[DRBD-user] mount a LVM with detached DRBD device - write data possible without corruption?

Ekkard Gerlach ekkardgerlach at aiai.de
Sun Jan 18 21:24:51 CET 2009


* Tomasz Chmielewski schrieb:

> Papp Tamas schrieb:
> >Ekkard Gerlach wrote:
> >>Can I write to the according lv-device when DRBD is stopped by 
> >>"drbdadm down /dev/drbd1"
> >>and lv is mounted by 'mount /dev/vg-ford2/vserver-home /mnt' without 
> >>data corruption? -
> >>DRBD is set up on  logical volumen!
> >>  
> >
> >Of course not.
> 
> It's enough to mount the device (without writing any "real files") to 
> make it inconsistent with DRBD metadata.

OK, thx. But if this device on a separate harddisk is NEVER in use with DRBD? - Its used
only if my system with 2 nodes crashes completely, when I need a copy to build a complete new
DRBD-system! For this case once again my question: if I write to that device on a separate
harddisk partition (that was formatted originally as DRBD-device) - is that device 
usable for a NEW DRBD-system where this harddisk is primary from beginning/ the start?


Perhaps you don't understand my aim. Background what I want: 
==============================================================
node2 of my DRBD-system has only ONE harddisk, node1 has 2 disks 
in soft-raid1. node2 is only a emergency and backup node. 
The second harddisk of node2  is a standalone disk with backup 
of the last day. Every night data ist rsync'ed to that second harddisk: 
 
node2:~>hb_standy all  # not necessary, node2 is always secondary on all vservers
node2:~>drbdadm down all
node2:~>mount -o ro /dev/vg-ford2/vserver-home /to_sync-ro
node2:~>rsync -aHx /to_sync-ro  ==> my second harddisk
node2:~>drbdadm up all 

The second harddisk is a dd-copy of the first one of node2! 
I changed UUID of physical volume and volume group and 
renamed volume group of the second harddisk. Now I have a entirely redundant
harddisk for node2. Do you understand me now? 
Only one logical volume carries user data that changes every day and I want 
to update my second harddisk every night.  So I've two benefits: 
   * data from yesterday. Perhaps for restoring lost (deleted) or corrupted files
   * a complete harddisk build a new DRBD-system if my running system crashes. 

==> once again my question: can I update my second emergency and last-day-backup 
harddisk every night without data corruption? 


BTW: I know that LVM is backup'ed with rsnapshot easily. I choosed the way
     with rsync because I've lots of good experiences with rsync. 

Ekkard



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