Does anyone extend the vmfs volume to get more disk space for the vm? Or simple create a larger LUN and move the VMDK files to it?
Do you know what the best size for a single LUN? 200GB, 500GB or 1TB, etc...
Yes, you can extend a LUN/Volume but it's not a good idea, this is called an extent, and is highly discouraged. You should create a new volume and move the VM's to the new LUN.
The size depends on the number of VM's per LUN, if they are small, your LUN should be smaller, but 500Gb to 1TB is a good range. There is a "theorhetical" limit to the number of VM's due to SCSI locks, but it depends on number of VM's, snapshots, and things that require getting a SCSI reservation.
I have standardized on a maximum of 500GB per LUN. I have seen many discussions about it. There are all kinds of opinion's on this one.
If you anticipate very large disk sizes on the VM's then lean towards 500G-1000G volumes. If your VM disks create a need to be larger than that the you may want to think about using raw volumes in addition to a vmfs LUN. Extents are safe, I have not seen issues with them but they are rarely done so you may find yourself on an island if there are issues with it. I have seen some indication in my SAN IO trends that too big is not a good thing, you can have some LUN doing very little IO and others going nuts so balance is they key here. There is a maximum number of 256 LUN's on ESX 3.x so if you have a large install then you will need to consider that as well because the LUN are shared across the server farm.
Hope that helps.
I would make a bigger LUN and move your VMs from the smaller one to a larger one as there is no way to 'resize' the VMFS on a given LUN. If you did resize the VMFS LUN to be larger you would then have either unused space or multiple partitions on the LUN. With the way ESX does its locking 1 partition per LUN for VMFS is a requirement.
As for LUN size make.laspina has some good advice. I tend to use 500GB LUNs or so.
Edward L. Haletky
VMware Communities User Moderator
Author of the book 'VMWare ESX Server in the Enterprise: Planning and Securing Virtualization Servers', Copyright 2008 Pearson Education. As well as the Virtualization Wiki at http://www.astroarch.com/wiki/index.php/Virtualization