VMware Cloud Community
RGS3
Enthusiast
Enthusiast

Best Practice RDM Sizing for File Server

Sweet spot for VMDK ive read is about 300-600GB. How about RDM's? I need to create a file server with several TB (probably 2TB) and figured RDM was the way to go due to size and not really performance reasons. My choices are the following:

1. Create 4 vmdk's at 500GB each create 4 virtual disks and use Windows Striping (RAID 0) to get my 2TB drive

2. Create 4 RDM's at 500GB each and use Windows Striping

3. Create one large 2TB LUN and create an 1 RDM

This will be for a file server but we might have a SQL database that we might want to go large with. I am leaning on option 3 for the file server since the software raid overhead of the options 1 and 2 might impact performance.

Is this the best method? Ive read several WP about VMFS and RDM with no mention of this.

Regards,

Richard

Reply
0 Kudos
9 Replies
BenConrad
Expert
Expert

Option #3 makes a lot of sense and will be easier to troubleshoot if you have any issues.

Reply
0 Kudos
CiscoKid
Enthusiast
Enthusiast

RDMs are extremely useful when heavy I/O is anticipated. Because you are anticipating SQL I would totally agree with option 3 to give SQL the I/O it needs. Creating a VMDK on a VMFS might cause issues especially if it is being shared among other running VMs. You might run into SCSI locking issues if other VMs on the same VMFS are also disk intensive. And I absolutely agree about the software RAID scenario...not only are you impact the performace of disk operation but you are also going to increase the amount of vCPU cycles ultimately impact the entire host under the right conditions. Go with option 3!!!

Reply
0 Kudos
RGS3
Enthusiast
Enthusiast

But is there any issues with presenting 2TB RDM to a VM? Do the same best practices that apply to VMFS not apply to RDM? So for VMFS 300-600GB is considered best practice, but RDM 2TB is fine?

THX

Reply
0 Kudos
BenConrad
Expert
Expert

The best practices don't apply. RDM != VMFS. A RDM is just a LUN from your storage presented to your ESX ESX servers & guest. When creating a RDM you essentially 'link' the guest to the LUN with the RDM. The guest hardware links to a scsi device. The guest scsi device get linked to a LUN in the guest config.

If you find that you need to move the RDM LUN from virtual <> physical you can easily do so since the filesystem on the RDM is directly managed and native to the guest.

Ben

Reply
0 Kudos
CiscoKid
Enthusiast
Enthusiast

The only issue with regards to size is dealing with sizes above 2TB. The MBR has known issues when trying to write signatures and initializing sizes over 2TB in the Windows operating system. But since you are only doing 2TB you will be fine.

Reply
0 Kudos
mitchellm3
Enthusiast
Enthusiast

One other consideration is backup window. If you present a 2TB lun to a file server and you fill it with 1.75 TB of data, can you back that up in enough time? Maybe 4 500G Drives will be easier to schedule and maintain with your backup software/method. Just a thought.

Reply
0 Kudos
ewannema
Enthusiast
Enthusiast

Most of the time the sizing best practices seem to be around limiting the number of VMs you are putting on a single LUN. This is to avoid contention and locking issues when updating file system metadata. In this case I am not sure that there is a benefit of using a RDM over a large vmdk. Just make sure to set the block size appropriately when you create the vmfs3 volume.

2TB is the maximum size of a SCSI disk and a file or RDM so you will probably need to think about your growth options if you exceed your initial allocation anyway.

If you do decide to go the RDM route make sure that you understand the two different types and pick the appropriate one. If you might use snapshots or VCB in the future this will be critical.

http://wannemacher.us
Reply
0 Kudos
lfchin
Enthusiast
Enthusiast

I had more than 2TB on my RDM per VM and running fine without any issue. Perfomance wise is good and I can easily zone it to another physical server if needed.

Craig http://malaysiavm.com
Reply
0 Kudos
ewannema
Enthusiast
Enthusiast

You have created one RDM presented as a single SCSI disk larger that 2TB to a VM? I have not tried to configure anything that large, but was basing my comment off of the configuration maximums document .

http://wannemacher.us
Reply
0 Kudos