Judging by some of the virtual machine (VM) management tools I have encountered lately, server backup software looks certain to be shortly replaced by a new breed of server replication tools.

The trouble with backups is they are rarely at hand when you actually need them. In contrast, replicated servers can be retrieved and deployed on virtual server farms in an instant.

Server replication tools work by copying a running VM onto a secondary server, monitoring the original for changes and copying the changes across at pre-defined intervals. The result is an up-to-date copy, or replica, of a VM that’s ready to run within a few moments’ notice.

The idea is attractive enough to storage managers as it stands given the budgets associated with traditional backup systems. But replication gains even more appeal when the secondary server is located in a distant city or on a different continent, at which point it also forms the basis of a disaster recovery strategy.

The downside is that this type of replication can only be done in virtual server environments because it relies on the presence of a hypervisor that monitors the VM’s operating system and can snapshot its disk at an appropriate moment. It also depends to a certain extent on the presence of specific SCSI and NICs in the secondary server ­ easy to arrange in virtual environments where the VMs use virtual rather than physical hardware resources.

VM replication is still very much in the early stages of its development, and the few products currently available tend to go about things in different ways. Of the two I have looked at so far, Vizioncore’s esxReplicator 2.0 uses data compression to minimise the amount of data at the network layer, for example, while Double-Take Software’s Double-Take for VMware Infrastructure leaves network data uncompressed in order to minimise the load placed on the host server’s hardware.

Both require the administrator to drive the failover process manually. But while esxReplicator does some of the work for you, Double-Take forces you to start the failover process from scratch.

The two tools differ in the way they monitor VMs for changes. DoubleTake
uses the snapshot capability in the VM’s hypervisor to track changes,
while EsxReplicator takes a different approach by comparing the virtual
disk files from the source and replica.

VM replication tools might sound like a whacky niche, but they address a growing need. No doubt in a year or two there will be far more to choose from.

‹ www.doubletake.com ‹ www.vizioncore.com
‹ roger@rogerh.com