stretched

What does Datastore Sharing/HCI Mesh/vSAN Max support when stretched?

Duncan Epping · Oct 31, 2023 ·

This question has come up a few times now, what does Datastore Sharing/HCI Mesh/vSAN Max support when stretched? It is a question which keeps coming up somehow, and I personally had some challenges to find the statements in our documentation as well. I just found the statement and I wanted to first of all point people to it, and then also clarify it so there is no question. If I am using Datastore Sharing / HCI Mesh, or will be using vSAN Max, and my vSAN Datastore is stretched, what does VMware (or does not) support?

We have multiple potential combinations, let me list them and add whether it is supported or not, not that this is at the time of writing with the current available version (vSAN 8.0 U2).

vSAN Stretched Cluster datastore shared with vSAN Stretched Cluster –> Supported
vSAN Stretched Cluster datastore shared with vSAN Cluster (not stretched) –> Supported
vSAN Stretched Cluster datastore shared with Compute Only Cluster (not stretched) –> Supported
vSAN Stretched Cluster datastore shared with Compute Only Cluster (stretched, symmetric) –> Supported
vSAN Stretched Cluster datastore shared with Compute Only Cluster (stretched, asymmetric) –> Not Supported

So what is the difference between symmetric and asymmetric? The below image, which comes from the vSAN Stretched Configuration, explains it best. I think Asymmetric in this case is most likely, so if you are running Stretched vSAN and a Stretched Compute Only, it most likely is not supported.

This also applies to vSAN Max by the way. I hope that helps. Oh and before anyone asks, if the “server side” is not stretched it can be connected to a stretched environment and is supported.

Do I need 2 isolation addresses with a (vSAN) stretched cluster for vSphere HA?

Duncan Epping · Sep 27, 2023 ·

This question has come up multiple times now, so I figured I would write a quick post about it, do you need 2 isolation addresses with a (vSAN) stretched cluster for vSphere HA? This question comes up as the documentation has best practices around the configuration of HA isolation addresses for stretched clusters. The documentation (both for vSAN as well as traditional stretched storage) states that you need to have two reliable addresses, one in each location.

Now I have had the above question multiple times as some folks have mentioned that they can use a Gateway Address with Cisco ACI which would still be accessible in both locations even if there’s a partition due to for instance an ISL failure. If that is the case, and the IP address is indeed available in both locations during those types of failure scenarios then it would suffice to use a single IP address as your isolation address.

You will however need to make sure that the IP address is reachable over the vSAN network when using vSAN as your stretched storage platform. (When vSAN is enabled vSphere HA uses the vSAN network for communications.) If it is reachable you can simply define the isolation address by setting the advanced setting “das.isolationaddress0”. It is also recommended to disable the use of the default gate of the management network by setting “das.usedefaultisolationaddress” to false for vSAN based environments.

I have requested the vSAN stretched clustering documentation to be updated to reflect this.

