I am new to Virtualisation, and attended a VM event today which cast the above in a different light. I spoke with the engineer following the event to be told something that contradicts the advice we received from our implementing partner.
So the question I have is, which is better applying diskspace to a Virtualised server using a Raw Mapped LUN, or applying disk space as a VMDK file?
I currently backup the disks of the Virtualised serversby presenting a snapshot of the disks to a Windows system with a local tape drive, would this be possible using the VMDK option or would we need an agent running on each VM system?
VMWare will probably tell you the performance difference between an RDM or a VMDK file is minimal. If you follow best practices for either method I would tend to agree with that. I guess a simpler way to look at it is either choice should be able to be configured to meet your requirements.
From a backup standpoint if you want to utilize snapshots from your storage system as part of your backup method, then you will need physical RDMs for your VM disks. If you want to use VMDK files you could look into using VMWare Consolidated Backup (VCB) which will similarly schedule a VMWare snapshot (on your VMFS filesystem) and present that snapshot in a way that your Windows storage system can read it and back it up to tape.
Hope some of this information helps and at least gets you going in the right path.
Mike nailed it on the head.
To me the best thing about the vmdk is portability...I can dmotion it anywhere I want with the system live and running. Try moving the raw mapping like that. BCV works fine for a lot of people.
And yes, performance is almost identical.
As mike siad it depends on how you want to used them
I store my Data and databases on an RDM ... so incase of a blue screen or windows upgrade I just detach the RDM from the VM and reattach to another Virtual machine... So my data and databses are always intact...
My less important servers I uses vmdks...
Ah, but you can do the same thing with vmdk's. As long as it's a different virtual disk (we split our OS and data partition into two different vdisks for this reason), you can simply add that disk to a different guest. We use vmdk's for everything, critical or non-critical....One less thing to manage.