[DRBD-user] protocol B in dual-primary mode

John Lauro john.lauro at covenanteyes.com
Tue Dec 13 20:00:15 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

None of the open source targets will handle reservations or block locking.
However, that point is moot when you have a single client multi-pathing to
drbd pair, but reservations are very critical for SAN based filesystems...
Assuming only a single client to two DRBD nodes and doing failover, it is
very risky as the slightest network hiccup can cause a split brain as the
default timeout retries are extremely low and despite having protocol C,
one node might thing the other is down when it is not really.  However, if
(and only if) you truly implement STONITH so split brain is impossible, as
far as I can tell you could do multi-pathing despite all of the comments
not to do it.

So two requirements:
	1. No multi-node to same resource.  (No SAN type filesystem).
	2. STONITH must be implemented.
If you meet those two requirements, then it should technically be
possible, but comes down to a why bother with active/active multipath DRBD
at all, and why not just fail over the service on top of DRBD.  

> -----Original Message-----
> From: drbd-user-bounces at lists.linbit.com [mailto:drbd-user-
> bounces at lists.linbit.com] On Behalf Of Felix Frank
> Sent: Tuesday, December 13, 2011 4:44 AM
> To: Marcus Sorensen
> Cc: drbd-user at lists.linbit.com
> Subject: Re: [DRBD-user] protocol B in dual-primary mode
> Hi,
> On 12/13/2011 05:35 AM, Marcus Sorensen wrote:
> > What if you're just
> > exporting drbd block, active-active devices via say, iscsi or SAS
> > target? Couldn't the initiator just treat them as the same disk,
> > multipathed?
> there was a lengthy discussion last week or the one prior on why you
> cannot do that. Long story short, two state of the art iSCSI targets
> cannot operate as one multipath target.
> I can't seem to find it quite on the spot, but a quick trawl through the
> archives should yield some info.
> Cheers,
> Felix
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