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Malleshmp
Contributor
Contributor

Virtual Disk or RDM

I am preparing a Virtal Machine to be file server with 9 TB of storage. I have created 4 LUNs with 1.9 TB and 1 with 1.4 TB.

My question is, should go ahead by creating virtual disk of this big disks or i should use RDM?

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mike_laspina
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Hi,

That's not a lot of info to go on. The answer will vary but generally that's large enough to justify using RDMs. The real question is are you willing to give up the ability to vMotion it in the future and are you willing to deal with the additional complexity.

You may want to consider NFS if it's available, e.g. NetApp or S7000 etc.

Regards,

Mike

vExpert 2009

http://blog.laspina.ca/ vExpert 2009
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StephenTJ
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Contributor

I would say because of the size RDMs are a better way to go, I say this mostly because I dont like the idea of entire datastores having 1 VMDK file filling them up (and I hope you don't consider extents...). Also, depending on if you use virtual or physical mode RDMs you either gain the benefit of it being treated like a VMDK file (cloning, snapshoting, etc) in virtual mode or in physical mode you can use virtual w/ physical server clustering. RDMs also allow you to re-present the disk to a physical host if you need to move the server to physical at a later time.

There is no risk to vMotion capability using RDMs in physical or virtual compatibility mode, excluding using some form of clustering solution (i.e MSCS).

As far as increased complexity, you just need to be aware that the LUNs are used as RDMs and that all the hosts in your cluster are presented the LUNs in the same fashion.

It really comes down to your preference, you can have a bunch of datastores dedicated to this one server (probably completely full and at their size limitation without extents). That, or you can have a few LUNs on each host as RDMs presented to this guest for access and have to be aware that its setup that way.

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mike_laspina
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I'm sorry I was referring to svMotion will no longer be an option.

vExpert 2009

http://blog.laspina.ca/ vExpert 2009
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StephenTJ
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That is true that SVMotion isnt an option if you are using physical compatibilty mode for your RDMs, SVMotion should still work if the RDMs are in virtual compatibillty mode (since they will support snapshoting capability).

This KB article addresses the concerns for RDMs and migration scenarios that you might encounter.

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mike_laspina
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Actually svMotion will not migrate any RDM, it only migrates the stub (aka vmdk definition of the RDM). So to transition the 9TB of storage to a new SAN it will be a Backup/Restore only.

vExpert 2009

http://blog.laspina.ca/ vExpert 2009
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StephenTJ
Contributor
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Sorry, I meant that as long as the RDMs are in virtual compatibility mode you can migrate other VMDK files that virtual guest might have to new storage. As far as migrating RDMs to a new SAN you are correct that it would require a backup/restore system or SAN based migration tools to accomplish.

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gurusimrankhals
Contributor
Contributor

Another alternative would be CIFS. If you have an array that supports it you'd get a lot of benefits from putting a file server of that size directly on the array.

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mike_laspina
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Yes, direct NAS attached CIFS is an option however we don't know what the application is at this point. Supporting CIFS on the NAS will also require out of band ICAP anti-virus protection which is somewhat new to most environments. ACLs are also a challenge.

I am exploring this direction with the s7000 series for general windows clients.

It's extremely fast! I am seeing ~300,000 peek IO operations with SSDs.

vExpert 2009

http://blog.laspina.ca/ vExpert 2009
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