The VMguy

I was talking to another great customer today who was excited to upgrade from two single ESX hosts to a cluster of 3 with vCenter.  We were talking back and forth about the storage and it turns out his current datastores were a bit unique.  The customer had migrated from physical slowly, perhaps a few physicals a week.  Each time a new host was converted, the customer created a new LUN and datastore and p2v’d the physical drives to a single LUN/datastore on their EVA SAN.  That LUN was also unmasked to just one of the hosts (remember, 2 single hosts – no vMotion yet).  As I talked thru their current configuration with them you can imagine the look on my face.  I was perplexed, surely there must be something completely wrong with this design.  My years at EMC and NetApp were failing me, I knew this was not a good idea but no good reason came to mind.

Then it hit me, a single ESX host currently can see up to 256 LUNs.  Initially I thought, “but they’re never going to run more than 256 VMs on a host.”  No, but they did want to start using vMotion.  Now the LUNs will need to be presented to all hosts.  This 256 LUN limit no longer relates to the single host but to the cluster as a whole.  With all LUNs presented to all hosts, as long as they keep provisioning one-LUN-per-VM, they will be limited to 255 VM’s for the cluster (one of the LUNs is for booting ESX).  This was a limit they were most certainly going to hit (and at an accelerated pace, now that they have vMotion).

This made sense quickly to the customer.  The story has a happy ending: next week we’re upgrading them to vSphere and going to storage vMotion those VMs to a place with a better design.  There’s one thing I’ve learned about storage and virtualization is that there are no wrong designs.  However, there are ones that limit functionality.

The moral of the story is to know thy vmware maximums!  Make sure to check if a single host’s limitation could affect the design of an entire cloud.

Happy Earth Day!