vSAN Stretched Cluster failure matrix
source link: https://www.yellow-bricks.com/2023/05/30/vsan-stretched-cluster-failure-matrix/
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vSAN Stretched Cluster failure matrix
Duncan Epping · May 30, 2023 · Leave a Comment
The last couple of weeks I was involved internally in a discussion around the different vSAN stretched cluster failure scenarios. I wrote a lengthy email about how vSAN and HA would respond in certain scenarios. I have documented many of these over the years on my blog already, but never really published them as a whole.
In some of the scenarios below, I discuss a “partition”, a partition is a scenario where both the L3 connection to the witness is down and the ISL to the other site for one of the locations. So in the diagram above for instance, if I say that Site B is partitioned then it means that Site A can still communicate with the witness, but Site B cannot communicate with the Witness and cannot communicate with Site A either.
For all of the below scenarios the following applies, Site A is the preferred location and Site B is the secondary location. When it comes to the table, the first two columns refer to the policy setting for the VM as shown in the screenshot below. The third column refers to the location where the VM runs from a compute perspective. The fourth discusses the type of failure, and the fifth and sixth columns discuss the behavior witnessed.
Time to list the various scenarios, and no, it doesn’t include all failures that could occur, but should discuss most which are important for a stretched cluster configuration. Do note, below discussed behavior will only be witness when the best practices, as documented here and here, are followed. Also note that the table has multiple pages, there are close to 30 scenarios described! If there are any questions feel free to leave a comment, if you feel a failure scenario is missing, also please leave a comment.
Site Disaster Tolerance | Failures to Tolerate | VM Location | Failure | vSAN behavior | HA behavior |
---|---|---|---|---|---|
None Preferred | No data redundancy | Site A or B | Host failure Site A | Objects are inaccessible if failed host contained one or more components of objects | VM cannot be restarted as object is inaccessible |
None Preferred | RAID-1/5/6 | Site A or B | Host failure Site A | Objects are accessible as there's site local resiliency | VM does not need to be restarted, unless VM was running on failed host |
None Preferred | No data redundancy / RAID-1/5/6 | Site A | Full failure Site A | Objects are inaccessible as full site failed | VM cannot be restarted in Site B, as all objects reside in Site A |
None Preferred | No data redundancy / RAID-1/5/6 | Site B | Full failure Site B | Objects are accessible, as only Site A contains objects | VM can be restarted in Site A, as that is where all objects reside |
None Preferred | No data redundancy / RAID-1/5/6 | Site A | Partition Site A | Objects are accessible as all objects reside in Site A | VM does not need to be restarted |
None Preferred | No data redundancy / RAID-1/5/6 | Site B | Partition Site B | Objects are accessible in Site A, objects are not accessible in Site B as network is down | VM is restarted in Site A, and killed by vSAN in Site B |
None Secondary | No data redundancy / RAID-1/5/6 | Site B | Partition Site B | Objects are accessible in Site B | VM resides in Site B, does not need to be restarted |
None Preferred | No data redundancy / RAID-1/5/6 | Site A | Witness Host Failure | No impact, witness host is not used as data is not replicated | No impact |
None Secondary | No data redundancy / RAID-1/5/6 | Site B | Witness Host Failure | No impact, witness host is not used as data is not replicated | No impact |
Site Mirroring | No data redundancy | Site A or B | Host failure Site A or B | Components on failed hosts inaccessible, read and write IO across ISL as no redundancy locally, rebuild across ISL | VM does not need to be restarted, unless VM was running on failed host |
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