I was reading an article last week by Ray Lucchesi on Virtual Volumes and queueing. In that article (and podcast) Ray (and friends on the podcast) describe Virtual Volumes and the benefits they bring but also a potential danger. I have written about Virtual Volumes before and if you don’t know what it is or does then I recommend reading those articles. I have been wondering as well, how all of this works, as I also felt that there could easily be a bottleneck. I had some conversations over the last couple of weeks and I figured I would share it with you instead of just leaving a comment on Ray’s blog. Lets look at an architectural diagram first:
In the diagram above (which I borrowed from the vSphere Storage blog, thanks Rolo) you see two important constructs which are part of the overall VVOL architecture namely the Storage Container aka Virtual Datastore and the Protocol Endpoint (PE). The Storage Container is where the VVOLs will be stored. The IO though is proxied through the Protocol Endpoint. You can imagine that if we would not do this and expose every single VVOL directly to vSphere that you would have 1000s of devices connected to vSphere, and as you know vSphere has a 256 device limit at the moment. This would never scale, and as such the Protocol Endpoint is used as an access point to a VVOL capable storage system.
Now think about a VMFS volume and look at the VVOL architectural diagram again. Yes, there is a potential bottleneck indeed. However, what the diagram does not show is that you can have multiple Protocol Endpoints. Ray mentions the following in his post: “I am also not aware of any VASA 2.0 requirement that restricts the number of PEs for a storage system’s support of a single vSphere cluster”. And I can confirm that VMware did not limit the number of Protocol Endpoints in any shape or form. I read the specifications and it literally states 1 PE at a minimum and preferably more. Note that vendor implementations of VVOL may differ, I have seen implementations that describe many PEs per storage system, but also implementations which have 1 PE per storage system. And in the case of 1 PE per storage system can that be a bottleneck?
The queue depth of the Protocol Endpoint isn’t limited to 32 like a regular LUN when multiple VMs are contending for IO (“disk.schednumreqoutstanding”) or 64 (typical device queue depth) but set to 128 by default. This can be increased when required however. Before you do, please consult your storage vendor. There are a couple of variables that need to be taken in to account like the max device queue depth for instance and then there also is the HBA max queue depth as well. (For NFS queue depth is no concern typically.) The potential constraint when there is only (uncommon) a single PE can be mitigated. What is important here is that VVOL itself does not impose any constraints.
I am hoping that clears up some of the misunderstandings out there.