Passthrough Disks –ARE– Still Supported By WS2012 Hyper-V Live Migration, But With Limits

I recently reported on a new KB article that says:

Changes and improvements in Windows Hyper-V Server 2012 no longer support pass-through disks if Live Migration is to be used.

In other words, Live Migration was allegedly not supporting the use of passthrough disks.

That article was incorrect

The story is:

1) Hans Vredevoort told me that found a contradicting blog comment/response by Jeff Woolsey where he stated that Live Migration of VMs with passthrough disks would be supported in what I’ll call legacy scenarios:

Pass through disks are still supported with Windows Server 2012 Hyper-V Live Migration (just like they were with Windows Server 2008 R2 Hyper-V) as long as the migration of a clustered VM and the pass through disk is managed by the cluster. For migrations outside of a cluster, such as:

  • Shared Nothing Live Migration or
  • Using standalone hosts with file on a SMB share (without clustering enabled)

…pass through disks aren’t supported because the pass through disk doesn’t have a way to move between hosts.

That makes total sense.  The passthrough disk (which is not a CSV) has to be passed from one host to another, and only a cluster-managed disk can do this.

Therefore the new scenarios in WS2012 Hyper-V cannot support Live Migration.  Again, that makes total sense:

  • Non-clustered SMB-stored-VM Live Migration – you can’t store a LUN on a file share!
  • Shared-Nothing Live Migration – until you can transport a LUN from one server to another Star-Trek-style, it isn’t happening.

2) Who was correct, Jeff Woolsey or the KB article?  We needed clarity so I reached out to the Hyper-V group in Redmond.  They responded overnight and Jeff was right.  The KB article was … a … bit enthusiastic (I personally loved the message – I’m quite clear in my training that I’ll smack anyone who I find using passthrough disks).  In fact, that KB article has been deleted by Microsoft.

So those of you who are panicking about your passthrough disks, you can calm down now.

However …

The advice from everyone who knows anything about Hyper-V is that you should switch to using VHDX files.  This isn’t just me.  Check out any of the Virtual Machine MVPs on this topic.  Read what Didier Van Hoye or Hans Vredevoort (both being the top 2 storage guys in our group IMO) have to say on the topic.

  • VHDX scales out to 64 TB
  • It has new health features to limit corruption
  • It supports 4K sector matching for performance
  • It offers the legacy VHD types, where Fixed will be nearly the same speed as the underlying disk

I heard loads of complaints over the “death” of passthrough disks in the last 7 days.  To you I say, you need to put down your toys of the 2000s. and join us in the 2010s.  We virtualise now for flexibility.  The business demands it, and passthrough disks are not flexible.

The one argument I’ll give some credence to is “I can’t hot expand a VHDX”.  If you are hot expanding LUNs every couple of days then you’ve got other issues, but I too would like this feature.

Anywho, panic over.  You can Live Migrate a VM with passthrough disks as long as both the VM and the passthrough disks are managed by a Hyper-V cluster.  I’m going back to my lazy vacation day now.

9 thoughts on “Passthrough Disks –ARE– Still Supported By WS2012 Hyper-V Live Migration, But With Limits”

    1. Not that many people are running DPM as VMs. Typically it’s cheaper to use a storage server with lower cost SATA disk in RAID5

  1. Now i’m officially relieved 8-)))

    as i expanding LUN… in some environment a down every year is seen as unacceptable ( if it’s's_Problem ) and cause of endless meeting and planning and overnight work ( and this is really, as you noted, my issue ) but sometimes LUN thin provision excite too much my customers so my habit to try to not overaprovision the LUN of a SAN can cause a few LUN resize but also save the day ( and maybe the year ) when the SAN is really full and the customer is in trouble until the budget is replenished 8-)))

    sorry about my rant but the number of explaining/changing backup strategies/replanning that this thing would have caused really droved me nuts.

    Only tinking about someone who ask me to restore from a year old tape a file and thinking that ( as i thinked in the rush ) i must restore to disk a 4 TB vhdx to extract a file, and maybe redo more than a time because the user fail to identify when this file vanished… sorry but this really depress me 8-)))

    Obviously i’ve not meant to Shoot the messenger, in facts i’m a fans of your work with this blog and i would thank you so much.

    I ‘ve also think really about travel from Italy the to follow the 2 days you posted some time ago but my English is not so good and the 16 seat available… so i pass

    But your latest book is already on my shelf 8-)))

  2. Regarding live expansion of VHD’s, what’s your opinion on spanning the partition to another vhd in disk management (adding another vhd to the scsi controller in Hyper-V) versus expanding the current vhd?

  3. Hi Aidan,

    Thanks for the input. I have one robotic arm tape library (IBM TS3200) which needs to be connected to a Hyper-V 2012 host using Fiber interface. Can I use the TL to be made available to one of the virtual machines in the host (which will be by media server i.e. BE 2012)? The entire infrastructure will be virtualized on Hyper-V 2012.

    Does Hyper-V support such a scenario of using a tape library connected to a virtual machines, being used as a media server?

    Thanks in advance.

  4. So your saying I should create 15TB LUNS and then stick a 15TB VHDX file on it. What’s the point, I might as well just use pass-through disks. Pass-through is still useful

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