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I figured I would ask before I built a test environment to try this out.

I have a physical server that has a somewhat large amount of physical storage. It is running SQL and would prefer to do a cold migration over a hot migration to ensure I don't have data consistency issues.

My question is, does free space transfer very quickly during the migration process or does it literally do a bit for bit copy? The storage has 600GB of physical space and just under 200GB of it is used.

I do not have a large period of time where I could take it down. Obviously one option is to build a separate SQL server and migrate the databases one at a time but for now let's focus on the P2V question regarding how empty space is transferred.

Mr. Lost IT Guy
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  • do You migrate to another physical machine or to You want to convert Your physical server to a hypervisor ? – Bartłomiej Zarzecki Jan 12 '14 at 20:29
  • I would recommend you do a hot p2v only capturing the OS drive and any other drives that do not contain the DB files or logs to keep everything online. Then do a SQL backup and restore to your VM and you can then coordinate a cutover to your VM. You can also create smaller VMDKs to restore to if you want to as well. This can also be changed during a P2V conversion in advanced disk options. Or you can change from the default of thick to think during the P2V conversion. Also instead of doing a cold migration you could just quiet the DB and perform the P2V that way as well. – mrlesmithjr Jan 12 '14 at 23:38
  • Bartlomiej: I am planning on turning the physical machine into a VM not a HyperVisor. – Mr. Lost IT Guy Jan 13 '14 at 18:57
  • mrlesmithjr: that is actually an interesting idea as well. I will run this past my DBA and see if he thinks the SQL processes will freak out if they don't see the data drives right away. – Mr. Lost IT Guy Jan 13 '14 at 18:58

2 Answers2

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Just quick share from my experience:

  1. Use thin provision disk will not bring empty space to newly created guest VM.
  2. Change to thick (whatever type if you want) if it is necessary or justify (usually not).

Side-notes: it have to be long enough process for the conversion to take place if you mentioned "consistency", ask your boss or system team to be realistic.

Joe Sze
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I have not tried this, but guessing from how VMDKs work you might get lucky with zeroing out the free space before conversion.

One tool for doing this is sdelete: http://technet.microsoft.com/de-de/sysinternals/bb897443.aspx (if this is a windows machine)

On linux you use dd if=/dev/zero of=/tmp/dummyfile to write a file with zeros until the drive is full. (You can delete the file afterwards)

However keep in mind that in any case the converter will have to read the whole disk anyways. So this will - if at all - increase conversion speed if the network pipe is the issue.

Also it will increase vmdk creation speed on the server if you select the disks to be "lazy zeroed" (not "eager zeroed").

Hanno S.
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