[ClusterLabs] Removing DRBD w/out Data Loss?

[Valentin Vidić]

On Wed, Sep 09, 2020 at 12:10:54PM +0000, Eric Robinson wrote:
> With DRBD stopped, wipefs only showed one signature...
> 
> [root at 001db01 ~]# wipefs /dev/vg0/lv0
> offset               type
> ----------------------------------------------------------------
> 0x438                ext4   [filesystem]
>                      UUID:  2035aabe-b6e1-4313-a022-9518eb7489e6
> 
> So I just mounted the filesystem...
> 
> [root at 001db01 ~]# mount /dev/vg0/lv0 /mnt
> [ 7712.478905] EXT4-fs (dm-2): 35 orphan inodes deleted
> [ 7712.484594] EXT4-fs (dm-2): recovery complete
> [ 7712.506344] EXT4-fs (dm-2): mounted filesystem with ordered data mode. Opts: (null)
> 
> ...and all my data seems to be there? Did I miss something? I don't
> understand. Where did the DRBD partition go?

DRBD metadata is always at the end of the device (or on a different
device if DRBD configuration sets external metadata parameter) so
the filesystem should be directly accessible once the drbd signature
is wiped. This also means it is possible to add DRBD to an existing
partition (if there is enough free space at the end or using an
external device).

I get a different output and behavior on Debian 10 (kernel 4.19,
DRBD 8.4.10, drbd-utils 9.5.0):

# wipefs /dev/vg0/drbd
DEVICE OFFSET     TYPE UUID                                 LABEL
drbd   0x3ffff03c drbd 4c26a7436a17d14                      
drbd   0x438      ext4 b708b27c-f6a6-4f07-8417-8c17d8a2febb 

# mount /dev/vg0/drbd /mnt
mount: /mnt: unknown filesystem type 'drbd'.

# wipefs -o 0x3ffff03c /dev/vg0/drbd
/dev/vg0/drbd: 4 bytes were erased at offset 0x3ffff03c (drbd): 83 74 02 6c

# wipefs /dev/vg0/drbd
DEVICE OFFSET TYPE UUID                                 LABEL
drbd   0x438  ext4 b708b27c-f6a6-4f07-8417-8c17d8a2febb 

# mount /dev/vg0/drbd /mnt

-- 
Valentin