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====== DRBD ======
http://www.sgenomics.org/~jtang/blog/posts/Experimenting_with_DRBD_for_data_replication_for_backup_purposes/

<blockquote>
Software used,
  * DRBD http://www.drbd.org - I downloaded the centos srpms for v8.3.x and rebuilt them for scientificlinux.
  * Scientificlinux 5.4 http://www.scientificlinux.org

My test configuration
<file python drbd.conf>
#
# please have a a look at the example configuration file in
# /usr/share/doc/drbd83/drbd.conf
#

global {
    usage-count no;
}

common {
    syncer { 
        rate 33M;
        verify-alg sha1;
    }
}

resource r0 {
    protocol C;
    startup {
        wfc-timeout 0;
        degr-wfc-timeout 120;
    }
    net {  
        cram-hmac-alg "sha1"; 
        shared-secret "mysharedkey"; 
    }
    on myhost.mydomain1 {
        device /dev/drbd0;
        disk /dev/sdb2;
        address 10.0.0.1:7788;
        meta-disk internal;
    }
}
</file>

Where myhost.mydomain1 is just my first host, I wish to just create a one node drbd cluster for now so I can add additional hosts to the pool to replicate my data at a later point. In short I want run the cluster in a degraded mode till I get more hosts to add redundancy for backup purposes.

/dev/sdb2 is just a small 50gb partition for testing purposes and the 10.0.0.1 address is just a private ip address that I intend on binding to the second network interface on my server such that I can use a cross over cable to connect to my backup host later on.

Once I had the above config file, I needed to run the following commands.

<code bash>
/etc/init.d/drbd restart
drbdadm create-md r0
/etc/init.d/drbd restart
drbdadm -- --overwrite-data-of-peer primary r0
/etc/init.d/drbd restart
mkfs.ext3 /dev/drbd0
mount /dev/drbd0 /mnt/r0
</code>

The about should give you a single node drbd setup, from reading the documentation It should be possible to add a second host to the setup, by adding this to the config file.

<code python>
on myhost.mydomain2 {
    device /dev/drbd0;
    disk /dev/sdb2;
    address 10.0.0.2:7788;
    meta-disk internal;
}
</code>

then copy the updated drbd.conf file to myhost.mydomain2, then issue these commands on myhost.mydomain1, assuming you want to make this the primary node

<code bash>
drbdadm disconnect all
drbdadm invalidate-remote all
drbdadm primary all
drbdadm connect all
</code>

On the secondary node myhost.mydomain2 issue these commands

<code bash>
drbdadm disconnect all
drbdadm invalidate all
drbdadm connect all
</code>

Please be careful on which nodes you issue these commands or else you might end up blowing away data. Once the above commands have been issued, have a look at /proc/drbd there will be some statistics on whats happening.

I'm not really using drbd for high availability, but rather for an easy way of providing a backup copy of my data. Note you would probably not want to automount the drbd partition on boot so I have set my system not to automount and to do no fsck's

So I now have this in my fstab for /home
<code fstab>
/dev/drbd0      /home           ext3    defaults,noauto,usrquota,grpquota 0 0
</code>

and to mount it on my primary machine, I do
<code bash>
drbdadm primary all
mount /home
</code>

Notes: I found that if both my disks are invalid (after some messing to test the backups) you might get something like this
<code>
[root@myhost.mydomain1 ~]# drbdadm primary r0
0: State change failed: (-2) Refusing to be Primary without at least one UpToDate disk
Command 'drbdsetup 0 primary' terminated with exit code 17
</code>

you could force one of the hosts' disks to be valid by doing
<code bash>
drbdadm -- --overwrite-data-of-peer primary all
</code>

make sure you do it on the primary node!
</blockquote>
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