[DRBD-user] DRBD with more than one secondary node

Hector Blanco white.lists at gmail.com
Fri Jul 4 13:31:00 CEST 2008

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.


Hello.

First of all, I'd like to salute everybody in this list, as this is my
first message to it. As you might guess, I'm a total newbie here, but
I'll try to cooperate and help as much as I can.

Now I'd like to raise one question. Basically, it is quite simple:
Does DRBD support more than two secondary nodes? I guess it doesn't.
I've never seen a configuration example with more than two nodes
(primary + secondary) and I thing that (for what I understood in the
documentation) more than one secondary is not supported because of the
high network traffic load. But I'm asking because I have seen an email
(http://osdir.com/ml/linux.highavailability.devel/2006-06/msg00143.html)
saying that the support of more than two nodes was intended... and
that was two years ago so maybe I didn't find the right web page :)

Now, if you have more time, I'll try to detail my problem:

I have currently two Xen virtual machines running into a physical
machine. Each of the virtual machines is running a MySql instance,
each of which write the data in a drbd partition (thus the Mysql data
are the same). Over this, I have a Virtual IP controlled by HeartBeat.
Now, if a machine or a mysqld process fails, all the MySql petitions
are redirected to the other machine. As you may have foreseen, the
Drbd resource is controlled by Heartbeat as well. I think this is a
very typical configuration (I did more or less what is explained here:
http://marksitblog.blogspot.com/2007/07/mysql-5-high-availability-with-drbd-8.html).
You should be able to see (if imageshack still keeeps it) a simple
diagram of what I currently have in the following address:
http://img388.imageshack.us/img388/6832/pict01tv3.png As I said Both
machines are Xen Virtual machines but it did't matter much... for the
moment.

Now I'd like to replicate the structure to another physical machine,
to have even more security (higher availability) and I don't know
exactly how I could do it... It would be nice if Drbd could replicate
in more than two partions. You can see what I want to get in
http://img379.imageshack.us/img379/2812/pict02hp5.png. What I want is
tho have everything running (is possible) in monitor01:database01. If
something in database01 fails, have heartbeat moving everything to
database02, and keep working connecting to that machine (in a
transparent way to the MySQL clients), but if monitor01 fails, or
database01 and database02 fails I have to move to monitor02. And there
comes my problem. I don't know which would be the best way to have the
same data in all the database0x machines. I thought in having a DRBD
unit in monitor01 and monitor02, and then mounting it (via NFS or
something like that) in the database0x machines. Something like:
http://img352.imageshack.us/img352/2688/pict03vv6.png but I don't know
if it is good idea. Could anyone give me a hint, or even a personal
opinion?

Thank you in advance!!



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