This week I experimented with DRBD (Distributed Replicated Block Device) which is “a block device which is designed to build high availability clusters. This is done by mirroring a whole block device via (a dedicated) network. You could see it as a network raid-1.”
DRBD can be used for build a “poor man’s SAN”, a RAID running over Ethernet. In our case, we want a warm spare of an important database system, i.e. a second set of hardware which can take over for the primary within a few minutes of failure, and DRBD might be a great way accomplish that. DRBD runs on commodity hardware (ordinary servers, network adapters, etc.) so it is inexpensive. It is also, if benchmarks and comments are to be believed, surprisingly good. I suspect this is because CPUs and networks (Gigabit Ethernet, ideally) are so much faster than disks.
The systems at hand for testing here run the Ubuntu distribution, itself based on Debian. It took a lot of reading and learning, but very little actual typing, to get DRBD going on Ubuntu. My writeup here should help the next person along. As I write this, the released version of DRBD is 0.7, and the DRBD site encourages ignoring the newer unofficial release, advice that I followed.
First, read the HOWTO and the FAQ, but don’t follow the instruction in them yet; then you are ready to begin. The guessable part is to install the module with apt-get (you’ll need the “Community” APT sources in Ubuntu; I don’t know offhand what is equivalent in Debian):
apt-get install drbd0.7-module-source
This installs the module’s source code, not the module itself. After doing this, I spent a couple of hours trying to figure out how to compile it – untarring it and running “make” does not work. There is an abundance of confusing and misleading advice on the web about this, including advice to compile your own kernel. If you don’t need a custom-compiled kernel for other reasons, ignore all that and discover the joy of Debian’s “Module Assistant”, invoked by the command “m-a”.
“prepare” will get your machine ready to compile modules that can link with the running kernel; it will prompt to download and install packages needed for this, including the correct kernel header packages. Now you are ready to build and install the module:
m-a a-i drbd0.7-module-source
Follow the prompts; if it asks to install more packages, agree to do so. On Ubuntu 6.10, it may fail with an error about “mv”. This is a known bug; the workaround is to expand the module source file (it lives in /usr/src/), add the line “SHELL=/bin/bash” at the top of each Makefile, then retar the module source.
By the way, this points out a stark and troubling aspect of Ubuntu “community” packages: not only are they not assured to work, they are not even casually tested to verify they compile on the Ubuntu version they are purportedly intended for use with… which I find very disappointing. I suppose the lesson is that Ubuntu is very slick for the set of officially supported packages, but if you’ll be using a lot of packages beyond that set, its appeal is much diminished.
Now you are ready to enable DRBD, following the HOWTO instructions, starting with setting up /etc/drbd.conf.
In my case I used LVM to make a 10 GB partition on each machine, put DRBD on top of that, put a filesystem (Reiserfs, for variety) on top of them, then moved/symlinked my PostgreSQL data there. It worked fine, including these tests: I started a long intensive DB operation (restoring a several-GB backup), then with that running, rebooted the secondary machine. This did not interrupt the DB operation, and when the secondary finished its reboot, the DRBD partition re-synced automatically. I then shut down the primary machine, and verified I could mount and read that same data on the secondary machine… all exactly as described.
One caveat though – in a few cases, machines with DRBD stopped during the boot sequence, waiting for user intervention (in this case, closing a shell) because they the DRBD device wouldn’t mount. I suspect this was because I don’t fully understand how to configure it.
Of course this was just a short initial look, but I am very impressed with DRBD so far.