Moving to from DRBD 7 to DRBD 8

General
1

So in HA + DR (Three DBs: one HA pair [Primary <> Secondary] and one DR [i.e. DR Secondary]) there is one ‘SOURCE-of-TRUTH’ which is the Primary DB. When you see UpToDate in a Secondary (drbdadm status all) you know that the UPToDate means that the Secondary has an EXACT-, SIMILAR COPY to the Souce-of-Truth (Primary) and this connection made via tcp/7788.

HOWERVER, I also see the TWO Secondaries (HA Secondary <> DR Secondary) communicate with each other (via port 7789) and here are my question:

  1. Why do they communicate each other ? What are the ‘benefits’ of two Secondaries communcate with each other if the source-of-truth is the Primary ?

  2. Are they ‘watching’ each other ? For what ? I know there is the ‘mesh structure’ between all nodes but what benefits it gives us if everything is UpToDate relative to the Primary ? Are they need to be also ‘UpToDate’ compare to the copy of the other Secondary (i.e. two Secondaries compare the copies they have).

  3. Finally, what kind of data they resync with each other via port tcp/7789 (bitmap and other Meta data) or somthing ‘else’ ?

Again, I will be happy for any link / clip to explain this idea.

Thanks.