HBase based block placement in DFS
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Key: HBASE-4755
URL: https://issues.apache.org/jira/browse/HBASE-4755
Project: HBase
Issue Type: New Feature
Reporter: Karthik Ranganathan
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.
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