FT sizing and HA Admission Control

[Igor_Nemilosti1]

Hi!

Plz, specify how to make a correct sizing FT.

For example, there are two virtual machines. Then, FT takes 4 slots of cluster. Plus 2 slots for SecondaryVMs from the documentation:

VMware HA includes the resource usage of Fault Tolerance Secondary VMs when it performs admission control calculations. For the Host Failures Cluster Tolerates policy, a Secondary VM is assigned a slot, and for the Percentage of Cluster Resources policy, the Secondary VM's resource usage is accounted for when computing the usable capacity of the cluster.

Total of 6 solts. Right?

If each virtual machine has 4Gb of memory, total will be 24Gb, right?

And what if the HA cluster with FT consists of only TWO hosts?

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Igor Nemilostivy

[admin]

No, each primary takes 1 slot and each secondary takes 1 slot - so 4 slots altogether. Also, the slot size is based on the memory (and cpu) reservations, not the configured memory (or number of vcpus).

[Igor_Nemilosti1]

Hm? On my little experiment is not so. You sure?

And for Primary and Secondary VMs of FT, reservations takes value the configured memory (or number of vcpus). Or it is not?

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Igor Nemilostivy

[admin]

Actually we're both right vSphere assigns a memory reservation to an FT vm that is equal to the configured memory (and you can't change this reservation). However, each vm should only be using 1 slot (unless you are using a user-defined slot size with HA advanced options). What gives you the impression that the primary is using 2 slots? In the cluster summary tab there is an "Advanced Runtime Info" link in the HA section. That should tell you how many slots are being used by vms that are currently powered on.

Elisha

[Igor_Nemilosti1]

Now my stand dismantled, and no possibility to check it.

As from the documentation

VMware HA includes the resource usage of Fault Tolerance Secondary VMs when it performs admission control calculations. For the Host Failures Cluster Tolerates policy, a Secondary VM is assigned a slot, and for the Percentage of Cluster Resources policy, the Secondary VM's resource usage is accounted for when computing the usable capacity of the cluster.

, 2 slots uses Secondary VM, if Host failures cluster tolerates is 1.

Other words, 1 slot for PrimaryVM + 1 slot for SeconadryVM + 1 slot for SecondaryVM_when_failure_occured = 3 slots. Not true?

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Igor Nemilostivy

[admin]

Not exactly. If just 1 primary and its secondary are powered on then the "used slots" field of the advanced runtime info for the cluster will equal 2 (1 slot for the primary and 1 slot for the secondary). In addition, a number of slots are reserved by HA in case of a failure. This number will depend on the configured failover level you have set as well as the number of slots in the biggest host (or hosts if configured failover level > 1). It doesn't depend on the number of vms powered on. Eg, if the biggest host in the cluster can provide 10 slots and you have a configure failover level of 1 then HA will reserve 10 slots to make sure that you can survive the failure of that host. You can determine how many slots HA has reserved for failure with this simple formula (based on numbers from the advanced runtime info):

reserved slots = total slots - used slots - available slots

[Igor_Nemilosti1]

I know how HA calculating slots in cluster. I not surely know how the HA calculating slots in cluster with FT feature.

Ok. For example:

If all hosts is identical;

If Admission Control is Host failures = 1;

And, if this is important, let HA calculating 6 slot per host.

Then, how many "used slots" takes for one VM, for which we will turn on FT?

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Igor Nemilostivy

[admin]

When you power on an FT vm, used slots should increase by 2 (1 for the primary, 1 for the secondary)

[Igor_Nemilosti1]

Hm, I'm not sure about this!

You can check it?

Or

ANYBODY else can help us?

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Igor Nemilostivy