From Rong-en who has two regionservers:
When the second HRS starts up, but when all regions
are still not assigned to the first server. The 2nd HRS keeps
silence (though it's visible at master's ui). Once all regions are
assigned to first server, the balancer kicks in. Finally, the
regions are evenly split to two servers.
And then on IRC:
[21:38] <rafan> st^ack: got my mail?
[21:48] <st^ack> You have two HRSs?
[21:49] <st^ack> There is an odd decision made in the master that goes something like "is there only one regionserver? or are there more". If it decides the answer is 1, it just dumps all to it.
[21:49] <st^ack> My guess is that in your case, it decided that all regions were for server 1.
[21:50] <st^ack> Mind making a JIRA? Let the master parcel out the regions... the rate can vary with the number of OPEN messages it gets back.
[21:50] <st^ack> Allow that other regions could come in meantime?
If many regions, master should allow that while its assigning, other regionservers could come online rather than make the binary decision "There is only one regionserver in this cluster"