Advice required on LUN's and Virtual Machines

Hello,

I am doing a lot of research into virtualisation and SAN's at the moment and hope to implement a solution once i ahve enough solid information to go on. At the moment I am confused as to the whole idea with LUN's and Virtual machines. I understand a LUN is essentially a partition of an array which is presented for clients to connect to.

It would seem to me that the best way to set luns up is one LUN per VM, for performance reasons, but from what I have read online it would seem this is not the case. Could somebody explain to me how this all works from a VM per LUN perspective? I also understand that in Hyper-V that have introduced cluster shared volumes in order to deal with multiple VM's per LUN so its obviously viable to put more than one VM each.

Could somebody explain this to me, and additionally explain how iSCSI targets are generated/what they are.

I feel like I almost have the idea and just need a correct explanation for things to 'click'.

Thanks.
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forzaawAsked:
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vmwarun - ArunCommented:
I am a subject matter expert in VMware so I would put things in VMware perspective.
LUNs can be configured in 2 ways, adaptive and predictive.
You can use one or many small LUNs ranging from 300GB to 650GB or you can use a single LUN of maximum 2TB-512B with a block size of 8MB when you are using VMFS (Virtual Machine File System).
VMFS is a clustered file system which is equal to the clustered shared volumes of Hyper-V.
If you are looking to configure Microsoft Clustering then you can use either RDMs (Raw Device Mappings) or VMFS depending on your method of clustering or Network Load balancing.
As per EE Terms of agreement, you are expected to ask only one question per thread so I suggest that you ask about the iSCSI Config in another thread.
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msmamjiCommented:
Hi,
LUNs (or logical devices) are virtual representation of (shared) storage, so that storage can be made available to desired machine(s).

"It would seem to me that the best way to set luns up is one LUN per VM, for performance reasons, but from what I have read online it would seem this is not the case. Could somebody explain to me how this all works from a VM per LUN perspective?"

That was the case in Windows Server 2008 and previous Server OSes, since we only had NTFS as filesystem available. NTFS disk cannot be simultaneously controlled (read or written to) by two ( or more) machines at the same time, therefore, if you wanted to make a (single) VM highly available, you needed to place it on a separate LUN. If you place more then one VM on a single LUN then failover of LUN from one to node to the other would trigger the movement all the VMs on the LUN. In other words since LUN is the unit of failover, therefore you move everything on the LUN, be it one VM or several.

I also understand that in Hyper-V that have introduced cluster shared volumes in order to deal with multiple VM's per LUN so its obviously viable to put more than one VM each.
With the advent of CSV (available on Win 2008 R2 only), the limitation of one VM per LUN is gone. CSV are LUNs configured clustered shared Volume, therefore making them available to more then one node for read and write operations. For more detail, you can have a look at this.

http://www.savilltech.com/Videos/CSVDeepDive/CSVDeepDive.wmv
http://www.savilltech.com/Videos/Clustering_in_a_virtual_world/Clustering_in_a_virtual_world.wmv

So to answer you question, yes it does make more sense to use CSVs. Win 2008 R2 and CSVs you give the ability to live migrate the VMs among other things.

Regards,
Shahid
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forzaawAuthor Commented:
Hello,

Thank you for your answers. So in a scenario that I have 3 physical host servers, working together as a cluster which have lets say 4 VM's each on them, how would I configure the LUN's? Would I only need one LUN which they all connect to? Surely having multiple VM's on one array or LUN section will cause a performance problem on the disks?
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coolsport00Commented:
I would create 3 RAID5 LUNs "forzaaw", with 4 VMs on each. You certainly could have all your VMs on 1 LUN, but that is opening you up for a single point of failure. Sure, you could create the LUN as a RAID5, but if more than 1 disk fails, all your VMs are out of commission. Also, you're correct, there would be a slight performance degregation with placing all VMs on 1 LUN. If the VMs are low I/O type servers, you probably wouldn't notice a performance issue. But, if your VM servers are diverse, such as Exchange, SQL (or some DB), etc, then you would 1. want to separate your high I/O VMs onto different LUNs/datastores  for performance as well as for 2. better failover.

I would not create 1 LUN per VM...that is most certainly not cost-effective nor provides best usage of your disks.

Hope that helps.

~coolsport00
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forzaawAuthor Commented:
Excellent, I have found some great video resources which I have been watching and what you have said backs up exactly what I have been watching.
I have also since answered my own question in regards to iSCSI targets. Thanks all for your help, I will assign points accordingly.
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forzaawAuthor Commented:
Both comments built upon the basic understanding I already have of the system. I asked this question in order to solidify what I thought to be true and this has made me feel more confident on my future Project.
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