Extents vs Storage DRS

I was meeting with a customer today and had to stop for a second when they said they were using 10 TB datastores in vSphere 4.1.

At first I was going through my head of maybe NFS? No they are an all block shop. Oh wait yeah, extents. They were using 2 TB -512 byte luns to create a giant Datastore. I asked, why? The answer was simple, “so we only manage one datastore.”

I responded with well check out Storage DRS in vSphere 5! It gives you that one point to manage and automatic placement across multiple datastores. Additionally you actually can find which VM lives where, and use Storage Maintenance mode to do storage related maintenance. Right now they are locked into using extents. If they change their datastores into a Cluster the gain flexibility while not losing the ease of management.

I wanted to use the opportunity to list some information I think about Extents with VMware.

  1. Extents do not equal bad. Just have the right reason to use them, and running out of space is not one.
  2. If you lose one extent you don’t lose everything, unless that one is the first extent.
  3. VMware places blocks on extents in some sort of even fashion. It is not spill and fill. While not really load balancing you don’t kill just one lun at a time.

An extent with a datastore is like a stack of luns. Don’t knock out the bottom block!


Some points about Storage DRS.

  1. Storage DRS places VMDK’s based on IO and Space metrics.
  2. Storage DRS and SRM 5 don’t play nice, last time I checked (2/13/12).
  3. Combine Storage DRS with Storage Policy and you have a really easy way to place and manage VM’s on the storage. Just set the policy and check if it is compliant.

A Storage DRS cluster is multiple datastores appearing as one.

Some links on the topics:

Some more information from VMware on Extents
More on Storage DRS (SDRS)

In conclusion, SDRS may be removing some of the last reasons to use an extent (getting multiple lun performance with single point of management). Add that to being able to have up to 64 TB Datastores with VMFS and using extents will become even rarer than before. Unless you have another reason? Post it in the comments!

2 thoughts on “Extents vs Storage DRS”

  1. I would add in favor of extents that each new LUN that you add up you actually increase the host LUN queue count as well. And as you mentioned it, VM placement rotates between the extents. Performance wise extents might be a good thing to do. But as usual it all depends on many factors. First one would be IMO how good are they at storage management for instance?

    1. You are right, I forgot to mention queues. Extents and SDRS provide you the extra storage queues. Another reason to move to Storage Clusters over extents in my opinion.

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