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Changes Coming to Thin Provisioning

Among the new features of VMware's highly anticipated vSphere platform is the new support for thin-provisioned disks from ESX 4. ESX 3 did not offer thin provisioning by default, but it was possible through the vmkfstools command.

Just to be clear, VMware is by no means behind the curve on thin provisioning of virtual disks -- many of their products have offered this for years. But for most VMware Infrastructure 3 (VI3) administrators, the storage-planning process revolves around full allocation of a VM's disks on the storage system. (Note also that Hyper-V, Sun xVM VirtualBox and XenServer all offer thin provisioning as well.)

How do we approach this from a technology management perspective? Frankly, I'm wary of the jump to thin provisioning, as it can quickly get away from an administrator -- especially if the well runs dry on the storage system. The storage system can help out, or complicate things in this regard. (By the way, Virtualization Review magazine columnist Chris Wolf offers a good primer on this important point in "Troubleshooting Trouble".)

We can get even more feature-rich (again -- maybe more trouble) now with storage systems that are virtualization-aware. These platforms can provide thin provisioning on a LUN, yet allow the host to write out the entire virtual disk file.

Candidates for thin-provisioned storage may be easy to identify in some environments, based on storage usage and consumption. Busier and more disk-intensive workloads may not be as suitable for thin-provisioning, however. Looking forward to ESX 4, VMware shops have an advantage due to the Virtual Machine File System (or vStorage VMFS), which can get you out of a jam. One of the new features coming in vSphere is Enhanced Storage VMotion, which permits a conversion from a fully-provisioned virtual disk to a thin-provisioned virtual disk.

Historically, I haven't used thin-provisioning for server virtualization. At this point, I am seriously considering it for development and lowest tier production systems. Like any decision, it comes down to a cost vs. benefit vs. risk calculation. Once I quantify the amount of unused space on all eligible guest VMs, my decision process will be underway. Taking into account these new features coming, what is your take on thin-provisioning for servers? Send me a note or drop a comment below.

Posted by Rick Vanover on 03/25/2009 at 12:47 PM


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