We have some clients who like to purchase storage well over what their expected use will be. We've started thin provisioning the data disks on any VMs from these particular clients, or for any new request over 100G. So we've built in some over-provisioning of our storage into the service while watching the capacity at both the current thin and theoretical thick levels.
And then I started watching the disk usage creep on a few VMs: Our Solaris 10 machines that I've been particularly keen to keep the performance and customers happy. Our Sol admins switched from UFS to ZFS root. ZFS and it's partner-in-crime, RAID-Z use Copy-on-write transactional model so that a new block is used upon every write.
So I've asked our Admins to bring the VMs back to UFS or I'd need to thick the disks out.
So there you have it. Every time the Solaris VMs that I'm trying to keep in top shape write something to disk, they're potentially extending the thin provisioned vHDD. VMware would like to think that there's no performance hit with thin disks, but I don't think they've planned for a copy-on-write file system.
The fact that Solaris is officially supported but there's little discussion or documentation has stymied our group's willingness to bring on more Sol in VI. It just looks like VMware and Solaris/Oracle are headed in different directions.