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    How many vCPUs can I have?

    Scheduled Pinned Locked Moved IT Discussion
    hyper-vhyper-v 2016vcpuvirtualizationhypervisor
    46 Posts 13 Posters 4.7k Views
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    • ObsolesceO
      Obsolesce @marcinozga
      last edited by

      @marcinozga said in How many VCPU’s can I have?:

      My rule of the thumb is start with 1 vCPU if it's Linux vm, 2 vCPUs if it's Windows, unless you enjoy Windows updates or Antivirus scans taking hours to complete....

      This is how I do it too. Unless it's something that does nothing 99.99999% of the time like a license server on Windows.

      1 Reply Last reply Reply Quote 0
      • JoelJ
        Joel
        last edited by

        So our host is HyperV 2016 core.

        So if this is a 20 thread processor, could I be assigning double the amount of vCPUs?

        The reason I asked is because we did have high cpu usage on our app01 and sqlserver but after adding one additional vcpu this massively helped so I was thinking, should I just double what I’ve got in there already and I’ll be okay?

        scottalanmillerS 1 Reply Last reply Reply Quote 0
        • scottalanmillerS
          scottalanmiller @Joel
          last edited by scottalanmiller

          @Joel said in How many VCPU’s can I have?:

          So if this is a 20 thread processor, could I be assigning double the amount of vCPUs?

          If you assign less than ~18, you are just throwing performance away. The threads will be idle. So yes, assign more.

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          • scottalanmillerS
            scottalanmiller
            last edited by

            In a situation like this, I would not assign more than 19 vCPU across the workloads.

            JaredBuschJ 1 Reply Last reply Reply Quote 0
            • JoelJ
              Joel
              last edited by

              Thanks all for the feedback. Useful to know.

              1 Reply Last reply Reply Quote 0
              • scottalanmillerS
                scottalanmiller @Joel
                last edited by

                @Joel said in How many VCPU’s can I have?:

                DC- 1
                FS - 2
                APP - 3
                SQL - 4

                Into...

                DC - 1
                FS - 2
                APP - 10
                SQL - 6

                Is probably how I would do it. That's 19.

                But check your SQL licensing, that might cost you a fortune to increase it even a little.

                ObsolesceO 1 Reply Last reply Reply Quote 0
                • ObsolesceO
                  Obsolesce @scottalanmiller
                  last edited by

                  @scottalanmiller said in How many VCPU’s can I have?:

                  @Joel said in How many VCPU’s can I have?:

                  DC- 1
                  FS - 2
                  APP - 3
                  SQL - 4

                  Into...

                  DC - 1
                  FS - 2
                  APP - 10
                  SQL - 6

                  Is probably how I would do it. That's 19.

                  But check your SQL licensing, that might cost you a fortune to increase it even a little.

                  Virtual MS SQL is 4, more than that costs more.

                  Reid CooperR 1 Reply Last reply Reply Quote 0
                  • Reid CooperR
                    Reid Cooper @Obsolesce
                    last edited by

                    @Obsolesce depends on the license. You can get SQL Server in 2, 4, 6, 8, etc. If licensing by core.

                    ObsolesceO 1 Reply Last reply Reply Quote 0
                    • ObsolesceO
                      Obsolesce @Reid Cooper
                      last edited by

                      @Reid-Cooper said in How many VCPU’s can I have?:

                      @Obsolesce depends on the license. You can get SQL Server in 2, 4, 6, 8, etc. If licensing by core.

                      No, it's a minimum of 4 core licenses per VM. I've been down this road already.

                      pchiodoP 1 Reply Last reply Reply Quote 0
                      • pchiodoP
                        pchiodo @Obsolesce
                        last edited by

                        @Obsolesce I have also found through experience that more memory on a SQL server does not necessarily improve performance. This is also true with VCPUs. I would monitor closely and change one thing at a time. Just my 2 pennies worth

                        ObsolesceO PhlipElderP 2 Replies Last reply Reply Quote 0
                        • travisdh1T
                          travisdh1 @scottalanmiller
                          last edited by

                          @scottalanmiller said in How many VCPU’s can I have?:

                          NUMA issues is a key reason why we often recommend a single proc with more cores than two procs with fewer cores each. No NUMA.

                          AMD has been hitting high core counts by using multiple CPU dies on a single package, thus they have NUMA happening "on chip". The high end EPYC CPUs have 4 dies inside each package, with 4 memory controllers.

                          To stay away from NUMA, you have to stay with Intel, currently.

                          AMD is also doing the 2 threads per core thing now as well. So current EPYC 32 core CPU = 64 threads.

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                          • ObsolesceO
                            Obsolesce @pchiodo
                            last edited by

                            @pchiodo said in How many VCPU’s can I have?:

                            @Obsolesce I have also found through experience that more memory on a SQL server does not necessarily improve performance. This is also true with VCPUs. I would monitor closely and change one thing at a time. Just my 2 pennies worth

                            You probably mean the OP, not me.

                            But you do want to start with the minimal recommended, I think 8GB... it does help, even if your database is 1GB. At least from what I've seen and read.

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                            • Reid CooperR
                              Reid Cooper
                              last edited by

                              Ideally you want enough RAM to hold the database and any temporary or cache tables in RAM without having to touch the disk. More than that, and it is just wasted.

                              1 Reply Last reply Reply Quote 2
                              • JaredBuschJ
                                JaredBusch @scottalanmiller
                                last edited by

                                @scottalanmiller said in How many VCPU’s can I have?:

                                In a situation like this, I would not assign more than 19 vCPU across the workloads.

                                What? This makes no sense.

                                This implies that I can never have more than 19 virtual machines with 1 vCPU each.

                                Unless all 19 are super busy this will never be a problem. When you sprawl to single purpose VMs, you will constantly have at least a few mostly idle VMs.

                                scottalanmillerS 1 Reply Last reply Reply Quote 1
                                • scottalanmillerS
                                  scottalanmiller @JaredBusch
                                  last edited by scottalanmiller

                                  @JaredBusch said in How many VCPU’s can I have?:

                                  @scottalanmiller said in How many VCPU’s can I have?:

                                  In a situation like this, I would not assign more than 19 vCPU across the workloads.

                                  What? This makes no sense.

                                  This implies that I can never have more than 19 virtual machines with 1 vCPU each.

                                  Unless all 19 are super busy this will never be a problem. When you sprawl to single purpose VMs, you will constantly have at least a few mostly idle VMs.

                                  No, in a situation like this there are only four VMs. It's the situation like this that causes the limit, not my use of only 19 threads.

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                                  • scottalanmillerS
                                    scottalanmiller
                                    last edited by

                                    Tags added. Title fixed.

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                                    • PhlipElderP
                                      PhlipElder @pchiodo
                                      last edited by

                                      @pchiodo said in How many vCPUs can I have?:

                                      @Obsolesce I have also found through experience that more memory on a SQL server does not necessarily improve performance. This is also true with VCPUs. I would monitor closely and change one thing at a time. Just my 2 pennies worth

                                      Concur. We do a lot of testing with VMFleet and other utilities with monitoring via Performance Monitor or Telegraf/Grafana.

                                      There is a sweet spot with so many variables coming into play. Disk subsystem being one key.

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                                      • wrx7mW
                                        wrx7m
                                        last edited by

                                        Conversely, I over-allocated vCPUs across several VMs (ESXi) when I was new to type 1 hypervisors and ran into performance issues associated with high CPU ready times. After scaling down, performance improved dramatically.

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