[DRBD-user] DRBD user guide - GFS2 - recommendation to lose data?

[Matt Kereczman]

On 8/2/22 22:48, Reid Wahl wrote:
> Section 10.2:
> "Also, it is recommended to use some of DRBD’s features for automatic
> recovery from split brain. To do all this, include the following lines
> in the resource configuration:
> ...
> By configuring auto-recovery policies, you are configuring effectively
> configuring automatic data-loss! Be sure you understand the
> implications."
> 
> If these options incur a risk of data loss, why are they recommended?
> I'm wary of recommending that to our users.
> 
> https://linbit.com/drbd-user-guide/drbd-guide-9_0-en/#ch-gfs

Hello again :)

As you've noticed, the "GFS" section of the DRBD User Guide could use 
some attention. I believe it was written prior to GFS2's release (so 
RHEL 5?).

Issues with prose aside, this is another case where clarification is 
needed. In a properly configured Pacemaker + DRBD cluster (i.e. with 
fencing) when the replication network splits DRBD will call for 
Pacemaker to fence the peer. Both peers will have been Primary but 
disconnected and therefore DRBD will be in a split-brain state when 
the peer reboots and reconnects. It will reconnect in the Secondary 
state and because of the `after-sb-1pri discard-secondary;` setting 
the data on the returning Secondary node will be overwritten with the 
data from the Primary. Technically, data on the returning peer could 
be lost but its probably not anything you'd want to keep.

I believe the comment is alluding to more dangerous settings, such as 
`after-sb-0pri discard-least-changes;`. For example, that setting in 
an improperly configured cluster could result in a video of an 
intern's cat (large) being saved while the CFO's pin number (small) is 
discarded (or something equally dramatic).

I've opened an internal issue to clean up this section for future readers.

Best Regards,
Matt

-- 
Matt Kereczman – Solutions Architect
matt at linbit.com

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