Before you shut down, reboot, or disconnect a host that is a member of a cluster, you must put the host in maintenance mode to avoid downtime on the VM's which are running on it. When we put the host in maintenance mode, it migrates all the powered on VM's to the other hosts in the cluster and then put the host into maintenance mode.
When working with maintenance mode, consider the following guidelines:
- You must choose a data evacuation method, such as Ensure accessibility or Full data migration, when you put an ESXi host into maintenance mode.
- A vSAN cluster's capacity automatically decreases when any member host enters maintenance mode because the host is no longer contributes storage to the cluster.
- A virtual machine's compute resources might not reside on the host that is being placed in maintenance mode, and the storage resources for virtual machines might be located anywhere in the cluster.
- When you select the Full data migration mode, your data is automatically re-protected against a failure, if the resources are available and the Failures to tolerate set to 1 or more. When in this mode, all components from the host are migrated and, depending on the amount of data you have on the host, the migration might take longer. With Full data migration mode, your virtual machines can tolerate failures, even during planned maintenance.
- The Ensure accessibility mode is faster than the Full data migration mode because the Ensure accessibility migrates only the components from the hosts that are essential for running the virtual machines. When in this mode, if you encounter a failure, the availability of your virtual machine is affected. Selecting the Ensure accessibility mode does not re-protect your data during failure and you might experience unexpected data loss.
- When working with a three-host cluster, you cannot place a server in maintenance mode with Full data migration. Consider designing a cluster with four or more hosts for maximum availability.
The following must be verified before you put a host in maintenance mode:
- Check that the cluster has enough hosts and capacity available to meet the failures to tolerate policy criteria if you are using the Full data migration mode.
- Ensure that the remaining hosts have enough capacity devices to handle the stripe width policy requirements, if selected.
- Make sure that the remaining hosts have adequate free space to accommodate the volume of data that has to be moved from the host going into maintenance mode.
- Check that the remaining hosts have enough flash space to accommodate any flash read cache reservations. To analyze the current capacity use per host, and whether a single host failure might cause the cluster to run out of space and impact the cluster capacity, cache reservation, and cluster components, run the following RVC command: vsan.whatif_host_failures.
The Confirm Maintenance Mode dialog box provides information to guide your maintenance activities. You can view the impact of each data evacuation option.
- Whether or not sufficient capacity is available to perform the operation.
- How much data will be moved.
- How many objects will become non-compliant.
- How many objects will become inaccessible.
Check a Host's Data Migration Capabilities
To evaluate the effects of data migration options before putting a host into maintenance mode or removing it from the cluster, use the data migration pre-check. You can use the test findings to analyze the effects on cluster capacity, anticipated health checks, and any objects that will violate the rules. Pre-check offers information on the resources required in the event that the operation would fail.
Procedure
1. Navigate to the vSAN cluster.
2. Click the Monitor tab.
3. Under vSAN, click Data Migration Pre-check.
4. Select a host, a data migration option, and click Pre-check.
vSAN runs the data migration precheck tests.
5. View the test results.
The pre-check results show whether the host can safely enter maintenance mode.
The Object Compliance and Accessibility tab displays objects that might have issues after the data migration.
The Cluster Capacity tab displays the impact of data migration on the vSAN cluster before and after you perform the operation.
The Predicted Health tab displays the health checks that might be affected by the data migration.
By clicking Enter Maintenance Mode, you can migrate the data and put the host into maintenance mode if the pre-check shows that you can do so.
In the upcoming blog, we'll go into more depth about the capabilities of the maintenance mode in vSAN.
Thank you for reading!
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