The Truth About VM SAN Transfers

Most enterprise virtualization deployments make use of a robust shared storage infrastructure.  A high-performance SAN allows multiple hypervisors to access virtual machine disks and is the foundation for amazing virtualization benefits such as VMware vMotion and HA.  There are other benefits, too.

Virtualization management tasks such as cloning existing virtual machines or deploying new ones from templates often involve slinging around multi-gigabyte chunks of data.  It’s preferable to move or copy data on the SAN instead of the LAN because it can be faster, but more importantly, doing so reduces impact to other services that rely on the LAN.

Use the SAN, ESX!

VMware ESX will transfer VMs and templates over a SAN connection whenever possible but, if necessary, it also does a fine job of moving those bits over a standard LAN.  This goes for VMs as well as templates because with VMware vSphere, templates live on SAN datastores, too — shared among multiple ESX hosts — not on network file shares.  If templates were stored on a file server, there would be no choice other than to copy those multi-gigabyte files over the LAN and potentially impact other production traffic.

Nice design choice, VMware!  [Actually, if you go back to the early days of VirtualCenter 1.x, templates could also be stored directly on the VC server and deployed over the network.  That option was removed as of VC 2.0.]

So far, so good.  Let’s take a look at how Hyper-V stacks up.

Obfuscating the Inequalities

In the tradition of the recent storage hot add/remove claims, Microsoft has again gone the extra mile to give the appearance of feature parity.

Take a look at this excerpt from the System Center Virtual Machine Manager 2008 R2 What’s New page:

SAN migration into and out of clustered hosts: This allows virtual machines to migrate into and out of clusters using a SAN transfer, which saves the time required for copying the virtual machine file over the network.

And the features page is even more dramatic, proclaiming:

Virtual machine images can be large and difficult to move over the network. VMM auto-detects SAN infrastructure and enables copying of virtual machine images over fiber at fast speeds, thus leveraging SAN investments.

Sounds good, just like VMware — and at one-sixth the price!  But there is a small problem: it is not true.

Hyper-V uses the BITS service to transfer VMs over the LAN in almost all cases.  There is a scenario where SCVMM can orchestrate the disconnection and reconnection of a SAN LUN, “transferring” a VM to another host.  Enabling the feature requires additional configuration and some software from your SAN vendor.  Of course, it also means going back to one VM per LUN and foregoing the wonders of CSV.  Any takers?

The SCVMM Library Server

SCVMM 2008 R2 provides VM template functionality for Hyper-V.  Templates  and ISO images are stored in a Library, which uses standard Windows file services.  Obviously, that means no SAN copying for template deployments, either — kind of like VMware VirtualCenter 1.0.


“SAN LUN disconnect and reconnect” just doesn’t have the same ring to it as “SAN transfer” but I’m not sure that is sufficient justification for these misleading marketing claims.  And to use the word “copying” is simply dishonest.

Don’t make the mistake of assuming Hyper-V is just like VMware ESX.  Compare for yourself — seeing is believing.

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14 Responses to The Truth About VM SAN Transfers

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  2. NiTRo says:

    With Hyper-V, the past is now 🙂
    Remember old school Eric :

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  4. I’d like to correct something. In VC1, you were always able to store templates on the SAN. It was merely optional, not mandatory to store template locally. Many did to save space on their SAN…

  5. Phil says:

    “Don’t make the mistake of assuming Hyper-V is just like VMware ESX. ”
    ….because it not just like ESX, its actually a 3rd of the price.

  6. NiTRo says:


    You’re right, Hyper-V is a 3rd of ESX 🙂

  7. Eric Gray says:

    Mike, thanks for pointing that out. I’ve updated the post.

  8. Fernando says:


    How about commenting the content of the post ? We all know Hyper-V *initial* cost is lower.

  9. tonyr says:

    ok I’m going to break the rules by discussing an as yet not available product (been testing works fine) by another (won’t say who,you can cry in your own soup) san vendor. The CSV/VSS is a done deal and so are the other points listed in this post.

  10. Phil says:

    Frernando, no worries, am happy to comment on the content of the post.

    The Microsoft recommendations are that you operate your SAN infrastructure seperately from the production LAN. When SCVMM is using BITS to transfer VMs into and out of clustered storage, it occurs in our environment over the iSCSI infratstructure, thus there is no load on the production LAN, its on the equipment that is there to serve storage purposes.

    At the end of the day, my experience is this it just a non-issue in practice.

    Having said that, the cold hard truth is that if VMWare dropped their prices to be more in line with that of other companies public sector/educational pricing, then we would obviously go for the more feature rich ESX option.

    As usual, as case of Hyper-v being “good enough” for most scenarios.

  11. Fernando says:

    Well, I agree that , depending on your demands, that can be a non-issue.
    If you provision servers constantly, that will penalize you greatly. It if provision a server per quarter, that is not a problem. Again, the same pattern we could see with Hyper-V in general: Good enough for smaller/less demanding deployments, a headache for huge/demanding enterprises.

    Even if it is not an issue for you, that is sill marketing misleading claims for MS, plain dishonest, as Eric mentions in the post.

    Don’t forget you have a free ESXi offering, and some very affordable SMB offerings. You don’t need Enterprise Plus for all scenarios !

  12. Phil says:

    “Good enough for smaller/less demanding deployments, a headache for huge/demanding enterprises.”

    – Agreed, and exactly where I see the long term for VMWare as a virtualisation provider, in the large scale datacentre environment, with Hyper-v in most SMB Windows environments.

  13. tonyr says:

    Don’t forget you have a free ESXi offering, and some very affordable SMB offerings. You don’t need Enterprise Plus for all scenarios !

    if you want vmotion the free offering doesn’t work!

  14. Vladan says:

    Yes, the Essential plus version is most adapted for SMBs where you do not “absolutely” need vMotion and all the whistles. In the really small structures you actually Can afford to power down the VM and start it on the other ESX server, so you’ll be able to do a maintenance etc…

    I can see it every day, in discussion with clients which never used virtualization in production environement. They just want something that’s cheap and provides them with the most reliability possible. vMotion for them sounds more like a comfort option…


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