VMDK vs. RDM

[danam]

Hello,

we want to deploy a VM on ESX 3.01 with DRM and HA , the full program. It is supposed to access 1.5 TB of Storage. Would we deploy a large vmdk file or rather RDM ? Gut feeling tells me "Go for RDM in Virtual Mode". But are there any existing Best Practices for large disks

TIA

Dan

[VirtualNoitall]

Hello,

I would only go RDM if you plan on using some of the special services provided by your SAN for this virtual machine; service such as: replication, snapshotting, or something like a virtual and physical node in a MSCS clustered pair. Otherwise stick with VMFS. There is virtually no performance difference and you retain the portability of your virtual machine.

[VirtualKenneth]

Agreed, there's no to very little performance difference. RDM is usually used in environments were they for example need very big fileserver storage.

Recently I installed a site in where they wished to use RDM for backup compatibility. They didn't wanted to use VCB stuff and just wanted to backup "normal" LUN's

[esiebert7625]

I was facing the same decision with some large 600GB LUN's we were going to assign to a few VM's that were replacing a NAS system, in the end we decided to go with RDM's. RDM's are usually used for MSCS clustering and for servers that need to replicate SAN functionality.

Good post by a VMware employee yesterday on RDM's -

http://www.vmware.com/community/thread.jspa?messageID=661499

ESX Server Raw Device Mapping - http://www.vmware.com/pdf/esx25_rawdevicemapping.pdf

Fyi…if you find this post helpful, please award points using the Helpful/Correct buttons.

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Thanks, Eric

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[VirtualNoitall]

Cool, thanks for validating my response?

[danam]

RDM is usually used in environments were

they for example need very big fileserver storage.

Wow, you must be working in some environment if 1.5 TB is not large to you. How about this one: RDM could be useful, if you want to reassign the LUN, maybe even to a physserv.

True or false ?

[VirtualKenneth · Accepted solution]

True. and it's easy to grow as well

[danam]

True. and it's easy to grow as well

Well ok, that gives me a clear picture. Thanks to you all.

Any more views/experiences are obviously welcome.

Thanks

Dan

(I have to get used to this "points" stuff, I hope no one's offended)

[VirtualNoitall]

Don't worry, they are just points. It is not like we can buy anything with them

[WBDMIC]

Remember You cannot HOT extend a VMDK file while (if supported bij your SAN) with RDM it is posible to do so. You can only extend a VMDK when the VM machine is not runing.

[chumeng]

I did a test using RAID 5 & RAID 10 and the speed is much more faster compare to white paper.

Again, my test show RDM is much much faster than VMFS.

[SomeoneSomewher]

I have to agree, vmdk files are fine when reading from them but we have extensively tested numerously and found the write speeds are rubbish. We recently implemented a file server 1.2TB with VMDK files on ESXi most users don't notice but our engineering team regularly upload and download Gbit size files, download is very fast upload speed is generally rubbish.

Have tested on all Server 1 & 2, workstation, ESX and ESXi all show same problem. Write cache hides the issues for Smaller files but when all is said and done vmdk files still have the additional levels of abstraction and translation that RDM's simly don't have. Even with the partitions aligned to the disk storage.