Yellow Bricks

by Duncan Epping

BC-DR

VMware Availability Survey

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I just received the following… If you have some spare time on your hands please fill out this survey, it would be much appreciated.

We are hard at work building our future products to better meet your needs. As part of this process we are developing a 3-year strategy for the VMware Business Continuity offerings, and are seeking your input to best align our strategy with your business objectives.

Please bring your voice to the table – if you have a few minutes today, would you please click on the link below and share your insights on the VMware Business Continuity road map. Answer as few or many of the questions as you’d like.

https://vmware.allegiancetech.com/cgi-bin/qwebcorporate.dll?idx=6KGA9Q

Site Recovery Manager survey… please help us out!

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I just received an email from the the Site Recovery Manager Product Management team. They created a new survey, and I was hoping each of you who is using, or will be purchasing SRM soon, could take the time to complete it. These types of surveys are very useful for Product Management when it comes to setting priorities for new features and identify gaps etc. Thanks!

We are conducting a survey about VMware vCenter Site Recovery Manager (SRM) to learn more about how people use our products. The survey will help us identify where we can improve the product to meet your needs and we would really appreciate getting your feedback.

The link to the survey is below, it typically takes less than 10 minutes to complete. http://www.surveymethods.com/EndUser.aspx?ECC8A4BDEDA6B9BAE7

Thanks!

Using FT in a stretched cluster environment

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I had a discussion around using FT in a stretched cluster (vSphere Metro Storage Cluster) environment. The main discussion point was around the use of “Host-VM” affinity rules. Some people appear to be under the impression that a Host-VM affinity rule can be created to ensure the primary and the secondary FT are divided between sites.

As I heard multiple people mentioning that this was possible I decided to test it. Unfortunately it is not possible. As soon as you enable FT on a VM and that secondary is started you will not see that secondary in the DRS Rules UI? Yes you can see the secondary if you look on a host level, but not in the DRS Rules workflow, this means it is not possible to ensure the secondary VM is bound to the second site.

Answering some admission control questions

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I received a bunch of questions on HA admission control in this blog post and I figured I would answer them in a blog post so that everyone would be able to find / read it. This was the original set of questions:

There are 4 ESXi Hosts in the network and 4 VMs (Same CPU, RAM Reservation for all VMs) on each Host. Admission Control is policy is set to ‘Host failure cluster tolerates’ to 1. All the available 12 slots have been used by the powered ON VMs, except the 4 reserved slots for failover.
1) What happens if 2 ESXi Hosts fails now? ( 2 * 4 VMs needs to fail over). Will HA restart only 4 VMs as it has only 4 slots available? And Restart of the remaining 4 VM fails?
Same Scenario, but Policy is set to ‘% of cluster resources reserved’ = 25%. All the available 75 % resources have been utilized by all the 16 VMs, except 25 % reserved for failover
2) What happens if 2 ESXi Hosts fails now? ( 2 * 4 VMs needs to fail over). Will HA restart only 4 VMs as it consumes 25 % of resources? And Restart of the other 4 VM fails?
3) Does HA check the VM reservation (or any other factor) at the time of restart ?
4) HA only restart a VM if the Host could guarantee the reserved resources or restart Fails?
5) What if no VM reservations are set VM level ?
6)What does HA takes into consideration when it has to restart VMs which has no reservation ?
7)Will it guarantee the configured Resources for each VMs ?
8)If not, How HA can restart 8 VMs (as per our eg) when it only has configured reserved resources for just 4 VM
9)Will it share the reserved resources across 8 VMs and will not care about the resource crunch or is it about first come first serve
10)Admission control doesn’t have any role at all in the event of HA failover ?

Let me tackle these questions one by one:

  1. In this scenario 4 VMs will be restarted and 4 VMs might be restarted! Note that the “slot size” policy is used and that this is based on the worst case scenario. So if your slot is 1GB and 2GHz but your VMs require way less than that to power-on it could be all VMs are restarted. However, HA guarantees the restart of 4 VMs. Keep in mind that this scenario doesn’t happen too often, as you would be overcommitting to the extreme here. As said HA will restart all VMs it can. It just needs to be able to satisfy the resource reservations on memory and CPU!
  2. Again, also in this HA will do its best to restart. It can restart new VMs until all “unreserved capacity” is used. As HA only needs to guarantee reserved resources chances of hitting this is very slim, as most people don’t use reservations at a VM level it would mean you are overcommiting extremely
  3. Yes it will validate if there is a host which can back the the resource reservations before it tries the restart
  4. Yes it will only restart the VM when this can be guaranteed. If it cannot be then HA can call,”DRS” to defragment resources for this VM
  5. If there are no reservations then HA will only look at the “memory overhead” in order to place this VM
  6. HA ensures the portgroup and datastore are available on the host.
  7. It will not guarantee configured resources, HA is about restarting virtual machines not about resource management. DRS is about resource management and guaranteeing access to resources.
  8. HA will only be able to restart the VM if there are unreserved resources available to satisfy the VMs request
  9. All resources required for a virtual machine need to be available on a single host! Yes resources will be shared on a single host, just as long as no reservations are defined.
  10. No Admission Control doesn’t have any role in an HA failover. Admission Control happens on a vCenter level, HA failovers happen on an ESX(i) level.

Maximum amount of FT virtual machines per host?

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There was a discussion yesterday on our Socialcast system. The question was what the max amount of FT virtual machines was and what dictated this. Of course there are many things that will be a constraint when it comes to FT (memory reservations, bandwidth etc) but the one thing that stands out and not many realize is that the amount of FT virtual machines per host is limited to 4 by default.

This is currently controlled by a vSphere HA advanced setting called “das.maxftvmsperhost”. By default this setting is configured to 4. This advanced setting is an HA advanced setting (in combination with vSphere DRS) and defines the max amount of FT virtual machines, either primary or secondary or a combination of both, that can run  on a single host. So if for whatever reason you want a max of 6 you will need to add this advanced setting with a value of 6.

I do not recommend changing this however, FT is a fairly heavy process and in most environments 4 is the recommended value.