HBase based block placement in DFS

The feature as is only useful for HBase clusters that care about data locality on regionservers, but this feature can also enable a lot of nice features down the road.

The basic idea is as follows: instead of letting HDFS determine where to replicate data (r=3) by place blocks on various regions, it is better to let HBase do so by providing hints to HDFS through the DFS client. That way instead of replicating data at a blocks level, we can replicate data at a per-region level (each region owned by a promary, a secondary and a tertiary regionserver). This is better for 2 things:

• Can make region failover faster on clusters which benefit from data affinity
• On large clusters with random block placement policy, this helps reduce the probability of data loss

The algo is as follows:

• Each region in META will have 3 columns which are the preferred regionservers for that region (primary, secondary and tertiary)
• Preferred assignment can be controlled by a config knob
• Upon cluster start, HMaster will enter a mapping from each region to 3 regionservers (random hash, could use current locality, etc)
• The load balancer would assign out regions preferring region assignments to primary over secondary over tertiary over any other node
• Periodically (say weekly, configurable) the HMaster would run a locality checked and make sure the map it has for region to regionservers is optimal.

Down the road, this can be enhanced to control region placement in the following cases:

• Mixed hardware SKU where some regionservers can hold fewer regions
• Load balancing across tables where we dont want multiple regions of a table to get assigned to the same regionservers
• Multi-tenancy, where we can restrict the assignment of the regions of some table to a subset of regionservers, so an abusive app cannot take down the whole HBase cluster.