A cost-saving feature introduced in VMware vSphere 4 is fully supported thin-provisioned virtual disks. Thin-provisioning decreases demand for SAN storage space by permitting virtual disks to consume just the space they actually use — and grow as needed — instead of pre-allocating all space up front.
What’s new is not necessarily the technology — it’s the management. In fact, veteran VMware ESX admins have been creating thin provisioned virtual disks for years — for controlled scenarios — by way of the vmkfstools command.
Before vSphere and ESX 4, however, thin disks came with risk — there was no simple way of accounting for the overcommitted storage on each LUN. Even with multiple gigabytes of free space, a small gang of thin-provisioned virtual machines could quickly quickly grow to exceed datastore capacity during a sudden demand spike.
Complete Storage Accounting
Now in vSphere 4 there is a new element in the capacity section of the datastore summary tab that shows total provisioned space — the maximum potential growth of all virtual machines if thin provisioned disks were fully utilized:
Virtual machines with snapshots have the potential of consuming even more datastore space, so vSphere accounts for this condition, too. Take a look at this VM Summary tab, where the total provisioned storage includes:
- Virtual hard disk (40GB)
- Snapshot (another 40GB potential, worst-case)
- VM swap file (1GB — sized according to RAM in theVM)
As you can see, a VM with a 40GB virtual disk can actually consume up to 81GB of space on your SAN because of a forgotten snapshot! Use the snapshot alarm to stay in control. And don’t forget that VMware vSphere snapshots are perfectly suitable for production.
New Datastore Alarms
New alarms in vSphere prevent out-of-space surprises. Administrators can monitor not only the free space on a datastore, but also the percentage overallocated — making it easy to adhere to policies concerning thin provisioning aggressiveness in your environment.
Flexible Virtual Disk Re-configuration
When creating a new VM, opting for thin-provisioned disks is as easy as checking a box. If you change your mind later, you don’t have to start over — during a Storage VMotion operation, administrators can opt to change the VM disk format on the fly (see below). It is also possible to inflate thin disks into thick via a new menu in the Datastore Browser.
You’ve probably heard the now-famous quote by Tom Bittman from Gartner:
Virtualization without good management is more dangerous than not using virtualization in the first place.
That goes double for thin-provisioned virtual disks. Without comprehensive accounting and monitoring in place, your virtual infrastructure may be heading for disaster. This level of insight is only available with VMware vSphere 4 — and it’s built right into the platform.
What about Microsoft virtualization? Hyper-V R2 thin provisioning — known as “dynamic disks” — is not a best practice. Perhaps due to the lack of accounting and monitoring of storage overcommitment — especially critical now with Cluster Shared Volumes and multiple VMs per LUN.
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