The disk format upgrade is optional. Your vSAN cluster continues to run smoothly if you use a previous disk format version.

For best results, upgrade the objects to use the latest on-disk format. The latest on-disk format provides the complete feature set of vSAN.

Depending on the size of disk groups, the disk format upgrade can be time-consuming because the disk groups are upgraded one at a time. For each disk group upgrade, all data from each device is evacuated and the disk group is removed from the vSAN cluster. The disk group is then added back to vSAN with the new on-disk format.

Note: Once you upgrade the on-disk format, you cannot roll back software on the hosts or add certain older hosts to the cluster.

When you initiate an upgrade of the on-disk format, vSAN performs several operations that you can monitor from the Resyncing Components page. The table summarizes each process that takes place during the disk format upgrade.

Table 1. Upgrade Progress
Percentage of Completion Description
0%-5% Cluster check. Cluster components are checked and prepared for the upgrade. This process takes a few minutes. vSAN verifies that no outstanding issues exist that can prevent the upgrade from completing.
  • All hosts are connected.
  • All hosts have the correct software version.
  • All disks are healthy.
  • All objects are accessible.
5%-10% Disk group upgrade. vSAN performs the initial disk upgrade with no data migration. This process takes a few minutes.
10%-15% Object realignment. vSAN modifies the layout of all objects to ensure that they are properly aligned. This process can take a few minutes for a small system with few snapshots. It can take many hours or even days for large a system with many snapshots, many fragmented writes, and many unaligned objects.
15%-95% Disk group removal and reformat when you upgrade vSAN versions lower than version 3.0. Each disk group is removed from the cluster, reformatted, and added back to the cluster. The time required for this process varies, depending on the megabytes allocated and the system load. A system at or near its I/O capacity transfers slowly.
95%-100% Final object version upgrade. Object conversion to the new on-disk format and resynchronization is completed. The time required for this process varies, depending on the amount of space used and whether the Allow reduced redundancy option is selected.

During the upgrade, you can monitor the upgrade process from the Resyncing Components page. See "Monitor the Resynchronization Taks in the vSAN Cluster" in vSAN Monitoring and Troubleshooting. You also can use the RVC vsan.upgrade_status <cluster> command to monitor the upgrade. Use the optional -r <seconds> flag to refresh the upgrade status periodically until you press Ctrl+C. The minimum number of seconds allowed between each refresh is 60.

You can monitor other upgrade tasks, such as device removal and upgrade in the Recent Tasks pane of the status bar.

The following considerations apply when upgrading the disk format:
  • If you upgrade a cluster with three hosts and you choose Evacuate all data, the evacuation can fail for objects with a Primary level of failures to tolerate greater than 0 (zero). A three-host cluster cannot reprotect a disk group that is being fully evacuated using the resources of only two hosts. You might be prompted to add another disk group to an existing host.

    For a three-host cluster, you can choose the Ensure accessibility data migration option. When in this mode, any hardware failure might result in data loss.

    You also must ensure that enough free space is available. The space must be equal to the logical consumed capacity of the largest disk group. This capacity must be available on a disk group separate from the one that is being migrated.

  • When upgrading a three-host cluster or when upgrading a cluster with limited resources, allow the virtual machines to operate in a reduced redundancy mode. Run the RVC command with the option, vsan.ondisk_upgrade --allow-reduced-redundancy.
  • Using the --allow-reduced-redundancy command option means that certain virtual machines might be unable to tolerate failures during the migration. This lowered tolerance for failure also can cause data loss. vSAN restores full compliance and redundancy after the upgrade is completed. During the upgrade, the compliance status of virtual machines and their redundancies is temporarily noncompliant. After you complete the upgrade and finish all rebuild tasks, the virtual machines will become compliant.
  • While the upgrade is in progress, do not remove or disconnect any host, and do not place a host in maintenance mode. These actions might cause the upgrade to fail.

For information about the RVC commands and command options, see the RVC Command Reference Guide.