It's a good idea to take a snapshot of your virtual appliances before applying any updates, just in case. When you have multiple vCenter appliances operating in Enhanced Link Mode, though, it's important to make sure that the snapshots are in a consistent state. The vCenter vmdird service is responsible for continuously syncing data between the vCenters within a vSphere Single Sign-On (SSO) domain. Reverting to a snapshot where vmdird's knowledge of the environment dramatically differed from that of the other vCenters could cause significant problems down the road or even result in having to rebuild a vCenter from scratch.

(Yes, that's a lesson I learned the hard way - and warnings about that are tragically hard to come by from what I've seen. So I'm sharing my notes so that you can avoid making the same mistake.)

Take these steps when you need to snapshot linked vCenters to avoid breaking replication:

1. Open an SSH session to all the vCenters within the SSO domain.

2. Log in and enter shell to access the shell on each vCenter.

3. Verify that replication is healthy by running /usr/lib/vmware-vmdir/bin/vdcrepadmin -f showpartnerstatus -h localhost -u administrator -w [SSO_ADMIN_PASSWORD] on each vCenter. You want to ensure that each host shows as available to all other hosts, and the message that Partner is 0 changes behind.:

    1root@vcsa [ ~ ]# /usr/lib/vmware-vmdir/bin/vdcrepadmin -f showpartnerstatus -h localhost -u administrator -w $ssoPass 
    2Partner: vcsa2.lab.bowdre.net
    3Host available:   Yes
    4Status available: Yes
    5My last change number:             9346
    6Partner has seen my change number: 9346
    7Partner is 0 changes behind.
    8
    9root@vcsa2 [ ~ ]# /usr/lib/vmware-vmdir/bin/vdcrepadmin -f showpartnerstatus -h localhost -u administrator -w $ssoPass 
   10Partner: vcsa.lab.bowdre.net
   11Host available:   Yes
   12Status available: Yes
   13My last change number:             9518
   14Partner has seen my change number: 9518
   15Partner is 0 changes behind.
   

4. Stop vmdird on each vCenter by running /bin/service-control --stop vmdird:

   1root@vcsa [ ~ ]# /bin/service-control --stop vmdird
   2Operation not cancellable. Please wait for it to finish...
   3Performing stop operation on service vmdird...
   4Successfully stopped service vmdird
   5
   6root@vcsa2 [ ~ ]# /bin/service-control --stop vmdird
   7Operation not cancellable. Please wait for it to finish...
   8Performing stop operation on service vmdird...
   9Successfully stopped service vmdird
   

5. Snapshot the vCenter appliance VMs.

6. Start replication on each server again with /bin/service-control --start vmdird:

   1root@vcsa [ ~ ]# /bin/service-control --start vmdird
   2Operation not cancellable. Please wait for it to finish...
   3Performing start operation on service vmdird...
   4Successfully started service vmdird
   5
   6root@vcsa2 [ ~ ]# /bin/service-control --start vmdird
   7Operation not cancellable. Please wait for it to finish...
   8Performing start operation on service vmdird...
   9Successfully started service vmdird
   

7. Check the replication status with /usr/lib/vmware-vmdir/bin/vdcrepadmin -f showpartnerstatus -h localhost -u administrator -w [SSO_ADMIN_PASSWORD] again just to be sure. Don't proceed with whatever else you were planning to do until you've confirmed that the vCenters are in sync.

You can learn more about the vdcrepadmin utility here: https://kb.vmware.com/s/article/2127057