HBASE-28097 Add documentation section for the Cache Aware balancer fu… (#5495)
Signed-off-by: Wellington Chevreuil <wchevreuil@apache.org>
diff --git a/src/main/asciidoc/_chapters/architecture.adoc b/src/main/asciidoc/_chapters/architecture.adoc
index 23d069c..12bdc09 100644
--- a/src/main/asciidoc/_chapters/architecture.adoc
+++ b/src/main/asciidoc/_chapters/architecture.adoc
@@ -1130,6 +1130,49 @@
The compressed BlockCache is disabled by default. To enable it, set `hbase.block.data.cachecompressed` to `true` in _hbase-site.xml_ on all RegionServers.
+==== Cache Aware Load Balancer
+
+Depending on the data size and the configured cache size, the cache warm up can take anywhere from a few minutes to a few hours. This becomes even more critical for HBase deployments over cloud storage, where compute is separated from storage. Doing this everytime the region server starts can be a very expensive process. To eliminate this, link:https://issues.apache.org/jira/browse/HBASE-27313[HBASE-27313] implemented the cache persistence feature where the region servers periodically persist the blocks cached in the bucket cache. This persisted information is then used to resurrect the cache in the event of a region server restart because of normal restart or crash.
+
+link:https://issues.apache.org/jira/browse/HBASE-27999[HBASE-27999] implements the cache aware load balancer, which adds to the load balancer the ability to consider the cache allocation of each region on region servers when calculating a new assignment plan, using the region/region server cache allocation information reported by region servers to calculate the percentage of HFiles cached for each region on the hosting server. This information is then used by the balancer as a factor when deciding on an optimal, new assignment plan.
+
+The master node captures the caching information from all the region servers and uses this information to decide on new region assignments while ensuring a minimal impact on the current cache allocation. A region is assigned to the region server where it has a better cache ratio as compared to the region server where it is currently hosted.
+
+The CacheAwareLoadBalancer uses two cost elements for deciding the region allocation. These are described below:
+
+. Cache Cost
++
+
+The cache cost is calculated as the percentage of data for a region cached on the region server where it is either currently hosted or was previously hosted. A region may have multiple HFiles, each of different sizes. A HFile is considered to be fully prefetched when all the data blocks in this file are in the cache. The region server hosting this region calculates the ratio of number of HFiles fully cached in the cache to the total number of HFiles in the region. This ratio will vary from 0 (region hosted on this server, but none of its HFiles are cached into the cache) to 1 (region hosted on this server and all the HFiles for this region are cached into the cache).
++
+Every region server maintains this information for all the regions currently hosted there. In addition to that, this cache ratio is also maintained for the regions which were previously hosted on this region server giving historical information about the regions.
+
+. Skewness Cost
++
+
+
+The cache aware balancer will consider cache cost with the skewness cost to decide on the region assignment plan under following conditions:
+
+. There is an idle server in the cluster. This can happen when an existing server is restarted or a new server is added to the cluster.
+
+. When the cost of maintaining the balance in the cluster is greater than the minimum threshold defined by the configuration _hbase.master.balancer.stochastic.minCostNeedBalance_.
+
+
+The CacheAwareLoadBalancer can be enabled in the cluster by setting the following configuration properties in the master master configuration:
+
+[source,xml]
+----
+<property>
+ <name>hbase.master.loadbalancer.class</name>
+ <value>org.apache.hadoop.hbase.master.balancer.CacheAwareLoadBalancer</value>
+</property>
+<property>
+ <name>hbase.bucketcache.persistent.path</name>
+ <value>/path/to/bucketcache_persistent_file</value>
+</property>
+----
+
+
[[regionserver_splitting_implementation]]
=== RegionServer Splitting Implementation
