4 Replies Latest reply on May 23, 2019 8:44 AM by psb1963

    Mixed VSAN/External SAN Hybrid

    psb1963 Novice

      Our Chief Engineer threw out an odd thought to me and I wanted to bounce it off some of the experts here on the blog.

      Up Front:

      Our systems are extremely SWaP constrained, no room for additional nodes or power for nodes in custom (i..e. shallower than normal) racks, so to increase our VSAN capacity we need to use some very high price 15.3 TB SSDs. Also All our VMs have multiple vmdks to support multiple drive folders

      Question:

      As a way to increase capacity without using the more expensive drives could be to limit the VSAN datastore to the "C" drives of each VM for the OS, and map all other drives to an external SAN with all flash (our studies have shown that the drives in the AFAs are cheaper than the drives used in our servers)

      I know that a hybrid like this is supported by VMware, but not sure about how mixing them within VMs would affect performance.

       

      So I'm looking for any advice/thoughts from some of the experts on this design possibility.

      Thanks, Paul

        • 1. Re: Mixed VSAN/External SAN Hybrid
          daphnissov Guru
          vExpertCommunity Warriors

          SWaP

          Size, Weight, and Power ? I've never seen this acronym before so am venturing a guess.

           

          While the idea of splitting out disks would work, there are some things to consider:

           

          1. You now very likely increase the troubleshooting time and MTTR for a given VM because you've spread its storage interests among different systems, so an issue with one system will impact everything to a greater or lesser extent (because you now have all [potentially] VMs with at least one disk stored there).
          2. You will have different performance characteristics as a side effect to saving on capacity.
          3. If using a VADP-based backup application, taking snapshots is now more complex (and some apps don't take kindly) because vSAN uses one disk format while your external VMFS-based array uses another.

           

          This use case is really almost like the "I-need-a-swap-VMDK-datastore" argument that sounds attractive on paper but, in reality and the givens that are failure scenarios, can come back to bite you in the rear.

          1 person found this helpful
          • 2. Re: Mixed VSAN/External SAN Hybrid
            depping Champion
            User ModeratorsVMware Employees

            I fully agree, troubleshooting will just be extremely painful in these scenarios, if not almost impossible. I wouldn't go there, although technically this is supported indeed, from an operational point of view it is a nightmare.

            1 person found this helpful
            • 3. Re: Mixed VSAN/External SAN Hybrid
              psb1963 Novice

              Thank you for your reply, I appreciate the answer, it lines up to my train of thought but wanted the second opinion. You were pretty close on the acronym, SWaP  refers to Space, Weight, and Power.

              • 4. Re: Mixed VSAN/External SAN Hybrid
                psb1963 Novice

                Thanks Duncan, I'm a huge fan of your blog and keep your "Essential Virtual SAN" book close at hand on my desk. And thanks for confirming my thoughts, and giving me solid evidence to show my leadership.