[DRBD-user] Questions I'm sure have been asked before..

Corey Edwards tensai at zmonkey.org
Mon Jun 6 17:12:37 CEST 2005

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

On Mon, 2005-06-06 at 12:07 +0200, nick wrote:
> I've got two identical machines (pentium 4, 2 80 gb SATA drives, 512mb 
> ram, drbd 0.7.10, CentOS 3.3, although with the ultramonkey kernel for 
> the heartbeat integration) which will be running as HA mailservers, with 
> two DRBD drives drbd0 which will contain /var and drbd1 which will 
> contain /home. The idea behind my setup is to make sure that no mail 
> gets lost either from the spool or the home users drives in case of a 
> system failure.

I recently brought up a DRBD volume on an existing /var partition and it
hasn't been without problem. I didn't have the luxury of repartitioning
since it was a critical mail server (hence the need for DRBD). On our
Debian system the most notable problems are due to apt & dpkg files
under /var. I really don't recommend mirroring /var if you can avoid it,
although with some extra effort it is doable.

> The thing is, I already have data and a filesystem present on md2 and 3, 
> ext3 to be exact, and most of the replies I've seen on this subject seem 
> to be aplicable to people with internal metadata, in that they advise 
> shrinking the filesystem first (I'm assuming here, correct me if I'm wrong).
> Before I take the leap in changing my fstab, and mounting the drbd 
> drives, what exactly do I need to know, then do? I'd prefer not to lose 
> any data (although it wouldn't be the end of the world), and I'm not 
> sure exactly what effect creating a filesystem would have on the 
> existing data. I'd be glad to read up on it, if anyone has any pertinent 
> links.

No need to shrink the filesystem. That's the benefit of using an
external metadata partition.

You also should not lose any data. When you start DRBD on the primary it
will come up as Secondary/Unknown, Inconsistent. It will not make any
changes to your partition. You have to force consistency with "drbdadm
-- --do-what-I-say primary <volume>". That also won't change your
partition. You now should be able to mount the drbd device. Once you
verify that is working you can continue to add the secondary as

I just went through this process on a couple production mail servers and
everything went pretty smoothly. I did practice a number of times
beforehand, which I strongly advise.


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