Backup challenges
"People don't put enough thought into the storage. They haven't thought through the details of the number of VMs. Then there are one-off situations that may require raw data mapping," which enables the storage admin to specify a LUN on the disk array for a given VM, says Ron Oglesby, director of virtualization and architecture services at GlassHouse Technologies Inc., Framingham, MA (see "Raw data mapping," below).

Also challenging are the different types of recovery: backing up and recovering the entire VM as a file, or backing up and recovering single files. To back up and recover the entire VM, you "copy it block by block, disk to disk and capture the VM in a specific state," says Oglesby. "Here, you have to restore the entire VM as a whole."

To back up and recover individual files you "do a file-level backup within the VM using a normal agent or VCB, like a traditional backup. In this case, you can restore a single file easily but can't bring the entire VM server back online easily," he continues. You have to recover the VM piece by piece.

Virtualization is essentially file-oriented; the VM consists of one or two encapsulated files. Restoring individual files remains a challenge. It's easy to back up and restore the entire VM. Restoring individual files isn't straightforward and, depending on the backup tools involved, may be quite cumbersome and involve restoring the entire VMware Disk file (VMDK).

"Everybody wants to rapidly back up at both the granular [file] and virtual machine level," says Mainland Information Systems' D'Costa. With VMware as it's configured today, that's not possible. "Right now you have to choose one or the other," he says. Granular backup means you can recover individual files without restoring and mounting the entire virtual server. With machine-level backup, you recover the entire virtual server as a s...

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• What it is
  
  • Enables a storage administrator to specify a LUN on the disk array for a given virtual machine (VM)
  • Prevents VMware from managing storage for that VM
  
  
  
• Advantages
  
  • Improves performance of the VM
  • Used mainly for VMs running critical database applications
  • Gives the storage administrator direct control over storage and backup for that VM
  
  
  
• Disadvantages
  
  • Negates some of the value of virtualization by complicating management tasks
  • Can't take advantage of VM mobility or Distributed Resource Scheduler (DRS)
  
  
  

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