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P2V Timesaver: Block Level, Then Storage VMotion

Converting systems with large amounts of storage has always been a tricky task. Frequently, there are limited amounts of quiet time to convert these systems. This can be made even more difficult when a raw device mapping (RDM) cannot be used. Here is a trick that will open up your playbook a little.

When converting a physical machine in VMware environments, it is generally good practice to resize the volumes to an appropriate geometry. This usually means making application or data volumes smaller to avoid disk space consumed for free guest operating system space on the virtualization storage system. When using VMware vCenter Converter, you have the option to size down the disks, but that makes the conversion process use a file-level copy -- much slower. If the disks retain their size or are made larger, a block-level clone of the source disk will be used. The latter configuration is noticeably faster; my experience puts it at around a factor of twice as fast.

The issue with the faster process is that there is potentially large amounts of wasted space on the SAN. This can be solved by vSphere's Enhanced Storage VMotion. The enhancements in vSphere allow you to perform a storage migration task from a fully allocated virtual disk to a thin-provisioned disk. This means that once the workload is converted, you can perform this task to reclaim that wasted space. This is a timesaver, as the Storage VMotion task is done online with the virtual machine. The only caveat is that there needs to be enough space on the storage platform to support all of the pieces in motion, which may add up if there is a lot of free space involved.

Have you come across that trick or do you have any others for large systems? If so, please share your comments here or send me an e-mail.

Posted by Rick Vanover on 10/27/2009 at 12:47 PM


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