[DRBD-user] confused with DRBD 9.0 and dual-primary, multi-primary, multi-secondary ...

[Lars Ellenberg]

On Wed, Aug 29, 2018 at 03:38:34PM +0200, Lentes, Bernd wrote:
> Hi,
> 
> i found: 
> "With current DRBD-9.0 version running in Dual-Primary mode is not recommended (because of lack of testing).
> In DRBD-9.1 it will be possible to have more than two primaries at the same time."

The answer to your actual question is "Yes", below.

But read on, I try to give a more general answer for
"Can DRBD 9 do multi-primary?" first.

Even with DRBD 9, we can NOT do "multi-primary", but only "two-primary".

If you start playing around, it may at first *seem* like it would work.
It does not.

And *if* you do go "two-primary", that also means two nodes only.

NOT two-primary plus multiple secondaries.
Does not work either, unfortunately.

With the notable exception of live-migration in a multi-peer setup,
where we have "two" primaries with potentially "many" secondary peers.
But only for a short time, and these are not actually concurrently
written to, so that's a special case.

What I think we want to support at some point even in the "9.0" series
is two-primaries plus one or more secondaries. Maybe.

"N >= 3" Primaries is something that will come only
in some later "9.1" (or 9.2 or 10.x or 23.y) version.
If at all.  We don't see a "real" use case for this.

> Clear statement. I'm using SLES 12 SP3 with DRBD 9.0.8.

If you want DRBD 9, you should be prepared to keep track of,
and regularly upgrade to, upcoming DRBD 9 releases.
Currently we are at 9.0.15.
You should certainly not stay on 9.0.8.

If you really want/need "real, concurrent" dual-primary DRBD,
(which is most likely a bad idea anyways;
there are very limited use cases where dual-primary is actually useful
beyond being "cool" or apparently, but not actually, "convenient")
For now I'd suggest to stay with 8.4,
because that emphasizes the "two node only" aspect.

If you only need the "short time, not really concurrent" dual-primary
for "live migration" in a virtualization environment,
both DRBD 8.4 and DRBD 9 can do that,
and this is one of the main use cases for LINSTOR with DRBD 9.

> The kernel modules are 9.0.x.
> The drbd-utils i found are 9.x.
> 
> Are these already dual-primary capable ?

Yes.

But *really* at least upgrade to 9.0.15,
and then .16 and .17 and ... when they come out.

If that's not something you can do in your environment,
stay with the stable-and-frozen DRBD 8.4 series.

Or look into our LINSTOR project, which requires DRBD 9, and is supposed
to make integration with various virtualization solutions much easier.
Still, also in that case,
prepare to regularly upgrade both DRBD 9 and LINSTOR components.
There will be bugs, and bug fixes, and they will be relevant for your
environment.

-- 
: Lars Ellenberg
: LINBIT | Keeping the Digital World Running
: DRBD -- Heartbeat -- Corosync -- Pacemaker

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