Posted by: Eric Siebert
Eric Siebert, VMware, vSphere
A reader was recently reading the VMware white paper What Is New in VMware vSphere 4: Storage and he came across this paragraph:
Improved Storage Resources Control
As the scope of storage resources have increased significantly with large deployments of virtualization environments, so has the need for greater automation and control of these resources. In the vSphere release, vCenter has been enhanced with several new storage specific capabilities to help the virtual administrator manage these environments with a higher degree of control. These enhancements provide administrators with proactive alerts and alarms to address issues before they interrupt the availability of applications running on those resources. vCenter allows setting permissions and quota limits on datastores, as well as per VM.
His question was to explain the meaning of the following line: “vCenter allows setting permissions and quota limits on datastores, as well as per VM.” In particular he wanted to know about setting quota limits on data stores and VMs. While I know a lot about the new permissions in vSphere, particularly for data stores, the part about quota limits confused me also because I have never heard of quota limits in vSphere. I did some research, checked all the vSphere documentation and couldn’t find anything about them. So I contacted VMware to get some clarification and I received the following response from the white paper author:
The quota limit applies to the storage given to a VM. There is not really an equivalent for a data store. One can set alarms to notify/alert one when a certain percent-full or overcommit is hit. But I am not aware of a means to stop allocations or placement of VMs when a certain percent overcommit is hit. That is a common request that I do not believe is there now.
So basically “quota limits” simply means the amount of disk space assigned to a VM. I guess technically it’s a quota but once you assign the space to the VM you can’t control how much of it that it can use. If you create a 20 GB virtual disk for a VM the guest operating system is going to see all 20 GB of it regardless of whether it is a thick disk or thin disk. If you do overcommit your data stores by using thin disks you need to carefully monitor datastore free space using alarms and reporting. Currently in vSphere there is no way to set any type of quotas for snapshots, virtual disks or datastores.
So while alarms and permissions are greatly improved in vSphere there are really no quota limits that you can use. Perhaps in a future release VMware will put some type of quota controls in place. Thanks to John Troyer and Paul Manning from VMware for the clarification on this.