[RHEV 3.5] Upgrading RHEL 6/RHEV-H 6 Hosts to RHEL 7/RHEV-H 7 Host without VM downtime

Overview

RHEV 3.5 supports both RHEL6 and RHEL7 hosts (RHEL or RHEV-H). It is recommended to upgrade your hosts to RHEL7 once on RHEV 3.5. However it is not possible to mix RHEL6 and RHEL7 hosts in a single cluster. How to keep Virtual Machines up and have the hosts upgraded without having VMs downtime then?

This article documents the procedure of creating a new cluster for RHEL7 based hosts and then live migrating virtual machines to the new cluster.

Note 1: it is recommended to schedule a cold restart of your VMs at some point after the upgrade, in order to enjoy the full functionality of the new release. If the VM is not shut down after upgrade, it will continue running in the same process created on the older version and would not be 100% compatible with the new features. (It will be still 100% operational and there is no problems to continue running the VM the way it was initially created.)

Note 2: In RHEV 3.6 this procedure can be done differently, via inClusterUpgrade feature, that allows to mix RHEL 6 and RHEL 7 hosts in the same 3.5 Compatibility Level cluster. Refer to the official documentation for details: This content is not included.Upgrade Guide or This content is not included.Self-Hosted Engine Guide.

Once you have created a version 7 cluster with 3.5 compatibility version and made sure the live migration prerequisites are met, you can migrate virtual machines from the version 6 cluster to the version 7 cluster without downtime.

IMPORTANT: This is the only supported use case of migrating virtual machines between different clusters. Migrating virtual machines back from the version 7 cluster to the version 6 cluster is not supported.

Live Migration Prerequisites

• The source and destination host must have a status of Up.
• The source and destination host must have access to the same virtual networks and VLANs.
• The source and destination host must have access to the data storage domain on which the virtual machine resides.
• There must be enough CPU capacity on the destination host to support the virtual machine's requirements.
• There must be enough RAM on the destination host that is not in use to support the virtual machine's requirements.
• The migrating virtual machine must not have the cache!=none custom property set.
• The migrating virtual machines must not only be currently running in a 3.5 Level Cluster in the RHEL6/RHEV-H 6 Host, but must also have been started in a 3.5 Level Cluster. If the Cluster was not upgraded to 3.5 Level, some machines might still be running with 3.4 Level definitions. In these cases, the Virtual Machines must be shutdown and power on again, so that the upgrade process to 3.5 is complete. A 3.4 Level VM running in a 3.5 Level RHEL6 cluster is not supported to migrate to a RHEL7 3.5 Level cluster.

Live Migrating Virtual Machines From a version 6 Cluster to a version 7 Cluster

1. In the Administration Portal, click the Virtual Machines tab and select a running virtual machine.
2. Click Migrate to open the Migrate Virtual Machine(s) window.
3. Click the Advanced Parameters button to expand the advanced settings and select a destination cluster.
4. Use the radio buttons to select whether to Select Host Automatically or to Select Destination Host, specifying the host using the drop-down list.
5. Click OK to commence migration and close the window.

You can choose to cancel the migration at any time by clicking the Cancel Migration tab.