25TB Disk... VMDK or pRDM?

[ranger203]

I have a client that wants a large single disk of 25TB attached to a VM. The ESXi server is 5.5U1, and everything is compatible for both.  pRDM or a 25TB VMDK, which one should I choose?  This will be the data drive for a content repository of many files.  I'm thinking because of that, I may want to use a client backup rather than VADP backups.  But which should I pick for the LUN type?  EMC VMAX for the storage.

[CoolRam]

Its always been suggested to VMDK untill you don't have the specific requirement for RDM. Since the performance wise both are nearly equall but you may lose the so many feature with the use pRDM.

[npadmani · Accepted solution]

I would also favour Virtual Disk, better that way.

use cases of Physical Mode RDM and limitations are listed bellow,

http://www.vmwarearena.com/2013/02/raw-device-mapping-rdm-use-cases.html

[a_p_]

With a virtual disk you do have all options to do the backup, i.e. you can use both VADP and/or client based backups, so I'd probably go with the virtual disk format.  However, IMO it's less a question of whether to use a virtual disk or an RDM, but more a question about backup and - maybe even more important - of restore. What are the requirements for RPO/RTO? Are you able to do the backup within a predefined backup window (especially with lots of files and a client/agent based backup)? Are there requirements for the backed up files like keeping a certain history for modified files, or allowing to restore deleted files for a certain amount of time? I'd suggest you put together (unless already don) the requirements for restoring single files up to restoring the whole volume, and develop a backup strategy based on these requirements.

André