Thin Provisioning

Just got word that I’m going to be doing a “Thin Provisioning” install next week.  I’ve not had a lot of experience with EMC’s implementation of this particular brand of virtualization so it’s going to be interesting.

Thin provisioning has been around for a while, I think NetApp and Compellant have had thin devices in one form or another, but it’s not my idea of a fun time.

Thin provisioning is basically pretending you have space that you don’t.  You create a storage pool, or a group of volumes that your “thin” disks can pull from, they fill up space as it’s used, taking from the pool of disks in the process.

The thin devices then pull from the common pool as tracks are used.  This way a 500GB pool of disks can easily be provisioned to the hosts as a terabyte or more of virtual disks.

But if you create a 500GB pool and 4 250G “thin” devices, you are only safe until the total used space hits 500GB.  (IE 125G in each thin device)

I can’t even begin to count the number of ways this can blow up on you.  The only reason for using thin provisioning would be to lie to your internal customers.  Now granted I can think of a great use for this in dealing with users who make unreasonable storage requests.  (You know, the “I need 250GB for a webserver that only has a 35MB webpage hosted on it” types)

The problem is, again.  When they decide to fill up the disk, you’re out of luck.


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    • on August 27, 2008 at 9:18 pm
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    I agree with you on thin provisioning. It’s over subscribing an array. What I do think Thin Provisioning or Virtual Provisioning (I’m an EMC guy) is useful for is for base OS installs like Win 2003/2008, RHEL, etc.

    But otherwise I really wouldn’t use it for anyting else. I don’t like the idea of over subscribing- but for the base OS, I think it’s a good idea.


    • on August 28, 2008 at 6:52 pm
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    You guys are missing obvious uses. Say I had 2000 desktop vm’s. You can’t give them only the amount of OS, because they save documents, and you might need to install future applications. But how much do you give them? You don’t know what the future holds. You may give what you think is a perfectly good amount of space to each vm(say 6 gigs, about 50 percent free after OS and base apps) only to find that the next app you have to deploy is an oracle client!

    But, if you give them space, plus what they LIKELY will need in the future, you have just wasted terabytes of space! Solution? Thin provisioning. I give them 12 gig disks, they won’t run out of room anytime soon, if ever(we use network shares for file storage), but only 3 gigs is used on the vast majority of the volumes. My waste has been trimmed by nearly 100%.

    This is only one example, there are plenty of them around, but looking at it logically, you can see where it fits.

    • on August 29, 2008 at 7:16 am
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    I’ve seen this beast before. For anyone familiar with CPanel, or web hosting, the term “overselling” comes to mind.

    Now, considering that it took experience (read: falling flat on your face) for web hostings companies to weigh up the right ratios or formula to avoid embarrassment and understand what overselling really meant, I think the real question is how good is the reporting on thin provisioning use?

    If the reports are useless, well, you know what happens in 3 mths time when Finance or the Dev team keep asking for VMs/space 😉

  1. It makes sense from a web/hosting kind of environment. When you sell someone 10G, they expect to see 10G available.

    However I believe it’s a lot like airlines overselling flights, a practice I always thought should be indictable. If you sell more than you have in just about any other industry, it’s called fraud.

    But you’re right, it’s been around for years, the Celerra has done it for as long as I can remember, and I suppose it’s common industry practice.

    Although I see a lot less risk doing it in a NAS device than with a block-level device. There is too much risk to data on a block-level device when you get to a point where there is no more space to be had..

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