[DRBD-user] 3 way replication : promoting the "stacked" node

Felix Frank ff at mpexnet.de
Wed Jan 12 10:28:05 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.

> You usually leave S disconnected, that's why you need a full sync to
> bring S up to speed, but normally what you would do when using stacked
> resources would be to configure S with protocol A, this is actually the
> recommendation in the drbd.org docs
> http://www.drbd.org/users-guide/s-three-nodes.html#s-three-node-config

Actually, this is only valid when
1. the third node is to stay in permanent sync and
2. there is a WAN link in between the third node and its peers

If the RTT is reasonably low, Protocol C is not supposed to be much
slower than A and stacking absolutely doesn't require Protocol A.

Also note that S never needs a full sync, unless you manage to break the
UUIDs on A and B. Normally, upon reconnecting, S will do a quicksync.

> You need A->B, and you can only setup one replication link that spans 2
> servers (e.g.: you can't set A's /dev/drbdx to replicate to both B and S
> at the same time), the only way to do it is to set normal A->B, and on
> top of that AB->S, because you don't know if A or B will be up and don't
> want to manually choose (if it was somehow possible) to sync A->S or
> B->S depending on which (A or B) is up. Stacked resources does that for
> you automagically, it syncs whomever is Primary to the stacked resource (S).

To answer the base concern: No, you cannot just reconfigure S to be an
unstacked peer for A or B (unless a full sync is OK for you). That's
because S does not share UUIDs with the resource replicated among A and
B. You can however leave the stacking in place on the surviving node (A
or B) and remain in a configuration of

A: (StandAlone/Primary) (Connected/Primary)
S: (------------------) (Connected/Secondary)

where your normal configuration is

A: (Connected/Primary)   (WFConnection/Primary)
B: (Connected/Secondary) (--------------------)
S: (-------------------) (Standalone/Secondary)

I hope this sketch is comprehensible.


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