When Microsoft released Hyper-V for Windows Server 2008, it shook the virtualization world with a cheap – and viable -- hypervisor option. Still, VMware remained the hypervisor leader in several technical areas, including storage.
With Windows Server 2008 R2, Microsoft made some changes to Hyper-V (now
Removal of old clustering limitations
In a standard Windows cluster, one machine in the cluster owns a resource at a time. As a result, any cluster that hosts a set of Hyper-V virtual machines on a storage area network (SAN) has to put each VM on a separate LUN so that a cluster node fails over all of its appropriate cluster resources. This puts a severe configuration management hurdle in the way of the highly-available Hyper-V model.
Cluster Shared Volumes (CSV) is a new built-in feature in Hyper-V R2 that allows VMs access to their virtual disk -- even when they are all stored on the same LUN. This allows you to take advantage of the storage configuration in place without having to ask storage administrators to rearrange their SAN in order to add a new virtual machine.
CSV requires at least two LUNs. One LUN is the primary volume that has the VMs with the configuration and virtual hard disk files, while the other LUN is the witness disk configured at the hardware level, but it is not a visible resource to use as a disk.
For those who have used Windows clustering, the witness disk is not new. The witness stores cluster data, but it doesn't need to be the same size. In fact, it only needs 1 GB maximum.
The LUNs provided need to be dedicated to this failover cluster: don't share the storage with other clusters. Failover clusters in Windows Server 2008 R2 can still be configured without Cluster Shared Volumes, though the previous restrictions exist. Therefore, it is important to ensure you get it setup before you allocate storage for Hyper-V machines.
Adding storage on the fly
In the original Hyper-V, the guest operating system (OS) had to be shut down in order to make almost any virtual hardware change.
Dynamic storage is new in Hyper-V R2 and allows storage to be added to -- and removed from -- a guest OS without having to shut the system down. This provides the same flexibility as with physical SCSI hot swap disks arrays.
Although dynamic storage may not seem like a very exciting feature since VMware has offered it for several years, administrators see this as a key factor in taking Hyper-V further into production scenarios where flexibility and better uptime is a prime consideration.
Hyper-V R2 features a new SCSI controller driver. This driver is made available to guest OSes through Integrated Services, but it is only recognized by more modern Microsoft systems like Windows Server 2003, Windows Server 2008 and Windows Server 2008 R2. Certain client OSes, like Windows XP SP3 64-bit edition, Windows Vista and Windows 7, are also included.
In the original Hyper-V, virtual hard disks (VHD) were represented by IDE disks. The new SCSI driver in Hyper-V R2 allows you to hang 64 disks off a single controller, with up to four SCSI controllers in each guest.
For additional storage, you just need to ensure a SCSI controller is available to the guest OS via the Hyper-V Manager. As long as the guest OS recognizes the storage miniport driver in Device Manager, you have the technology. To figure this out, go to the VM's properties, select SCSI Controller, click Hard Drive and then Add. The Wizard guides you through adding a new disk. After creating the VHD, open Disk Management in the guest OS and configure it just like a new hard drive: bring it online, format it, and use it.
Furthermore, this disk can be removed on the fly. Just go to the VM's properties of the SCSI controller and choose to remove the hard drive. It will disappear from the guest OS. Adding and removing VHD disks on the fly moves data more quickly than physically swapping disk drivers. This can be useful when your availability needs have you add a new server in place of an older one -- but you still need those files and folder as they were on another VM. Just add the existing VHD file as a hard disk and you're back up with that same disk in a new VM.
Some VMware shops may have no need to look at the new Hyper-V features since they already have similar features in place. However, many others are looking to Hyper-V as a cost driver to do virtualization cheaper.
If your virtualization plans rest on Hyper-V having high-availability features, Cluster Shared Volumes and dynamic storage are two enhancements that achieve this.
ABOUT THE AUTHOR
Eric Beehler has been working in the IT industry since the mid-90's, and has been playing with computer technology well before that. His experience includes over nine years with Hewlett-Packard's Managed Services division, working with Fortune 500 companies to deliver network and server solutions and, most recently, I.T. experience in the insurance industry working on highly-available solutions and disaster recovery. He currently provides consulting and training through his co-ownership in Consortio Services, LLC.
This was first published in December 2009