DRBD on a disk with existing file system that takes all the place

I'm currently trying to simulate the environment via XEN. I have installed two debian systems with such FS layout:

cltest1:/etc# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda2            6.0G  417M  5.2G   8% /
tmpfs                 257M     0  257M   0% /lib/init/rw
udev                   10M   16K   10M   1% /dev
tmpfs                 257M  4.0K  257M   1% /dev/shm

Host cltest2 is identical.

Here's my drbd.conf

global {
  minor-count 1;
}

resource mysql {
  protocol C;

  syncer {
    rate 10M; # 10 Megabytes
  }

  on cltest1 {
    device    /dev/drbd0;
    disk      /dev/xvda2;
    address   192.168.1.186:7789;
    meta-disk internal;
  }

  on cltest2 {
    device    /dev/drbd0;
    disk      /dev/xvda2;
    address   192.168.1.187:7789;
    meta-disk internal;
  }
}

I have not created filesystem on drbd0

Starting DRBD via init.d script errors out with:

Starting DRBD resources:    [ d(mysql) /dev/drbd0: Failure: (114) Lower device is already claimed. This usually means it is mounted.

[mysql] cmd /sbin/drbdsetup /dev/drbd0 disk /dev/xvda2 /dev/xvda2 internal --set-defaults --create-device  failed - continuing!

Running: drbdadm create-md mysql gives:

cltest1:/etc# drbdadm create-md mysql
md_offset 6442446848
al_offset 6442414080
bm_offset 6442217472

Found ext3 filesystem which uses 6291456 kB
current configuration leaves usable 6291228 kB

Device size would be truncated, which
would corrupt data and result in
'access beyond end of device' errors.
You need to either
   * use external meta data (recommended)
   * shrink that filesystem first
   * zero out the device (destroy the filesystem)
Operation refused.

Command 'drbdmeta /dev/drbd0 v08 /dev/xvda2 internal create-md' terminated with exit code 40
drbdadm aborting

As I understand, all of my problems are because I don't have unallocated disk space on xvda2.

What are my options besides shrinking FS and connecting a separate physical disk? Can't the meta-data be stored on a file in the local filesystem?

Solution 1:

drbd can sometimes pickup 'existing' filesystems even without one being on the sytem. You could of dd if=/dev/zero of=/dev/xvda2 (and control-C after a few secs) and it will clear the partition that could of held a filesystem before.

Solution 2:

You are not suppose to mount the disk with any filesystem format; In Ubuntu, using gparted I specified the /dev/xvda2 to "Cleared" partition. and running the same command removes the above error.