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From: Apache Wiki <wikidiffs@apache.org>
To: core-commits@hadoop.apache.org
Date: Sat, 04 Apr 2009 18:13:27 -0000
Message-ID: <20090404181327.12623.70892@aurora.apache.org>
Subject: [Hadoop Wiki] Trivial Update of "Hbase/DesignOverview" by
 EvgenyRyabitskiy

Dear Wiki user,

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The following page has been changed by EvgenyRyabitskiy:
http://wiki.apache.org/hadoop/Hbase/DesignOverview

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  == Cluster initialization ==
  
- 
+ While first start master tries to get root and root region directories from HDFS and after fail creates them and first meta region directory. In next start master will get information about cluster and it's regions.
  
  === Assigning regions to HRegionServers ===
  
- The first region to be assigned is the ''ROOT region'' which locates all the META regions to be assigned. Each ''META region'' maps a number of user regions which comprise the multiple tables that a particular HBase instance serves. Once all the META regions have been assigned, the master will then assign user regions to the H!RegionServers, attempting to balance the number of regions served by each H!RegionServer.
+ Each region is assigned to only one H!RegionServer or not assigned yet. The first region to be assigned is the ''ROOT region'' which locates all the META regions to be assigned. Each ''META region'' maps a number of user regions which comprise the multiple tables that a particular HBase instance serves. Once all the META regions have been assigned, the master will then assign user regions to the H!RegionServers, attempting to balance the number of regions served by each H!RegionServer.
+ 
+ ==== Assigned region ====
+ 
+ If region is assigned to some H!RegionServer it means that this region served by this server. While serving region H!RegionServer handles read/write requests to this region, caches last (not yet flushed) modifications in memcache and writes all changes to WAL but persistent reliable storage of regions is provided by HDFS.
  
  ==== The META Table ====