2.1.3 Disks and CVM

The nodes in a cluster must always agree on the status of a disk. In particular, if one node cannot write to a given disk, all nodes must stop accessing that disk before the results of the write operation are returned to the caller. Therefore, if a node cannot contact a disk, it should contact another node to check on the disk's status. If the disk has failed, no node will be able to access it and the nodes can agree to detach the disk. If the disk has not failed, but rather the access paths from some of the nodes have failed, the nodes cannot agree on the status of the disk. Some policy must exist to resolve this type of discrepancy. The current policy is to detach the disk, even though it has not failed. While this resolves the status issue, it leaves mirrored disks with fewer mirrors and non-mirrored volumes with no access to data.

This policy is the current implementation option. In a future release of CVM, the system administrator will be able to select another policy. For example, an alternative policy would be for any node that cannot access the disk to leave the cluster; this maintains integrity of data access at the cost of keeping some nodes outside the cluster.