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DKramkowski
Enthusiast
Enthusiast

VMDKs with RDMs

I've got a few servers with RDMs, and have notived that there are VMDK files that 'appear to be' the same size as the RDM. I have beed doing some reading and near as I can gather, the files aren't really that large, but in fact are probably only a few KB. This makes some sense becasue I have two VMs on a 300GB LUN that have RDMs, one with a 250GB RDM & the other with a 400GB RDM, and obviously, 650GB won't fit on a 300GB LUN. But if that's the case, why does it take as much time to move said VMDK from one LUN to another as it would if it really was 250GB? I started migrading one of the VMs from that LUN to another and ended up having to cancel the move because it was taking WAY to long and I had to bring the server back up.

Another thing I read is supposedly the VMDKs are related to virtual mode RDMs, not Physical mode RDMs, but all my RDMs are physical mode. I've never used virtual mode RDMs.

If these 'mapping files' really aren't very big, why do they 'appear' to be and why to they behave like they really are that big when you try to migrate the VMs data files to a different datastore?

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keiooz
Enthusiast
Enthusiast

That is right. I have tried and check it with both virtual and physical RDM and it seems like there are a lot of difference when it comes to speed.

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kjb007
Immortal
Immortal

A cold migration of strictly RDM's should not take very long.  Worst case scenario, you can remove the RDM's and add them back in after you move your main vm disk.

Virtual mode RDM's have their SCSI commands still go through ESX, which allows them to support snapshots, which physical RDM's don't.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
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DKramkowski
Enthusiast
Enthusiast

That's the thing; I'm using strictly 'Physical mode' RDMs, and the time it was taking with the VM Powered down (that's the only way I can do is as we don't have the license for storage vMotion was every bit as long as it would be if the file was really 250GB.

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kjb007
Immortal
Immortal

That is very odd indeed.  If nothing else, use the 2nd approach, remove the RDM, move the vm, add the RDM back in.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
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