If you’re like me, then your first thoughts after discovering thin provisioning were similar to: “Hmm, that sounds like my SAN space is getting out of control.” Nobody likes the idea of losing precise overview of his systems. And that’s what thin provisioning does — while the array promised 100TB of space to you (on your command, of course) it actually only has six 1TB drives inside. What if I fill them up? I decided to test it for you and show what happens to a Windows server system when a thin-provisioned LUN runs out of physically available space.

1. I’ve configured the storage system so that it only had about 2 GBs of free space. I’ve created a 10GB thin-provisioned LUN.
2. The LUN was attached to Windows Server 2003, formatted as a NTFS volume.
3. I started copying approximately 3 gigabytes of large files to the volume. When the written size crossed the 2GB limit, the system stopped responding.
4. It remained alive but refused to perform any task. I’ve let it stay as such for about an hour.
   
   
5. As a last step I decided to free up some space by deleting other LUN from the array. Almost immediately, the Windows finished the copy operation like there was no interruption.

What’s a result of the test? Well, the Windows behaviour was strange.  Apparently the system was not ready for a situation when the drive remains online, claims to have enough free space but doesn’t accept more writes. On the other side, it doesn’t fall into BSOD nor did it close any application. It just slept. I can’t say it will not hurt your application. Nor do I think you shouldn’t monitor your real capacity. I just wanted to test what happens in that situation. Knowing it may help you overcome this common fear of thin provisioning.

I love thin provisioning. I’ve taught a few customers to love it too. They don’t have to calculate their storage space requirements for a tens of months perspective more. They just purchase as much storage as they need now. Still they configure their volumes many times bigger than they currently need without having to pay for the white space. The average oversubscription I’m seeing is around 2-3 times of physical capacity. The fear of less precise space control is gone!

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