APIC-EM - High Redundancy

[sjean2013]

Hello,

Why do you need to deploy 3 VM for APIC-EM redundancy?

If I have 2 ESX Servers, 2 VM are enough ?

Thanks

[aradford · Accepted solution]

Hi Sebastien,

This is based on a fundamental concept in distributed computing called "split brain".

If one VM is not able to see the other, it could be due to the other node failing or the link to the other node failing.

In the scenario  of the link failing, both sides could potentially continue to process requests (split brain) which would cause issues when they were reconnected.  Which version of the "truth" would you trust?

To address this, if a cluster is partitioned, it needs to have >50% of the members active.

The minimum cluster size you need to do this is 3.

Which is why 3 is usually the minimum number of members for any HA cluster.

It is also why clusters often have odd numbers of members.

With APIC-EM, if you only have 2 VM in a cluster, and one VM goes down, the cluster goes down.

Adam

[sjean2013]

Hi Adam,

Thank you for the explanation !

[sjean2013]

Hi Adam,

For Multi hosts deployment, in Release notes 1.4, it is necessary to have 8x 900 GB HDD = 7,2 TB for each host in the APIC-EM cluster. It is outsize ...!

Is it a mistake or is it correct ?

[aradford]

let me check.  I do not think that is the intention.

[sjean2013]

Hi Adam,

Do you have any news about HDD size on APIC-EM cluster ?

/Seb

[aradford]

no, but chasing it... thanks for reminder.

[sjean2013]

Hi Adam,

We have a multihost cluster with 3 nodes running release 1.4.3.1009.

This infrastructure worked for several months (we use Plug and Play application).

Our APIC-EM is not reachable trough HTTPS, we have a “502 Bad Gateway” error message (we tried with Internet Explorer, Chrome and Firefox but we have the same issue). SSH still working without any problems.

Did you already have this problem ?