Thin provisioning pros and cons

At first glance, thin provisioning is a no brainer, but even with all of the benefits that come with it, administrators need to pay careful attention to the downside, too. Here, I will discuss some of the pros and cons related to thin provisioning.


  • Provides longer term flexibility to individual virtual machines. Whenever you set up a new virtual machine, you face the resource challenge: How much RAM and disk space do you allocate for the new virtual machine? It’s very easy to add RAM after the fact, but it can be a bit more challenging to modify storage allocation. if you use thin provisioning, you can allocate all of the storage that you think you’ll need and more worry about wasting expensive SAN storage space.
  • Allows an organization to absolutely maximize the use of space in a storage array. Many administrators make sure that each individual virtual machine has some opportunity for growth. While this might not be a big deal for one or two machines, as you scale to dozens or hundreds of virtual machines, it starts to cost real money in the form of wasted space. Thin provisioning gives you the best of both worlds – the ability to continue managing individual vital machines through providing some growth space while making sure that such space does not result in waste. Although storage prices continue to drop, I don’t know of many organizations that are interested in wasting money.


  • Requires closer storage oversight than thick provisioning. One major administrative need that is introduced with thin provisioning is much closer oversight of available storage space. With thin provisioning, it becomes possible to inadvertently over provision space and run out in production. This is bad! So, if you decide to use thin provisioning, keep an eye on your space.
  • Eliminates the possibility of using some of vSphere’s advanced features. As soon as storage in a virtual machine is thinly provisioned in vSphere, you lose the ability to use advanced features such as Fault Tolerance, which requires thickly provisioned storage.
  • Requires administrators to think a bit differently about normal disk maintenance. “Back in the day” administrators constantly defragmented disks and performed other storage maintenance duties. Today, administrators need to take care with these kinds of duties if considering performing them against thinly provisioned storage. As a thinly provisioned disk is defragged, it becomes possible to trigger vSphere to add space since vSphere may see this activity as modifying disk blocks. This would completely negate the benefits to be had from thin provisioning. The guidance here: Choose storage maintenance tools with care and make sure that they don’t ruin your thin provisioning plans.
  • May carry a performance penalty as space has to be made available on the fly and prepare for use. As new space is made available for thinly provisioned storage expansion, vSphere has to perform operations to make this happen, which includes reserving the space and zeroing it out. If you are in an environment where absolute top performance reigns paramount and the cost/benefit of wasted space still demands better performance, don’t use thin provisioning.

What other pros and cons can you think of? Leave a comment.

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