Difference between a VMDK disk and Virtual RDM disk

[durjay]

I am trying to understand the difference between above two disks. From vmware documents what I found is

1. The mapping. A VMDK disk is a file within a VMFS file system and a virtual RDM(.vmdk) is a mapping file to a RAW LUN(no file system).

1. Usability: VMDK disks only supports cluster in a box but virtual RDM Disks supports both Cluster-in-a-Box and Cluster-Across-Box.

If the above are the only difference between them, what is the specific scenario when to go for each of them. If there are any other difference please let me know.

Thanks in advance

-Durjay

[AntonVZhbankov]

VMDKs can be used in cluster-in-a-box and cluster-across-boxes, depending of SCSI bus sharing compatibility (virtual or physical).

VMDKs and virtual RDMs have almost the same functions, both supports snapshots etc. But you don't have any snapshots if you have shared SCSI bus in VM.

So, if you want to set up cluster with VMs from different ESXes you can use virtual RDMs or VMDKs.

There is also physical RDM needed for virtual-physical cluster.

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VMware vExpert '2009

http://blog.vadmin.ru

[aleph0]

Hello,

RDM do not implies Hypervisor tasks, so it's suitable for High performance filesystem (DB transaction logs partition, high disk IO processes,...)

\aleph0

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[AntonVZhbankov]

As per performance tests difference between RDM and VMDKs is negligible.

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VMware vExpert '2009

http://blog.vadmin.ru

[durjay]

Anton,

Thanks for the reply.

Do you mean to say cluster across boxes (MSCS) supports non-scsi passthrough disks like VMDK(disks) ?

-Durjay

[AntonVZhbankov]

There are clusters other than MSCS, and technically you can set up MSCS on VMDKs. It will work, but it's not supported by Microsoft.

Moreover, MSCS is not supported if MSCS VMs are placed on cluster with HA or DRS enabled.

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VMware vExpert '2009

http://blog.vadmin.ru

[AndreTheGiant · Accepted solution]

For performance difference see:

http://www.vmware.com/files/pdf/performance_char_vmfs_rdm.pdf

http://malaysiavm.com/blog/vmware-vmfs-vs-rdm-raw-device-mapping/

Andre

[petedr]

vm/etc has a short article on performance differences with sql between vmdk and rdms

http://vmetc.com/2008/12/08/virtualizing-high-performance-sql-vmfs-or-rdms/

www.phdvirtual.com, makers of esXpress