I'm playing around with virtual disks and thin provisioning and have a question about it.

I've created several disks and allocated space to with thin provisioning. Is it possible to allocate more disk space than the maximum amount of the datastore space. So that for example, my VM's will stop functioning when the disks slowly start filling up?


Yes, absolutely. That is what thin provisioning is all about!

If you are using thin provisioning, you need to have a contingency plan in place to purchase and add more storage when your overall utilization passes a pre-determined threshold (e.g. 80%), and you need to have a monitoring system in place to ensure that you don't miss the moment. You also need to be sure that you are protected against situations wherein a single VM could begin to consume disk space explosively (e.g. Exchange or MSSQL transaction logs). For example, if you've decided that your threshold for buying more storage is 200GB of free space in your thin provisioned storage pool, it would be beyond foolish to thin provision >=200GB of extra space on any virtual server drive, even if you currently have terabytes of free space available.

If the above is too much for your organization (e.g., perhaps you cannot be entirely confident that an acquisition of additional storage would be approved when it is needed), then thin provisioning may not be an appropriate choice.

If you have already thin provisioned more space than you actually have, the first step to "undo" thin provisioning would be to reduce the sizes of the allocated partitions within the virtual disks such that, collectively, their sizes no longer exceed your physical storage limits. Then you would be able to convert to fixed-size virtual disks.

Note that converting to a fixed-size virtual disk is both time-consuming and potentially unnecessary: even if the virtual disks themselves are still technically thin provisioned in VMware, the VMs will never use unpartitioned space. For example, if you thin provisioned a 100GB virtual disk for a virtual machine that is only using 20GB of a 100GB partition, you could shrink the allocated partition within the virtual disk to 30GB. Under no circumstances would the VM use more than 30GB: the VM will still think it has a larger disk, but the unallocated space will never be used.

  • You're welcome! If you don't already have suitable partitioning software on hand, one workable approach is simply to "insert" the GParted Live or PartedMagic ISO file into a VM's virtual DVD-ROM drive, boot from it to make the changes, and then shut down and "eject" it. Of course, it is always important to make sure that you have a current backup of the virtual disk before doing this, just in case. – Skyhawk May 9 '10 at 19:44
  • Worth mentioning also that if you do run out of space on a LUN when using thin provisionion - Your VMs will pause. – Chris Thorpe May 9 '10 at 21:03

Yes, and kinda yes - they may well do if they expect to write but can't. They won't just fall over instantly though, probably pretty quickly after though.

  • mmh, Is there a way to undo thin provisioning for a disk? – Datapimp23 May 9 '10 at 18:13
  • Oh yes, moving back and forth is an option in the storage vmotion dialogue box, it can take a while however. There are command line ways of doing this but it's easier via the GUI. – Chopper3 May 9 '10 at 18:39
  • With vSphere there is also now an option to convert thin disks to thick with the DataStore browser. – Helvick May 9 '10 at 21:07

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