[DRBD-user] DRBD and LVM Snapshot with 2 nodes configuration

[Andreas Semt]

Lars Ellenberg wrote:


> you could have set it up as LVM2 on top of DRBD instead.
> 
Oh, really? Thought I've read somewhere don't do this. Very good to 
know. Why LVM2 or is LVM1 also acceptable?
> Basically the problem is that you won't get a "clean" but only a
> "consistent" snapshot if you have LVM below DRBD, because LVM expects to
> be the topmost layer right below the file system, and if DRBD (or any
> other stacking block device driver, for that matter) is in between, LVM
> is not able to contact the filesystem and tell it to flush its journals
> and metadata stuff,
> 
Yes, that makes sense.
> 
> "stop drbd on primary without making the peer primary"
> is
> "take down that device, and all services depending on the data on it".
> 
> this is the easy way, and of course will give you a clean and consistent
> view of the data, because the data won't be used by anyone but the
> backup now...

Not so good, i believe ...
> so what should work is backup/snapshot during downtime:
> 
> 	node-A: Primary; node-B: Secondary
> node-A# stop_all_services && umount /dev/nb0 && drbd stop
> 	node-A: Secondary; node-B: Secondary
> either node# lvmcreate {snapshot}
> either node# drbd start ; mount_that_device && start_all_services 
> 
> then do anything you want to do with the snapshot, and delete the
> snapshot after you are done with it.  you should be able to access the
> drbd device as usual while accessing the snapshot.
> 
> the difficult thing is: doing a clean and consistent snapshot while drbd
> is online.

Mhm, i see ... stop_all_services won't be a good solution, cause these 
services should be high available, it's a HA cluster ;-)
The snapshot should be created _while_ all services are running.
> The failover/failback thing, and "just inbetween" doing the snapshot,
> as you outline above, won't be such a good idea, because you probably
> will get a consistent, but not a *clean* snapshot.

What exactly means *clean*? Is that the problem you above-mentioned, 
i.e. making a snapshot while drbd is online?

Thanks for help, possible these questions should go on a "drbd-newbie" 
list.

-- 
Best regards,
Andreas Semt