The Virtualization Room

Oct 3 2008   4:38PM GMT

Take the time to learn direct disk access to a virtual machine

Rick Vanover Rick Vanover Profile: Rick Vanover


Recently I had the opportunity to go through a series of tests revolving around the use of a raw disk or dedicated logical unit number (LUN) to a virtual machine. I think that any virtualization administrator should go through the drill. The basic principle is to seamlessly move storage on a storage area network (SAN) from a physical system to a virtual machine. This can accommodate an emergency physical-to-virtual (P2V) conversion of a system with large storage, saving costly time on a system conversion.

The experience I reference was a VMware ESX Server and VirtualCenter configuration with Windows Server 2003 systems. With a LUN made available to a physical system, moving that storage over to a virtual machine was actually quite easy and uneventful. There are a few pointers to remember when doing this configuration, namely because the storage is not residing on a virtual machine file system (VMFS) LUN, it will not be eligible for a lot of VMware’s management and high-availability (HA) functionality. This includes Storage VMotion, host-to-host VMotion and HA features for failover onto another host automatically.

Taking the time to become familiar with the process and expected functionality is a worthwhile investment. This is a configuration that may not be something that you foresee using, but it may come in handy. The figure below shows a virtual machine configured with two LUNs made available to the host and assigned to a virtual machine:
Figure 1

One important step is not to add storage as you normally would for a VMFS volume. This will destroy the contents of the drive and make your preserved data unreadable. In the configuration shown below, the drive arrives seamlessly to the guest operating system and will be ready for use. There may be a drive letter change, but that is an easy correction.
Most storage systems and virtualization platforms should be able to support this functionality in some capacity. Taking the time to get the specifics down in your storage environment can avoid any surprises if this configuration is required due to an unplanned migration

3  Comments on this Post

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  • Scott
    If you attached the raw LUN in virtual compatibility mode (which you should in most cases) then you still get HA & VMotion as long as the LUN is presented to all hosts. You can also use snapshots (but don't try storage vmotion unless you want your disk converted to a VMDK). The only downside to raw LUNs in virtual compatibility mode is that you need a 1:1 ratio of disks to LUNs whereas VMFS3 LUNs can have a many:1 ratio of disks to LUNs.
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  • coobox
    Can I mount the same physical hdd to two virtual machines? I need it for a MySQL cluster. pls help me
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  • Rick Vanover
    Coobox - I would test the configuration with a different LUN. I just did something similar for a Windows MS SQL Server cluster, if you do this - you'll need to either remove cluster service before the swing - or get to reset the drive signatures so that the Windows drives are visible to the guest OS. As for having two VM's hit the same LUN - you could not do that unless the clustering service would manage it for Windows systems.
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