Following up on my last column on what you could do with GeoCluster (now part of Double-Take Availability); I’d like to take some time to explain an interesting phenomenon that end-users see in GC all the time. While inside the cluster manger GUI, GC disks behave exactly the same as shared-cluster disks, when you create a GC between two or more servers, you can see the disk volumes “attached” to the passive nodes in Windows. This is much different behavior than a traditional shared-disk cluster, where the volumes only appear on the active node for that disk resource.
The reason this happens is due to the fact that GC needs to replicate data from the owning node for that disk resource to all passive nodes that could potentially become owners if resources are moved or fail over. In a shared-disk cluster, no replication is performed, so the disks only need to be attached to one node at a time; but GC needs the volumes mounted for that replication to occur. The end result is that if you open up My Computer or Windows Explorer on a passive node, you will see the volumes mounted at all times. However, you will not be able to do anything with those volumes except view the contents on the drive. This is less than even read-only privileges, as you won’t be able to do things necessary to open and read files, like change the Last Access or Last Modified dates.
The technology that allows a volume to be mounted, but not accidentally altered, is part of the GC solution set itself. During configuration, GC inserts a blocking file system filter driver. This allows GC to control the flow of read and write I/O to disk. The system can be manually invoked in regular Double-Take Availability or Backup connections as well, but it is turned on by default for GC connections. Only the node that currently owns the disk resource can bypass the block to read and write to the disk, replicating changes a the byte-level to all other potential owning nodes.
When an arbitration occurs, and the resources move to another node, the block is removed on that node only so that the cluster can resume normal operations. Once the failed node is brought back into the cluster, GC will block that node’s volumes, allowing it to be re-synchronized and brought back into quiescence with the cluster as a whole.
The practical upshot of all this is the answer to two of our most common GeoCluster questions. 1) No, you cannot use the passive-node data for any purpose, even though you are able to view it in Windows Explorer. 2) Even though you can see the volumes and directories in Windows Explorer, replication will safely occur and the cluster will not be able to accidentally attach to the wrong node’s copy of the data.
Filed under: Double-Take Availability, GeoCluster
