why
along with the time goes,as unreasonable table design,or some crash nodes etc,these will cause some regionservers have overloaded regions and/or much underloaded regions,so a balancer is necessary to balance the load(regions count) per regionserver to avoid uncomplete resource usages.
how to
in hbase ,there is a balancer which can be run by manual or a period checker.for the later,the period will be adjusted by below property:
<property>
<name>hbase.balancer.period</name>
<value>300000</value>
<description>Period at which the region balancer runs in the Master.
</description>
</property>
look at the balance flow in HMaster, u will see a approx process like below:
1.generate region plans.that is which and where region to be moved.
2.close the region by regionserver notified by master
3.assign the closed region(s) by master in a normal procedure.
1.generate region plans in master
there is a slop called "hbase.regions.slop" to accommodate the balance accuracy.the less this value is,the more accurate results in.
also,the ceil and floor are generated by the averge load * (1 +/- slop).
but at last ,the calculation of underloaded regions and overloaded is crude:
use region count / number of regionservers as the floor,and
use (region count-1+number of servers) / number of servers as the ceil
2.close the region by regionserver
when all region plans are all ready,then a loop will iterate per plan to send close info to regionserver.after the later closed the region and updated the zk state,then a zk-related event is received in master in AssignmentManager#nodeDataChanged(),so a normal region assign process is issued.
3.assign the closed reigon to a new regionserver
after finishing the step 2 above,but how the master knows which regionserver to assign?
in the step 1,before sending a close info to regionserver,the master has kept up the region-plan(which contains the src and dest server),so when receiving a zk event,this plans info are stayed in memory in master also.
FAQs:
? rs的负载不均衡，已经有同事做了些改进，将同一table的region尽量分配到不同rs上
--decrease the slop factor below to 0.1,or adjust the crude min and max calculation.
? hot region的均匀分布。考虑根据region最近所服务的请求数作为balance的依据，使每台rs上的region所服务的请求数相
--i think this is a design fault.in general ,a even-consistent algorithm will not result in this case.
in the other hand,un-even region assigment will cause unconsistent resource usage.
? ignore zk event,let regionserver notify master to assign directly
--this is a improved feature in 0.96+?
? fix some regions in a regionserver,that is exclude new regions to be assigned to specified reginservers
--look at [3]
ref:
[1]５。hbase高级部分:compact/split/balance及其它维护原理--the flow of creating table
[2]改善HBase的Balance策略
[3] Support to drain RS nodes through ZK :suppress assignment of new regions