The Weakest Link

Although the SAN solution above sounds plausible in theory, it can quickly become an annoyance in the sys admin's daily grind. The weakest link in the Libvirt, SAN, Pacemaker, and GFS2/OCFS2/CLVM combination is undoubtedly CLVM, partly because the distributed locking manager (DLM) stack on Linux has been repeatedly modified in recent years. Thus, SLES, RHEL, Debian, and Ubuntu each have different tasks to handle, if a setup of this kind is to get off the ground.

Moreover, DLM-based components are highly complex and affect both the services themselves as well as management by a cluster manager, which is a must-have in this kind of setup.

Bastian Blank, who is heavily involved in maintaining the Linux kernel for Debian, recently even disabled the CLVM service in the Debian kernel, commenting that it had never worked reliably anyway. If you have a SAN that supports NFS, you would do well to use it instead of resorting to a cluster filesystem or distributed LVM.

DIY or Buy?

Admins who rely on one of the prebuilt virtualization solutions, such as VMware or Red Hat oVirt, no longer need to worry about the issue of migration and live migration as a general rule.

All of the ready-made solutions here offer live migration. If a small virtualization setup is desired on the basis of DRBD, a combination of DRBD, Pacemaker, and Libvirt is all you need for live migration.

However, if you have SAN storage, you can only hope that the device offers NFS capabilities; otherwise, you will need to resort to tools that convert a SAN into distributed storage.

Ceph is definitely an alternative; it has relied on Libvirt live migration from the outset (Figure 5).

Figure 5: The current version of Libvirt offers native Ceph connectivity. Live migration is no longer a problem in this kind of setup.

Now that its manufacturer, Inktank, has also officially declared the required components – Ceph, RADOS, Librados, and DRBD – to be stable, Ceph certainly should be considered within the scope of a setup like that discussed here.

The Author

Martin Gerhard Loschwitz is Principal Consultant at hastexo, where he is intensively involved with high-availability solutions. In his spare time, he maintains the Linux cluster stack for Debian GNU/Linux.