LUNs and virtual machines

Question

I have HP P2000 + XenServer solution working. What I'm thinking about is how to create LUNs for 20 virtual machines. Separate LUN for every VM ? It gives me ability to backup the specific virtual machine, however 20 vm = 20 shared storages in XenCenter and moreover I'm afraid of manageability of these LUNs or am I wrong ? Or I can have one bigger LUN for 20 VMs. What is the better option?

score 5 · Answer 1 · answered May 10 '11 at 18:03

I also think you need to ask yourself a few questions which will help you decide how many LUNs you want to create and how many VMs to deploy to each:

What are my organization's data retention and provisioning policies?
What type of data will this LUN be holding (Database, Web, just operating systems)?
Do we anticipate any growth in data or systems (perhaps you have 3 web servers, will you be adding a 4th?) (You may have 1TB worth of database data. Could that potentially grow?)
What are the business goals and objectives?

For example, if I have a heavily used database server that has a long data retention policy I will create a LUN for just the database data and perhaps put the operating system VM in a bucket LUN that holds other non-intrusive/not heavily used VMs.

Make sense?

score 4 · Answer 2 · answered May 10 '11 at 17:57

I don't know anyone who creates a separate LUN for each VM, not on Xen and not on any other hypervisor. You don't want TOO many VMs per LUN (I tend to limit my ESX datastores to 500GB to ensure that not to many can go into each) but one per LUN would just be a massive headache. If you have 20 VMs maybe think about 4 or 5 LUNs and spread them out across them.

score 1 · Answer 3 · answered May 10 '11 at 18:24

1

You can backup your VMs without having a separate LUN for each. There is no good reason to have a dedicated LUN per VM, and frankly I think it's asking for trouble.

answered May 10 '11 at 18:24

I will caveat this by saying that if you are using StorageLink with XenServer, it will create and manage a separate LUN for each VM. Since XenServer handles all the SAN operations for you, this is a pretty practical solution. But very ugly looking when you see a bunch of volumes named 'Xen' followed by a bunch of hex gibberish. – Jul 12 '11 at 16:16

score 1 · Answer 4 · answered May 10 '11 at 18:58

It's also worth noting it depends on your storage device. With NetApp SAN's for instance, you can have multiple LUNs on the same disk aggregate, so separating VMs into multiple LUNs won't really help if they're all running on the same spinning disks. Also consider dedupe options, which are normally LUN based, as a factor for LUN grouping.

To echo what everyone else has said, it's probablly better from a management perspective to group like VMs into consolidated LUNs. We limit our LUNs to 1TB.

score 0 · Answer 5 · edited Apr 13 '17 at 12:14

Are you using NPIV to pass-thru these LUNs to the virtual machines? In this case, what you're doing makes sense if you have a particular requirement for doing so. (you haven't listed any in your question)

If not, then what you're doing doesn't make sense and will bite you in the long run when you're trying to manage it.

Chopper3's answer: LUNs and virtual machines is good advice in this regard.

LUNs and virtual machines

5 Answers5