Double-Take has enhanced its replication products so that an entire set of VMware virtual machines in a physical server can be replicated. Previously each virtual machine had to have its own Double-Take replication product (and license) for the guest Windows operating system inside it.

Double-Take for VMware Infrastructure (DTVI) runs on the host Windows operating system in the physical server in a virtualised environment. From a Double-Take perspective, VMware Server and virtual machines running on it are like any other application that resides on top of the operating system, file system and hardware components of the server.

Bob Roudebush, Double-Take's director of solutions engineering, said: "By offering protection of entire virtual machines... Double-Take for VMware Infrastructure serves as a viable solution for any organisation working with business-critical systems running in VMware environments."

VMware server virtualisation technology enables a single physical server to run multiple virtual machines simultaneously. Each virtual machine has its own processor(s), memory, disks and network interfaces. Each virtual machine can have its disks protected by Double-Take replication software with the target server for the replicated disk writes being an actual physical server or another virtual machine.

Double-Take captures disk-writes at the host’s file system layer while all applications (including virtual machines writing to virtual disks) operate above the actual file system layer. This allows Double-Take to protect that data by replicating it via its normal mechanisms to a target server running another copy of Double-Take.

In addition, replicating from the host OS (running VMware Server for Windows) offers the benefit of replicating non-Windows guest operating systems (eg Linux, Solaris x86, or NetWare). This allows customers to extend the protection of Double-Take to additional operating systems.

However the replication does not provide continuous or byte-level data protection. The transmission of replication data is scheduled fifteen minute intervals or longer. It could be scheduled whenever there is 32MB or more of data to transmit. To provide byte-level replication in the new product the VMware ESX code needs enhancing. Work is ongoing to provide that.

Now it is simpler - and cheaper - to protect an entire set of virtual machines and recover from a physical server crash. During an outage, the replicated virtual machines can be started on the target server with the latest replicated data available.

Double-Take for VMware Infrastructure is compatible with VMware ESX Server versions 3 and later, and VirtualCenter version 2 or later. It costs $2,495 for 10 virtual machines.