Yellow Bricks

by Duncan Epping

ft

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.

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.

vSphere FT and Dynamically Mirrored Disks?

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I was just browsing through our documentation and stumbled on something which has got some cool potential.

http://pubs.vmware.com/vsphere-50/index.jsp?topic=/com.vmware.vsphere.storage.doc_50/GUID-F78EB579-B11C-4E65-9EE3-145888A005F6.html

This describes how to setup the vSphere side of things for a mirrored disk within a Windows 2008 Guest. Just imagine doing in-guest mirroring of your data while doing FT on the outside. This means you would be able to span multiple sites without the need for a replication mechanism.

I asked around and unfortunately this scenario is not supported today for vSphere FT, but it definitely has potential… Another way to solve this problem would be if we could somehow leverage the Mirror Mode driver that is used by Storage vMotion today. Once again, this is not available today and I don’t even know if people are working on it… just something that popped up and something that has great potential and seems like a small step.

New Academic/Tech Paper on FT

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I received this Paper a while back and think it is an excellent read. I just copied a random part of the paper to give you an idea of what it covers. There’s not much more to say about it then just read it, it is as in-depth as it can get on FT. I read it several times by now and still discover new things every time I read it.

The Design and Evaluation of a Practical System for Fault-Tolerant Virtual Machines

There are many possible ways to attempt to detect failure of the primary and backupVMs. VMware FT uses UDP heartbeating between servers that are running fault-tolerantVMs to detect when a server may have crashed. In addition, VMware FT monitors thelogging traffic that is sent from the primary to the backup VM and the acknowledgmentssent from the backup VM to the primary VM.

VMware FT/HA, we need your feedback!

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Yesterday I received an email from product management asking if I could plug the VMware FT/HA survey like I did with the VMware Snapshot survey. The VMware Snapshot survey literally had hundreds of responses with so much valuable feedback that they decided to re-open this survey and give you guys, my readers, the chance to speak your mind on VMware FT/HA!

Please take the time to fill out the survey and help VMware to improve FT and HA! It will only take a couple of minutes: http://www.surveymethods.com/EndUser.aspx?AD89E5F7ABEEFDF8

This survey will collect information relevant to VMware Fault Tolerance and VMware HA, a new solution that shipped with VMware vSphere 4. VMware Fault Tolerance provides 100% uptime for a VM in the course of an ESX host failure. This feature will ensure your VM continues running without data loss or noticeable downtime should you encounter a single host failure. As we continue to advance this solution, we hope you can provide us information about your current deployment that will help us design Fault Tolerance and HA with your most missions critical VMs in mind.