One of VMware’s partner engineer raised this query on an internal group. He wanted to understand and learn the best practices or the Do’s and the Don’ts while using RDM (Raw Device Mappings) Luns in a vSphere environment.
I hope you being a reader understand what an RDM is and what role does it play in a vSphere Environment. In case, you are not aware of RDM, then kindly refer to the following document – vSphere 5.x Storage Guide
and read about Rae Device Mapping (RDM).
During this discussion we will consider the following requirements which we need to meet :-
- We need to provision RDM’s for more than 20 VM’s and size of disks will vary from 1TB to 19TB.
- The RDM is decided for configuring MSCS on VMs like MS Exchange, MSSQL, File Servers etc.
A usual topic of discussion is choosing between RDM and VMDK. Since we have already solved that mystery, there is not much to worry about. We are already following the best practices around application layer by choosing RDM’s instead of VMDK. Now since we are playing with Luns mapped to your virtual machines, there are a few things we should take care of:-
- Choosing between Physical Compatibility Mode & Virtual Compatibility mode for RDM – A physical RDM is more storage array driven and virtual machine controlled. The VMkernel has limited or no role to play and it literally becomes a postman who delivers IOs from the OS to the LUN (just like an OS running on a physical server saving data on a storage LUN). This will restrict you from using VM level snapshots and other file locking technologies of VMKernel. Since you are talking about file sizes of more than 2TB, please ensure you are on VMFS 5 and use Physical compatibility mode only as VMFS 5 does not support RDM with Virtual compatibility mode for Luns greater than 2TB – 512bytes. On the other hand you would be able to use RDM with Physical compatibility mode for up to 64 TB. (VMFS 5 required)
- Ensure you have the correct Multipathing and failover settings
- Please ensure you are zoned appropriately. Test heavily before pushing things into production
- Follow the MSCS on vSphere guide without fail to avoid any last minutes surprises
Well this should help you do the right things with RDM’s. Ensure you go through the document which I mentioned before and you should be good to go.