RDM (Full Version)

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atc -> RDM (9.Dec.2011 1:55:48 PM)

RDM is to allow application to have full control of the disk but why do we need virtual and physical mode for RDM?

Thanks for the clarification.

Scott Lowe -> RE: RDM (11.Jan.2012 2:51:47 PM)

An RDM in virtual mode gains VMFS features, including the ability to be snapshotted and use fault tolerance. A virtual RDM gives you a lot more flexibility than a straight physical RDM.


Scott Lowe -> RE: RDM (29.Feb.2012 6:29:05 AM)

I should be more clear... an RDM that remains fully physical is not not subject to some limits. An RDM in virtual mode has a size limit of 2 TB minus 512 bytes. An RDM in physical mode has a 64 TB size limit. That's quite the difference!


atc -> RE: RDM (12.Mar.2012 5:54:54 PM)

Thanks for the clarification. :)

Scott Lowe -> RE: RDM (1.Jul.2012 5:14:48 PM)


No problem! Also, it's been a myth for quite some time that RDMs have better performance than VMFS. This may have been true at one point and people may still believe it to be true since "raw" implies lack of any abstraction. However, in testing, this myth has been busted. RFDMs and VMFS volumes have very similar performance characteristics. So, if you're using RDMs because they supposedly perform better, stop! Use VMFS instead.


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