Yellow Bricks

by Duncan Epping

durability components

Does vSAN Enhanced Durability work when you have a limited number of hosts?

· ·

Last week I had a question about how vSAN Enhanced Durability works when you have a limited number of hosts. In this case, the customer had a 3+3+1 stretched cluster configuration, and they wondered what would happen when they would place a host into maintenance mode. Although I was pretty sure I knew what would happen, I figured I would test it in the lab anyway. Let’s start with a high-level diagram of what the environment looks like. Note I use a single VM as an example, just to keep the scenario easy to follow.

In the diagram, we see a virtual disk that is configured to be stretched across locations, and protected by RAID-1 within each location. As a result, you will have two RAID-1 trees each with two components and a witness, and of course, you would have a witness component in the witness location. Now the question is, what happens when you place esxi-host-1 into maintenance mode? In this scenario, vSAN Enhanced Durability will want to create a “durability component”. This durability component is used to commit all new write IO to. This will allow vSAN to resync fast after maintenance mode, and enhances durability as we would still have 2 copies of the (new) data.

However, in the scenario above we only have 3 hosts per location. The question then is, where is this delta component created then? As normally with maintenance mode you would need a 4th host to move data to. Well, it is simple, in this case, what vSAN does is it creates a “durability component” on the host where the witness resides, within the location of course. Let me show you in a diagram, as that makes it clear instantly.

By adding the Durability component next to the witness on esxi-host-3, vSAN enhances durability even in this stretched cluster situation, as it provides a local additional copy of new writes. Now, of course I tested this in my lab. So for those who prefer to see a demo, check out the youtube video below.

vSAN 7.0 U2 Durability Components?

· ·

Last week I published a new demo on my youtube channel (at the bottom of this post) and it discussed an enhanced feature called Durability Components. Some may know these as “delta components” as well. These durability components were introduced in vSAN 7.0 Update 1 and provided a mechanism to maintain the required availability for VMs while doing maintenance. That meaning that when you would place a host into maintenance mode new “durability components” would be created for the components which were stored on that host. This would then allow all the new VM I/O to be committed to the existing component, as well as the durability component.

Now, starting with vSAN 7.0 Update 2, vSAN also uses these durability components in situations where a host failure has occurred. So if a host has failed, durability components will be created to ensure we still maintain the specified availability level specified within the policy as shown in the diagram above. The great thing is that if a second host fails in an FTT=1 scenario and you are able to recover the first failed host, we can still merge the data with the first failed host with the durability component! So not only are these durability components great for improving the resync times, but they also provide a higher level of availability to vSAN! To summarize:

  1. Host fails
  2. Durability components are created for all impacted objects
  3. New writes are committed to existing components and the new durability components
  4. Host recovers
  5. Durability components are merged with the previously failed components
  6. Durability components are deleted when resync has completed

I hope that help providing a better understanding of how these durability components help improving availability/resiliency in your environment with vSAN 7.0 Update 2.

I can understand that some of you may not want to test durability components in their own environment, this is why I recorded a quick demo and published it on my youtube channel. Check out the video below, as it also shows you how durability components are represented in the UI.