inefficient provisioning. This practice not only wastes the capital spent to acquire the disks and storage arrays, but also means additional expenditures for more disks and systems to keep the business running, because customers cannot access their stranded storage. Thin provisioning overcomes these problems, improving storage utilisation and saving money. To adopt thin provisioning services, you need to understand the benefits and limitations of thin provisioning and where the technology is headed in the future.
How does traditional provisioning work?
All end-user applications require storage to operate. With traditional storage management, a logical unit number (LUN) is created and assigned to each enterprise application -- a process called "provisioning." Creating a LUN (similar in principle to creating a partition on a computer's hard drive) basically carves out some portion of empty physical space in a storage array, which is then formally assigned to the particular application. The application runs over time, gradually filling the storage space allocated for the LUN.
But traditionally provisioned applications face several problems. Traditional LUNs cannot grow easily, and applications that run out of space will suffer performance problems or even crash outright until the LUN is resized. Expanding a traditional LUN requires the application to be quiesced and taken offline while the original LUN is backed up. Then the old LUN is deleted and a new larger one is created to provide additional storage space. The original LUN contents are restored to the new LUN, and the application can be restarted.
This time-consuming and error-prone process encourages excessive LUN sizes up front to avoid the need to resize for months or years. At the same time, that big LUN serves only one application. Ultimately, traditional provisioning results in poor storage utilisation and wastes money, since customers wind up buying substantially more storage to set aside in large, empty LUNs -- storage professionals often dub this "fat provisioning."
What is thin provisioning? What are the benefits of thin provisioning?
Thin provisioning overcomes this waste using a trick called "oversubscription" (sometimes called "overallocation"), in which a LUN, in essence, is created from a common pool of storage. The LUN is larger than the amount of physical storage that is actually available. For example, thin provisioning might allow a 1 TB LUN to be created, even though only 100 GB of physical storage are actually set aside. Thin provisioning also relies heavily on automation to map LUNs and then create or resize volumes, reducing the management overhead typically needed for provisioning tasks. "It's about managing -- about offloading and simplifying it so you're not constantly going in and changing the volume mapping or allocation, or expanding volumes and file systems," said Greg Schulz, founder and senior analyst with The StorageIO Group. "You're doing all of that up front."
The application uses the LUN normally, and eventually the 100 GB of actual storage will fill up. As actual space runs short, more physical storage can be added to supply additional free space as actual utilisation grows -- the application (in our example) still "thinks" it has 1 TB, but the actual storage that you buy and plug into the storage array can be far less. The actual space grows without altering the LUN or disrupting the application, making thinly provisioned storage faster and easier to manage. This saves money since you buy more storage only as needed, and drives up the storage utilisation because very little of the LUN's disk space is left empty.
What are the limitations or disadvantages of thin provisioning?
It's important to note that thin provisioning does nothing to prevent storage shortages or their effects. Applications can still suffer performance problems and crash if storage is not added to the thinly provisioned LUN in time to meet storage needs. "Now you have to react quickly to free up or reallocate space or do something if you're caught short," Schulz said.
Consequently, thin provisioning benefits from capacity planning to gauge LUN utilisation and predict the timing of storage upgrades based on usage patterns. Storage arrays that support native thin provisioning features may include tools that monitor storage utilisation and send alerts when volumes reach predefined limits -- basically monitoring the storage system 24/7.
While thinly provisioned volumes can grow, they remain extremely difficult to shrink without using the newest operating systems. "That's a reason why folks go to Windows [Server] 2008, because you can shrink the volumes back down," said Keith Norbie, director of storage and virtualisation at Nexus Information Systems.
Performance and scalability are additional concerns. Since thinly provisioned LUNs come from a pool of common storage, it is possible for multiple LUNs to overlap the same disk or disks, and this could create performance problems as applications compete for drive access.
"You're forcing the sharing of a fewer number of drives, so you have the potential for a performance bottleneck," Schulz said, noting that this behavior can also limit scalability when deployed in large petabyte or multipetabyte storage environments. However, performance and scalability issues are not guaranteed -- results will vary depending on the environment and the manufacturer's implementation of thin provisioning.
What are the future trends in thin provisioning?
Thin provisioning is expected to become a standard feature of storage systems -- systems without those features will be considered legacy products. While the basic features and capabilities of thin provisioning are not likely to evolve significantly, adoption is expected to expand, particularly among larger organisations with vast amounts of storage to provision.
Observers like Norbie go further -- pointing out that technologies like thin provisioning are simply heralds of a broader trend toward automation and dynamic management that has been taking place in the IT industry. "Overall, IT resources should be thinly or dynamically provisioned on an as-needed system," he said. "Each 'module' is justifiable to the CFO."
Another notable technology that may affect thin provisioning in the future is online compression, used to reduce data storage demands. Companies like Storwize already produce appliances that reside in front of storage to compress and decompress data at wire speeds. Even application makers like Oracle include compression in their software.
For example, compressing data an average of 3:1 effectively makes 1 TB of storage look like 3 TB, and this reduces the demand for new storage as well. "If you're thinking about thin provisioning, you should also be looking at online compression -- particularly for unstructured files where rapid and uncontrolled growth has taken place (i.e., NAS)," Schulz said, also noting the consulting value of being able to offer clients a variety of creative alternatives to their storage problems.