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.


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.


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.

(Visited 406 times, 1 visits today)
This entry was posted in Virtualizationism and tagged , , , , , , . Bookmark the permalink.

12 Responses to The Truth About Storage Hot Add and Remove

  1. Pingback: Tweets that mention The Truth About Storage Hot Add and Remove | VCritical --

  2. Pingback: uberVU - social comments

  3. Jason Boche says:

    Yes, I was in fact wondering about Hyper-V’s approach. How did you read my mind? Thank you for the information. I was just about to roll out Hyper-V but now I have changed my mind based on this information. 🙂

  4. Marcel says:

    A timely article for me as well. I got a question from a student about this, when we covered this topic on the VMware platform. Again this proves to me that Hyper-V is mostly a “defensive” technology and there’s no real innovation, just me-too functionality.

  5. PiroNet says:

    @Marcel Funny you say ‘just me-too functionality’. It is so true actually. Since the beginning of this great era Microsoft is trying to catch up. No innovation, just copying others. They missed the wagon of virtualization, bundled an hypervizor bought from Connectix into a general purpose OS and tells everybody ‘me-too’ but once you read between the lines you discover that it’s pretty much a ‘almost me-too functionality’ … Bummer 🙁

  6. Bobbi Perrin says:

    The hot add – extend features to me are the coolest innovations since sliced bread. I had the experience of having to hot add space to a running VM the other day. I extended the space in VM settings then used CLI commands to add the space to the OS while it was running. So totally cool! Microsoft Hyper-V can’t touch this!

  7. Phil says:

    Marcel: “…just me-too functionality”

    Nothing wrong with me-too functionaility when it meets your needs at quarter of the price of the competition.

  8. Ed says:

    How does one extend a VHD with Hyper-V R2, then? Is there really no way to do it?

    Meeting needs is definitely important. Maybe it’s just a coincidence but most of my work the past 4-5 months has revolved around extending C: and D: drives on our Windows VMs and using Storage VMotion to move VMs to new storage arrays and new datastores. If we had to have downtime for all of those operations I’d still be buried under the requests (or working every night!)

  9. Pingback: The Truth About VM SAN Transfers | VCritical

  10. Marcel says:

    If it’s good enough, that’s the whole point. I do agree agree on the pricing (Enterprise Plus is just a lame SKU) of ESX 4.

    I’m playing with it in a serious setup for a presentation on Hyper-V R2 and for people coming from a VMware environment you quickly get a big list of WTFs:
    Why no proper support for storing ISO files on a SMB/CIFS (or my preference:) NFS share?
    Why not simply license something like MelioFS? CSV more or less works but it requires a lot of configuration steps and it’s not very transparent. It feels very much like a hack.
    Why no support for OVF files?
    And I’ve run into more things which rather annoyed me.

    I expect a more polished product from Microsoft.

  11. thomasseyar says:

    Most NAS have iSCSI capabilities today, so no support for storing ISO on a SMB/CIFS NFS share is not a big deal.
    CSV works just fine => FUD
    OVF => SCVMM can manage ESX machines as well as Hyper-V ones

Comments are closed.