ISCSI disks - single vs multiple

[MalcO]

Is there a preferred method for these, ie. is it better to have one large ISCSI disk containing all the virtual machines or is it better to have smaller disks 1 for each virtual machine? Any ideas suggestions welcome.

many thanks

[George_B]

Do you mean VMFS volumes storing the VMDK files of your VM ? Or do you mean RDM volumes?

With respect to having a VMFS volume, you want to work out the size of the VM's you are going to be deploying, an estimate of the swap size , the snapshot space per VM that maybe required and the number of VM's per VMFS volume. You want to try and limit your VMFS volumes to 10-15 VMs so that SCSI locking does not start to cause a problem.

In my configuration I have a 200GB LUN with a VMFS volume for 10 VMs each with disks of 15GB. The 50GB remaining in the volume is reserved for each VMs swap file, snapshots and a little overhead for extra disks.

[MalcO]

We currently have 32 virtual machines each VMFS volume is on a separate LUN (32 LUNS), is it better for performance to have 1 large LUN with the 32 VMFS volumes in it?

[George_B]

How many VMs (VMDKs) do you have per VMFS volume? Is it one VM per VMFS volume on each LUN? This seems a rather complex way of organising it. I am not sure about performance but management would be difficult.

[MalcO]

Yes currently we have 1 VM per VMFS vol per LUN, is there a limit to the disk space that can be allocated to a LUN?

[cmanucy · Accepted solution]

2TB without using extents.

There is a limit of the number of LUNs an ESX host can access as well - so your 1:1 model doesn't scale well.

(Current) best practice reccomendation is 500GB LUNs, but you really need to make sure this fits with your needs. I would say 10-15 VM's per LUN is generally a good rule of thumb.

If you're running MS OS's, you can also use the iSCSI initiator for additional disks (think data) which will provide better I/O than using VMWare's iSCSI intitator.

[Dave_Mishchenko]

A single LUN can be up to 2 TB, but as mentioned you'll want to limit the total number of VMs per LUN to minimize problems with SCSI reservations. A reservation occurs for certain file operations when a host has to for example create a file or expand one. When that happens, the entire LUN is locked momentarily and I/O to all other VMs is paused. It's very brief, but if you have too many VMs on a LUN then it can start to be an issue if it happening too often.

[MalcO]

Great thanks for this, will go for 500Gb wih 10 machines on each.