Yellow Bricks

by Duncan Epping

6.0

I want vSphere HA to use a specific Management VMkernel interface

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This comes up occasionally, customers have multiple Management VMkernel interfaces and see vSphere HA traffic on both of the interfaces, or on the incorrect interface. In some cases, customers use the IP address of the first management VMkernel interface to connect the host to vCenter and then set an isolation address that is on the network of the second management VMkernel so that HA uses that network. This is unfortunately not how it works. I wrote about this 6 years ago, but it never hurts to reiterate as it came up twice over the past couple of weeks. The “management” tickbox is all about HA traffic. Whether “Management” is enabled or not makes no difference for vCenter or SSH for instance. If you create a VMkernel interface without the Management tickbox enabled you can still connect to it over SSH and you can still use the IP to add it to vCenter Server. Yes, it is confusing, but that is how it works.

If you want the interface to not be used by vSphere HA, make sure to untick “Management”. Note, this is not the case for vSAN, with vSAN automatically the vSAN network is used by HA. This only pertains to traditional, non-vSAN based, infrastructures.

vSphere HA virtual machine failed to failover error on VMs in a partitioned cluster

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I received two questions this week around partition scenarios where after the failure has been lifted some VMs display the error message “vSphere HA virtual machine failed to failover”. The question that then arises is: why did HA try to restart it, and why did it fail? Well, first of all, this is an error that in most cases you can safely ignore. There’s a KB on the topic which gives a bit of detail to be found here, but let me explain to also in a bit more depth.

In a partition scenario, each partition will have its own master node. If there is no form of communication (datastore / network) possible, what the HA master will do is it will list all the VMs that are currently not running within that partition. It will also want to try to restart those VMs. A partition is extremely uncommon in normal environments but may happen in a stretched cluster. In a stretched cluster when a partition happens a datastore only belongs to 1 location. The VMs which appear to be missing typically are running in the other location, as typically the other location will have access to that particular datastore. Although the master has listed these VMs as “missing and need to restart” it will not be able to do this. Why? It doesn’t have access to the datastore itself, or when it has access to the datastore the files are locked as the VMs are still running. As a result, this will, unfortunately, be reported as a failed failover. Even though the VM was still running and there was no need for a failover. So if you hit this during certain failure scenarios, and the VMs were running as you expected, you can safely ignore this error.

HA Admission control: How can I check how much reserved resources are used?

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I had this question twice in the past three months, so somehow this is something which isn’t clear to everyone. With HA Admission Control you set aside capacity for fail-over scenarios, this is capacity which is reserved. But of course VMs also use reservations, how can you see what the total combined used reserved capacity is including Admission Control?

Well, that is actually pretty simple it appears. If you open the Web Client, or the H5 client, and go to your cluster then you have a “Monitor” tab, under the Monitor tab there’s an option called “Resource Reservation” and it has graphs for both CPU as well as Memory. This actually includes admission control. To verify this I took a screenshot in the H5 client before enabling admission control, and after enabling it, as you can see a big increase in “reserved resources”, which indicates that admission control is taken into consideration.

Before:

After:

This is also covered in our Deep Dive book, by the way, if you want to know more, pick it up.

What happened to MaxCostPerEsx41DS? It doesn’t seem to work in vSphere 6.x?

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Today I received a question which also caught me by surprise, someone updated from vSphere 5.0 and he noticed that when doing an SDRS Maintenance Mode that the setting MaxCostPerEsx41DS did not work. This setting actually limits the number of active SvMotions on a single datastore. You can imagine that this can be desired when you are “limited” in terms of performance. I was a bit surprised as I had not heard that these settings changed at all. Also, a quick search on internal pages and externally did not deliver any results. After a discussion with some support folks and some more digging, I found a reference to a naming change. Not surprising I guess, but as per vSphere 6.0 the setting is called MaxCostPerEsx6xDS. So if you would like to limit the number of SvMotion’s active at the same time, please note the change in names.

For more background on this topic I would like to refer to Frank’s excellent blog on this topic here.

Trigger APD on iSCSI LUN on vSphere

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I was testing various failure scenarios in my lab today for the vSphere Clustering Deepdive session I have scheduled for VMworld. I needed some screenshots and log files of when a datastore hit an APD scenario, for those who don’t know APD stands for all paths down. In other words: the storage is inaccessible and ESXi doesn’t know what has happened and why. vSphere HA has the ability to respond to that kind of failure. I wanted to test this, but my setup was fairly simple and virtual. So I couldn’t unplug any cables. I also couldn’t make configuration changes to the iSCSI array as that would rather trigger a PDL (permanent device loss), so how do you test and APD scenario?

After trying various things like killing the iSCSI daemon (it gets restarted automatically with no impact on the workload) I bumped in to this command which triggered the APD:

  • SSH in to the host you want to trigger the APD on, run the following command 
    esxcli iscsi session remove  -A vmhba65
  • Make sure of course to replace “vmhba65” with the name of your iSCSI adapter

This triggered APD, as witness in the fdm.log and vmkernel.log, and ultimately resulted in vSphere HA killing the impacted VM and restarting it on a healthy host. Anyway, just wanted to share this as I am sure there are others who would like to test APD responses in their labs or before their environment goes in to production.

There may be other easy ways as well, if you know any, please share in the comments section.