Sunday, September 07, 2014

vSphere HA Cluster Redundancy

All vSphere administrators and implementators know how easily vSphere HA Cluster can be configured. However sometimes quick and simple configuration doesn't do exactly what is expected. You can, and typically you should, enable Admission Control in vSphere HA Cluster configuration settings. VMware vSphere HA Admission Control is control mechanism checking if another VM can be powered on in HA enabled cluster and still satisfy redundancy requirement. So far so good however complexity starts from here because you have several options what algorithm you will use to fulfill your spare capacity redundancy requirement. So what options do you have?

Admission Control can be configured for following three algorithms:
  1. Define fail-over capacity by static number of hosts
  2. Define fail-over capacity by reserving a percentage of cluster resources
  3. Use dedicated fail-over hosts
Let's deep dive into each option ...

Algorithm 1 is generally N+X host redundancy 
When N+X redundancy is required most vSphere designers go with this option because it looks like most suitable choice. However, it is important to know that this particular algorithm is working with HA Slot Size. HA Slot Size is calculated based on  defined reservations on powered VMs. If you don't use CPU/MEM reservations per VM than default reservation values (32 MHz, memory virtualization overhead)  are used for HA Slot Size calculation. By the way, VMware recommends to set  reservations per resource pools and not per VM so there is relatively high probability you don't have VM reservations and you will have very low HA Slot Size which means that Admission Control will allow to power on lot of VMs which introduce high resource over-allocation and your N+1 redundancy can significantly suffer. On the other hand if you have just one VM with huge CPU/MEM reservations it can significantly impact and skew HA Slot Size with negative impact on your VM consolidation ratio.  

How can we solve this problem? One solution are HA Cluster Advanced Options described below.

Maximum HA Slot size can be limited by two following advanced options.
  • das.slotcpuinmhz - Defines the maximum bound on the CPU slot size. If this option is used, the slot size is the smaller of this value or the maximum CPU reservation of any powered-on virtual machine in the cluster.
  • das.slotmeminmb - Defines the maximum bound on the memory slot size. If this option is used, the slot size is the smaller of this value or the maximum memory reservation plus memory overhead of any powered-on virtual machine in the cluster.
It helps in situation when you have one VM with high CPU or RAM reservations. Such VM will not increase HA Slot Size but it consumes more smaller HA Slots.

Default VM reservation values for HA slot calculation can be defined by another two advanced options.
  • das.vmcpuminmhz - Defines the default CPU resource value assigned to a virtual machine if its CPU reservation is not specified or zero. This is used for the Host Failures Cluster Tolerates admission control policy. If no value is specified, the default is 32MHz.
  • das.vmmemoryminmb - Defines the default memory resource value assigned to a virtual machine if its memory reservation is not specified or zero. This is used for the Host Failures Cluster Tolerates admission control policy. If no value is specified, the default is 0 MB.
Default VM reservation values can help you to define HA Slot Size you want but it doesn't automatically correspond with required overbooking and planed spare fail-over capacity because HA Slot Size is not proportional to VM sizes on particular cluster. If you really want to have one real spare host fail-over capacity you have to go with option 3 (Use dedicated fail-over hosts).

Algorithm 2 : percentage cluster spare capacity
This algorithm doesn't use HA Slot size but it simply calculate total cluster CPU/MEM resources and decrease these cluster resources by spare capacity defined in percentage.  The rest of cluster available resources is also decreased  by powered on VM reservations and new VMs can be powered on only when some cluster resources are available. Quite clear and simple, right? However it also requires to have VM reservations otherwise you will end up with over-allocated cluster and your overbooking ratio will be too high which can introduce some performance issues. So once again, if you really want to have one real spare host fail-over capacity without dealing with VM reservations the best way is to go with option 3 (Use dedicated fail-over hosts).
Note that algorithm 2 doesn't use HA Cluster Advanced Options related to HA Slot mentioned above. However das.vmCpuMinMHz and das.vmMemoryMinMB can be used  to set default reservations. For more details read this.

Algorithm 3 : dedicated fail-over hosts
This algorithm simply dedicate specified hosts to be unused during normal conditions and used only in case of ESXi host failure. Multiple fail-over dedicated hosts are supported since vSphere 5.0. This algorithm will keep your capacity and performance absolutely predictable and independent on VM reservations. You'll get exactly what you configure.

CONCLUSION
So what option to use? The correct answer is as usually ...  it depends :-)   

However, if VM reservations are not used and absolutely predictable N+X redundancy is required I currently recommend Option 3.

If you have mental problem with not using some ESXi host during non-degraded cluster state (isn't it exactly what is required?) I recommend Option 1 but VM reservations must be used to have realistic size of HA Slot. In this options artificial HA Slot can be designed leveraging advanced options.

If you don't want elaborate with HA Slot and use all ESXi hosts in the cluster you can use Option 2 but VM reservations must be used for some capacity guarantee to avoid high overbooking ratio.

FEATURE REQUEST
It would be great if VMware vSphere has some kind of Cluster Reservation policy for VMs. For example, if you want to guarantee cluster resources overbooking 2:1 you would setup 50% CPU and 50% RAM reservations for each VM running in HA Cluster. This policy should be dynamic so if someone change VM size from CPU or RAM perspective reservations would be recalculated automatically.

Let's break down our example above. We are assuming following HA CLUSTER RESERVATION POLICY => CPU 50%, RAM 50% assigned into our HA Cluster. Let's powered on VM with 2x vCPUs and 6GB RAM. Dynamic reservation calculation is quite easy from RAM perspective because memory reservation would be 3GB (50% from 6GB). It is little bit more complicated from CPU reservation perspective. CPU dynamic reservation have to be calculated based on physical CPU where VM is running. So let's assume we have Intel Xeon E5-2450 @ 2.1GHz. So 50% from 2.1GHz is 1.05GHz but we have 2 vCPUs so we have to multiply it by 2. Therefore dynamic CPU reservation for our VM is 2.1GHz.  I believe with such dynamic reservation policy we would be able to guarantee overbooking ratio and define cluster redundancy more predictable from overbooking and performance degradation point of view.

FEEDBACK 
I would like to know what is your preferred HA Cluster Admission Control setting. So, don't hesitate to leave a comment and share your thoughts with community. Any feedback is very welcome and highly appreciated. 

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