Best Practice for iSCS SAN LUN sizes for VMs.

[daveclaussen]

I have a best practices question for you all.  At this time, our VMware (4.0 and 4.1) servers, HP BL460c’s, utilize our Equallogic iSCSI SAN to house virtual machines.  We have been creating 500GB LUNS and allowing each VMware server to access each LUN – for vMotion.  We are up to nine, 500GB LUNs.

My question is; would we be better off using 1TB LUNs rather than 500GB LUNs?  Or even larger perhaps?

I have no problem continuing our present path and things are working fine, I just wanted to ask for experienced opinions.

As always, thanks for your time.

- Dave

[vmroyale]

Hello.

How many VMs are you averaging per LUN currently, and are you using the 5.0.2 firmware on the EQ?

Good Luck!

[LarryBlanco2]

IMHO, 500GB are great.  I use that same size to hold my VMDK's.  If I have a VM that requires 500GB of storage or more then I swap over to a RDM.  Pretty much the break point is 500GB for me.

You can up a couple of LUNS to 1TB just to provide space for any larger (100GB+) VMDK's you may have.

It's just a matter of prefernce and what works best for your environment.

Larry B.

[daveclaussen]

I am getting an average of five VMs per LUN.  I do not know about the EQ firmware - thant's another group.

What brought all of this on is that I have a few 250GB VMs to be created and I hate waiting for something small enough to fill up the rest of that 500GB LUN while leaving snapshot space.

Thanks!

[daveclaussen]

I have many 100GB + VMs on these 500GB LUNs.  I need to be careful to leave space for snapshots.  I leave about 80GB for snapshots on each 500GB LUN.  Is that too much space for snapshots?  Too little?

[LarryBlanco2]

I would assume you are talking about snapshots from the EQ San.  Yes 15%-20% is good for snapshot reservation.  80GB is about 16% or so.

Others may chime in as well to give you their opinions. 

Like stated before..  anywhere between 500GB and 1TB luns are good.

Larry B.

[daveclaussen]

Actually, I mean leaving ~80GB, per LUN, for VMware snapshots.  The SAN guys take care of provisioning space for the EQ snapshots.

[vmroyale]

Five sounds a bit on the low to me.  Larger LUNs would allow you more flexibility.  If the EQ is on the latest firmware release (and all your hosts are at 4.1), then the VAAI is there which should also help you move to larger LUNs without running into problems.  Keep in mind the VMFS block sizes as well, as these dictate how large individual VMDKs can be.

[vmroyale]

This is going to depend on a couple of things, but it really comes down to how safe you want to be or how much monitoring you have in place.  If you never use snapshots on the VMs, then 80GB is probably fine.  A snapshot can grow to the max size of its base disk, so with 80GB you could run into trouble here if a snap were to go neglected.  As mentioned earlier, it is about finding what works best in your envrionment and going with it.