we're currently using Phoenix 4.2 with Hbase 0.98.6 from CDH5.3.2 on our cluster and we're experiencing some perf issues.

What we need to do is a full table scan over 1 billion rows. We've got 50 regionservers and approximatively 1000 regions of 1Gb equally distributed on these rs (which means ~20 regions per rs). Each node has 14 disks and 12 cores.

A simple "Select count(1) from table" is currently taking 400~500 sec.

We noticed that a range scan over 2 regions located on 2 different rs seems to be done in parallel (taking 15~20 sec) but a range scan over 2 regions of a single rs is taking twice this time (about 30~40 sec). We experience the same result with more than 2 regions. 

Could this mean that parallelization is done at a regionserver level but not a region level ? in this case 400~500 seconds seems legit with 20~25 regions per rs. We expected regions of a single rs to be scanned in parallel, is this a normal behavior or are we doing something wrong ?

Thanks for your help