[DRBD-user] primary/primary with OCFS2 for the first time, questions

drbd at bobich.net drbd at bobich.net
Thu Mar 27 14:14:21 CET 2008

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

> I wonder how does OCFS2 or DRBD 8.0.* deal with failed failed replication link when the
> same file is accessed.

In primary/primary, it would splitbrains. From that point, the FS images 
would diverge.

> I mean, in the primary/secondary if the replication link went down on secondary no data
> was written. If for some strange reason the secondary resource became primary and
> mounted if we got split brain, solution was pretty straight forward. Data got
> (automatically or manually) replicated back from first primary node or the node with
> most current data on resource basis. r1 could be replicated one way, while r0 was in
> the other way.

In the case of primary/primary, you'd have to decide which copy of the 
data you want after the splitbrain.

> But what happens when i have only one resource in primary/primary and the same file
> gets written to or created on both with different content while resource is
> disconnected?

One clobers another on resync.

> I cannot say sync from first to second node since each node has some of the data
> correct and doing this in either way would make me loose half of the data. I'm I not
> understanding something here or is this expected?

You are understanding fine. There is no recovery from a splitbrain 
condition without losing one half of the data. If this is unacceptable, 
make sure your fencing works. If it does, one node will fence another by 
powering it off so the splitbrain cannot occur. Working fencing in a 
shared storage cluster is mandatory. GFS will halt all writes until 
fencing is successful. OCFS2 probably does something similar.


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