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This chapter outlines the use of DRBD as a Virtual Block Device (VBD) for virtualization envirnoments using the Xen hypervisor.
Xen is a virtualization framework originally developed at the University of Cambridge (UK), and later being maintained by XenSource, Inc. (now a part of Citrix). It is included in reasonably recent releases of most Linux distributions, such as Debian GNU/Linux (since version 4.0), SUSE Linux Enterprise Server (since release 10), Red Hat Enterprise Linux (since release 5), and many others.
Xen uses paravirtualization — a virtualization method involving a high degree of cooperation between the virtualization host and guest virtual machines — with selected guest operating systems for improved performance in comparison to conventional virtualization solutions (which are typically based on hardware emulation). Xen also supports full hardware emulation on CPUs that support the appropriate virtualization extensions, in Xen parlance, this is known as HVM (“hardware-assisted virtual machine”).
At the time of writing, CPU extensions supported by Xen for HVM are Intel's Virtualization Technology (VT, formerly codenamed “Vanderpool”), and AMD's Secure Virtual Machine (SVM, formerly known as “Pacifica”).
When a DRBD resource is used as a replicated Virtual Block Device (VBD) for Xen, it serves to make the entire contents of a domU's virtual disk available on two servers, which can then be configured for automatic fail-over. That way, DRBD does not only provide redundancy for Linux servers (as in non-virtualized DRBD deployment scenarios), but also for any other operating system that can be virtualized under Xen — which, in essence, includes any operating system available on 32- or 64-bit Intel compatible architectures.