LUN to VM and Host Ratio

Posted on 2013-02-06
Last Modified: 2016-11-23
We are currently planning to virtualize our physical environment. We have purchased a dell equallogic PS4100X (iscsi) with 24 x 600GB SAS disks which will be configured in a RAID 50 Configuration with two hot spares. Total usable storage will be approx 13 TB.

We will be running Vsphere 5.1 essentials plus across two Physical Vmware hosts.

15 physical servers will be virtualized and another 5 servers will be deployed over the next 3 months.

Our current data storage usage is 2TB and approx storage to be used up after virtualization will be approx 3 TB.

We are planning to dedicate a LUN for disk backups (before spinning off to tape)

Initally we were thinking of have two luns and two datastores. One for production servers and the other as the backup storage area However after some research we have some concerns if this will impact performance.

After reading a best practice white paper from vmware it was recommended to have a "consolidation with some level of isolation"

Our servers are not very resource hungry we have a typical windows environment with  file servers, DCs, SQL 2005, 2 x citrix servers with approx 60 citrix users and Exchange.

As a rough guideline how many LUNS per VM should we have? What whould we be considering?

Is there an issue with two vm hosts accessing the same datastore/LUN ?

I think we should carve out LUNs as required so maybe we should start with 2 or 3 LUNs and spread our servers across there then create additional LUNs as required.

any advice would be appreciated.
Question by:BA-IT-2000
LVL 119

Accepted Solution

Andrew Hancock (VMware vExpert / EE MVE^2) earned 250 total points
ID: 38862987
We prefer smaller LUNs to larger LUNs, just because of disaster recovery, larger the LUN, you can have more VMs, but if there is a disaster with a LUN failure you loose more VMs.

we work on 5-6 VMs per LUN, and create LUNs between 500-800GB. Unless we require a supersized LUN for specific VM.

There is no issue with multiple ESXi hosts accessing the same LUN.

We also spread the VMs across LUNs, so we do not put all our DCs on the same LUN, same with SQL servers, Exchange servers, spread them across LUNs.

Assisted Solution

xperttech earned 250 total points
ID: 38865801

Just to clarify, VMs are not traditionally assigned LUNs directly. It is possible but usually we try to avoid that. Mapping LUNs directly to VMs is called RDM (Raw Disk Mapping). VMs usually get virtual disks (VMDKs).

When you present a LUN to your VMware hosts, the first one to map it will format it with the VMFS filesystem. This format allows for multiple host access (read/write). This is VMware's filesystem as opposed to NTFS from Microsoft where you cannot have more than one host reading the LUN unless you are using clustering technology. The rest of the ESXi hosts will see the new VMFS and also map it. For DRS (distributed resource scheduler) and HA (high availability) to work properly, shared storage is key.

Hanccocka already answered above the recommendation about sizing and this coincides with my experience too.

Once a LUN is formatted with VMFS, it becomes a datastore. You can have your local datastores and your shared datastores.

What VMware admins need to do when deploying VMs is find a balance in performance and sizing when selecting among multiple datastores (shared storage).

When you create a new VM and select a datastore, the host creates a folder with your VM's name and there it places the files that make up your VM. See here the different files you can find there. The key files are VMDK (virtual disks) and VMX (your VM config descriptor file), but there are many there...

Hope this helps....

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