The Truth About Storage Hot Add and Remove

VMware vSphere 4 supports a range of hot storage management technologies:

• vStorage VMFS Volume Grow
• Hot Extend for Virtual Disks
• Hot Virtual Disk Add/Remove

With these capabilities, if space gets tight in your vSphere environment, it is easy to be proactive and address the issue before anyone notices.  The process goes something like this:

• Allocate additional physical hard disk space on your SAN to an appropriate LUN
• Grow your VMFS datastore onto the newly added free space
• Extend, or add new, virtual disks for the VMs that need more storage
• Expand the volume inside the guest to create more usable space

All without a reboot: zero downtime.

Not All Virtualization Platforms are Created Equal

Don’t jump to the conclusion that all hypervisors offer the same flexibility.  Perhaps you are wondering about Hyper-V capabilities?

First, let’s take a look at some Microsoft System Center Virtual Machine Manager 2008 R2 (SCVMM) marketing statements:

The What’s New page announces:

Hot addition/removal of Storage: Allows the addition and removal of storage to virtualized infrastructure without interruption. Additionally, “live” management of virtual hard disk (VHDs) or iSCSI pass through disks, allows administrators to take advantage of additional backup scenarios and readily use mission critical and storage-intensive applications.

The Top Benefits list proclaims:

Hot addition/removal of storage: With this capability, administrators can quickly and efficiently respond to changing storage requirements of virtual machines. This ability to hot-add additional storage eliminates the previous need to take the host down to upgrade storage thus increasing business continuity for end users and reducing complexity for administrators. Additionally it allows administrators to confidently deploy mission critical applications (in which up-time is of paramount importance) that may have rapidly changing storage requirements such as web, database or other business applications.

An IT decision-maker just might get the impression that both ESX and Hyper-V have essentially the same features. They do not.

You may be surprised to find out that all of the descriptions above merely refer to adding a new virtual disk to a VM — providing the conditions are right.  You cannot grow an existing VHD, and you can’t safely remove a VHD with SCVMM.

Plan Ahead

First things first.  If your Hyper-V VM does not have a virtual SCSI adapter — templates and VMs from Hyper-V R1 do not — you won’t be able to hot add a new VHD until you correct that shortcoming.  Hello downtime.

SCVMM can add a new blank virtual disk to a VM or it can copy an existing one across the network from the Library — if you copied it there beforehand, but there is no way to add a VHD that may already be present on your SAN — even if it is already sitting right next to the destination VM.

Removal?

Up-time may be of paramount importance, but preventing data-loss was evidently not part of the original design.  Removing a VHD with SCVMM results in the immediate deletion of the underlying VHD file.  Ouch!  Thankfully, a recent patch improves administrator job security by throwing up a warning before this happens, providing an option to cancel.  There is no way to simply disconnect a VHD using SCVMM.

It turns out that if you really want to take advantage of those “Additional Backup Scenarios” by hot adding and removing virtual disks, you need your trusty Hyper-V Manager utility.  Still think System Center is a single pane of glass?  And that’s not the only task that requires administrators to switch between Hyper-V interfaces.

Conclusion

Microsoft is trying hard to ride the coattails of VMware ESX.  The latest release of Hyper-V R2 still does not have capabilities enjoyed by VMware administrators since the ESX 3.5 days.

Don’t believe the obfuscated marketing literature.  VMware vSphere is for real.