vSAN Stretched Cluster failure matrix

Duncan Epping · May 30, 2023 ·

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 inter site / inter switch link 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 scenarios which are important for a stretched cluster configuration. Do note, the below-discussed behavior will only be witnessed 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
Site Mirroring	RAID-1/5/6	Site A or B	Host failure Site A or B	Components on failed hosts inaccessible, read IO locally due to RAID, rebuild locally	VM does not need to be restarted, unless VM was running on failed host
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Full failure Site A	Objects are inaccessible in Site A as full site failed	VM restarted in Site B
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Partition Site A	Objects are inaccessible in Site A as full site is partitioned and quorum is lost	VM restarted in Site B
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Witness Host Failure	Witness object inaccessible, VM remains accessible	VM does not need to be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site B	Full failure Site A	Objects are inaccessible in Site A as full site failed	VM does not need to be restarted as it resides in Site B
Site Mirroring	No data redundancy / RAID-1/5/6	Site B	Partition Site A	Objects are inaccessible in Site A as full site is partitioned and quorum is lost	VM does not need to be restarted as it resides in Site B
Site Mirroring	No data redundancy / RAID-1/5/6	Site B	Witness Host Failure	Witness object inaccessible, VM remains accessible	VM does not need to be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Network failure between Site A and B (ISL down)	Site A binds with witness, objects in Site B becomes inaccessible	VM does not need to be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site B	Network failure between Site A and B (ISL down)	Site A binds with witness, objects in Site B becomes inaccessible	VM restarted in Site A
Site Mirroring	No data redundancy / RAID-1/5/6	Site A or Site B	Network failure between Witness and Site A (or B)	Witness object absent, VM remains accessible	VM does not need to be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Full failure Site A, and simultaneous Witness Host Failure	Objects are inaccessible in Site A and Site B due to quorum being lost	VM cannot be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Full failure Site A, followed by Witness Host Failure a few minutes later	Pre vSAN 7.0 U3: Objects are inaccessible in Site A and Site B due to quorum being lost	VM cannot be restarted
Site Mirroring	No data redundancy / RAID-1/5/6	Site A	Full failure Site A, followed by Witness Host Failure a few minutes later	Post vSAN 7.0 U3: Objects are inaccessible in Site A, but accessible in Site B as votes have been recounted	VM restarted in Site B
Site Mirroring	No data redundancy / RAID-1/5/6	Site B	Full failure Site B, followed by Witness Host Failure a few minutes later	Post vSAN 7.0 U3: Objects are inaccessible in Site B, but accessible in Site A as votes have been recounted	VM restarted in Site A
Site Mirroring	No data redundancy	Site A	Full failure Site A, and simultaneous host failure in Site B	Objects are inaccessible in Site A, if components reside on failed host then object is inaccessible in Site B	VM cannot be restarted
Site Mirroring	No data redundancy	Site A	Full failure Site A, and simultaneous host failure in Site B	Objects are inaccessible in Site A, if components do not reside on failed host then object is accessible in Site B	VM restarted in Site B
Site Mirroring	RAID-1/5/6	Site A	Full failure Site A, and simultaneous host failure in Site B	Objects are inaccessible in Site A, accessible in Site B as there's site local resiliency	VM restarted in Site B

How to convert a standard cluster to a stretched cluster while expanding it!

Duncan Epping · Sep 27, 2022 ·

On VMTN a question was asked about how you could convert a 5-node standard cluster to a stretched cluster. It is not documented in our regular documentation, probably as the process is pretty straightforward, so I figured I would write it down. When you create a stretched cluster you will need a Witness Appliance in a third location first. I would recommend deploying that Witness Appliance before doing anything else.

After you deployed the Witness Appliance add the additional hosts to vCenter Server. DO NOT yet add them to the cluster yet though! First, configure each host separately. After you have configured each host, place the host into maintenance mode. After the host is placed into maintenance mode, move it into the cluster and do not take it out of maintenance mode!

Now, when all hosts are part of the cluster you can create the Stretched Cluster. This process is simple, you pick the hosts that belong to each location, and then you select the witness. After the cluster has been created you simply take the hosts out of maintenance mode and you should be good! Note, you take the host out of maintenance after the Stretched Cluster has been created to ensure that you don’t have any rebalancing happening while you are creating the stretched cluster. Simply avoiding unneeded resyncs from occuring.

Do note, all VMs will have the same storage policy assigned still, so you will need to change that policy to ensure that the vSAN objects are placed and replicated according to your requirements! (RAID1 across locations and RAID-1/5/6 within a location for instance.)

Nested Fault Domains on a 2-Node vSAN Stretched Cluster, is it supported?

Duncan Epping · Jun 20, 2022 ·

I spotted a question this week on VMTN, the question was fairly basic, are nested fault domains supported on a 2-node vSAN Stretched Cluster? It sounds basic, but unfortunately, it is not documented anywhere, probably because stretched 2-node configurations are not very common. For those who don’t know, with a nested fault domain on a two-node cluster you basically provide an additional layer of resiliency by replicating an object within a host as well. A VM Storage Policy for a configuration like that will look as follows.

This however does mean that you would need to have a minimum of 3 fault domains within your host as well if you want to, this means that you will need to have a minimum of 3 disk groups in each of the two hosts as well. Or better said, when you configure Host Mirroring and then select the second option failures to tolerate the following list will show you the number of disk groups per host you need at a minimum:

Host Mirroring – 2 Node Cluster
- No Data Redundancy – 1 disk group
- 1 Failure – RAID1 – 3 disk groups
- 1 Failure – RAID5 – 4 disk groups
- 2 Failures – RAID1 – 5 disk groups
- 2 Failures – RAID6 – 6 disk groups
- 3 Failures – RAID1 – 7 disk groups

If you look at the list, you can imagine that if you need additional resiliency it will definitely come at a cost. But anyway, back to the question, is it supported when your 2-node configuration happens to be stretched across locations, and the answer is yes, VMware supports this.