XenServer and Citrix Hypervisor use GFS2 to provide the ability to set up thin provisioning with a shared block storage device. Pools that use GFS2 to thin provision their shared block storage are clustered and behave differently to pools that use shared file-based storage or LVM with shared block storage.  For more information about GFS2 and clustered pools, see the product documentation .

Use the following information to troubleshoot minor issues that might occur when using this feature.

Q: All my hosts can see each other, but I can't create a cluster. Why?

The clustering mechanism uses specific ports. If your hosts cannot communicate on these ports (even if it is able to communicate on other ports), you cannot enable clustering for the pool.
Ensure that the hosts in the pool can communicate on the following ports:

• TCP: 8892, 21064
• UDP: 5404, 5405

If there are any firewalls or similar between the hosts in the pool, ensure that these ports are open.

Q: What if I already have an LVM partition set up on the block-based storage device?

If you have previously used your block-based storage device for thick provisioning with LVM, this is detected by XenServer. XenCenter gives you the opportunity to use the existing LVM partition or to format the disk and set up a GFS2 partition.

Q: Why am I getting an error when I try to join a new host to an existing clustered pool?

A: When clustering is enabled on a XenServer pool, every pool membership change must be agreed by every member of the cluster before it can be successful. If a cluster member is not contactable, operations that change cluster membership (such as host add or host remove) fail.
To add your new host to the clustered pool:

• Ensure that all of your hosts are online and contactable.
• If an offline host cannot be recovered, mark it as dead to remove it from the cluster. 

Q: How do I know if my host has self-fenced?

If your host self-fenced, it might have rejoined the cluster when it restarted. To see if a host has self-fenced and recovered, you can check the /var/opt/xapi-clusterd/boot-times file to see the times the host started. If there are start times in the file that you did not expect to see, the host has self-fenced.

Q: Why is my host offline? How can I recover it?

There aremany possible reasons for a host to go offline. Depending on the reason the host either can be recovered or not.
The following reasons for a host to be offline are more common and can be addressed by using information in the product documentation.

• Clean shutdown
• Forced shutdown
• Temporary power failure
• Reboot

The following reasons for a host to be offline are less common:

• Permanent host hardware failure
• Permanent host power supply failure
• Network partition
• Network switch failure

These issues can be addressed by replacing hardware or by marking failed hosts as dead. 

Q: A host in my clustered pool is offline and I can't recover it. How do I remove the host from my cluster?

A: You can mark a host as dead. This removes the host from the cluster permanently and decreases the number of live hosts required for quorum.

Q: I have repaired a host that was marked as dead. How do I add it back into my cluster?

A: A XenServer host that has been marked as dead cannot be added back into the cluster. To add this system back into the cluster, you must do a fresh installation of XenServer. This fresh installation appears to the cluster as a new host.

Q: What do I do if my cluster keeps losing quorum and hosts keep fencing?

If one or more of the XenServer hosts in the cluster gets into a fence loop as a result of continuously losing and gaining quorum, you can boot the host with the nocluster kernel command-line argument. To do this, connect to the hosts physical or serial console and edit the boot arguments in grub.

Use the information in this article to troubleshoot minor issues that might occur when using thin provisioning for shared block storage.