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From st...@apache.org
Subject svn commit: r1088879 - in /hbase/trunk: CHANGES.txt bin/graceful_stop.sh bin/hbase bin/hbase-daemons.sh bin/region_mover.rb src/docbkx/book.xml
Date Tue, 05 Apr 2011 04:08:25 GMT
Author: stack
Date: Tue Apr  5 04:08:24 2011
New Revision: 1088879

URL: http://svn.apache.org/viewvc?rev=1088879&view=rev
Log:
HBASE-3071 Graceful decommissioning of a regionserver

Added:
    hbase/trunk/bin/graceful_stop.sh
    hbase/trunk/bin/region_mover.rb
Modified:
    hbase/trunk/CHANGES.txt
    hbase/trunk/bin/hbase
    hbase/trunk/bin/hbase-daemons.sh
    hbase/trunk/src/docbkx/book.xml

Modified: hbase/trunk/CHANGES.txt
URL: http://svn.apache.org/viewvc/hbase/trunk/CHANGES.txt?rev=1088879&r1=1088878&r2=1088879&view=diff
==============================================================================
--- hbase/trunk/CHANGES.txt (original)
+++ hbase/trunk/CHANGES.txt Tue Apr  5 04:08:24 2011
@@ -134,6 +134,7 @@ Release 0.91.0 - Unreleased
    HBASE-3559  Move report of split to master OFF the heartbeat channel
    HBASE-3573  Move shutdown messaging OFF hearbeat; prereq for fix of
                hbase-1502
+   HBASE-3071  Graceful decommissioning of a regionserver
 
 
   NEW FEATURES

Added: hbase/trunk/bin/graceful_stop.sh
URL: http://svn.apache.org/viewvc/hbase/trunk/bin/graceful_stop.sh?rev=1088879&view=auto
==============================================================================
--- hbase/trunk/bin/graceful_stop.sh (added)
+++ hbase/trunk/bin/graceful_stop.sh Tue Apr  5 04:08:24 2011
@@ -0,0 +1,83 @@
+#!/usr/bin/env bash
+#
+#/**
+# * Copyright 2011 The Apache Software Foundation
+# *
+# * Licensed to the Apache Software Foundation (ASF) under one
+# * or more contributor license agreements.  See the NOTICE file
+# * distributed with this work for additional information
+# * regarding copyright ownership.  The ASF licenses this file
+# * to you under the Apache License, Version 2.0 (the
+# * "License"); you may not use this file except in compliance
+# * with the License.  You may obtain a copy of the License at
+# *
+# *     http://www.apache.org/licenses/LICENSE-2.0
+# *
+# * Unless required by applicable law or agreed to in writing, software
+# * distributed under the License is distributed on an "AS IS" BASIS,
+# * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# * See the License for the specific language governing permissions and
+# * limitations under the License.
+# */
+ 
+# Move regions off a server then stop it.  Optionally restart and reload.
+# Turn off the balancer before running this script.
+function usage {
+  echo "Usage: graceful_stop.sh [--config <conf-dir>] [--restart] [--reload] <hostname>"

+  echo " restart     If we should restart after graceful stop"
+  echo " reload      Move offloaded regions back on to the stopped server"
+  echo " debug       Move offloaded regions back on to the stopped server"
+  echo " hostname    Hostname of server we are to stop"
+  exit 1
+}
+
+if [ $# -lt 1 ]; then
+  usage
+fi
+
+bin=`dirname "$0"`
+bin=`cd "$bin">/dev/null; pwd`
+# This will set HBASE_HOME, etc.
+. "$bin"/hbase-config.sh
+# Get arguments
+restart=
+reload=
+debug=
+while [ $# -gt 0 ]
+do
+  case "$1" in
+    --restart)  restart=true; shift;;
+    --reload)   reload=true; shift;;
+    --debug)    debug="--debug"; shift;;
+    --) shift; break;;
+    -*) usage ;;
+    *)  break;;	# terminate while loop
+  esac
+done
+
+# "$@" contains the rest. Must be at least the hostname left.
+if [ $# -lt 1 ]; then
+  usage
+fi
+
+hostname=$1
+filename="/tmp/$hostname"
+# Run the region mover script.
+echo "Unloading $hostname region(s)"
+HBASE_NOEXEC=true "$bin"/hbase org.jruby.Main "$bin"/region_mover.rb --file=$filename $debug
unload $hostname
+echo "Unloaded $hostname region(s)"
+# Stop the server. Have to put hostname into its own little file for hbase-daemons.sh
+hosts="/tmp/$(basename $0).$$.tmp"
+echo $hostname >> $hosts
+"$bin"/hbase-daemons.sh --hosts ${hosts} stop regionserver
+if [ "$restart" != "" ]; then
+  "$bin"/hbase-daemons.sh --hosts ${hosts} start regionserver
+  if [ "$reload" != "" ]; then
+    echo "Reloading $hostname region(s)"
+    HBASE_NOEXEC=true "$bin"/hbase org.jruby.Main "$bin"/region_mover.rb --file=$filename
$debug load $hostname
+    echo "Reloaded $hostname region(s)"
+  fi
+fi
+
+# Cleanup tmp files.
+trap "rm -f  "/tmp/$(basename $0).*.tmp" &> /dev/null" EXIT

Modified: hbase/trunk/bin/hbase
URL: http://svn.apache.org/viewvc/hbase/trunk/bin/hbase?rev=1088879&r1=1088878&r2=1088879&view=diff
==============================================================================
--- hbase/trunk/bin/hbase (original)
+++ hbase/trunk/bin/hbase Tue Apr  5 04:08:24 2011
@@ -276,5 +276,9 @@ if [ "x$JAVA_LIBRARY_PATH" != "x" ]; the
   HBASE_OPTS="$HBASE_OPTS -Djava.library.path=$JAVA_LIBRARY_PATH"
 fi
 
-# run it
-exec "$JAVA" $JAVA_HEAP_MAX $HBASE_OPTS -classpath "$CLASSPATH" $CLASS "$@"
+# Exec unless HBASE_NOEXEC is set.
+if [ "${HBASE_NOEXEC}" != "" ]; then
+  "$JAVA" $JAVA_HEAP_MAX $HBASE_OPTS -classpath "$CLASSPATH" $CLASS "$@"
+else
+  exec "$JAVA" $JAVA_HEAP_MAX $HBASE_OPTS -classpath "$CLASSPATH" $CLASS "$@"
+fi

Modified: hbase/trunk/bin/hbase-daemons.sh
URL: http://svn.apache.org/viewvc/hbase/trunk/bin/hbase-daemons.sh?rev=1088879&r1=1088878&r2=1088879&view=diff
==============================================================================
--- hbase/trunk/bin/hbase-daemons.sh (original)
+++ hbase/trunk/bin/hbase-daemons.sh Tue Apr  5 04:08:24 2011
@@ -38,7 +38,7 @@ bin=`cd "$bin">/dev/null; pwd`
 . $bin/hbase-config.sh
 
 remote_cmd="cd ${HBASE_HOME}; $bin/hbase-daemon.sh --config ${HBASE_CONF_DIR} $@"
-args="--config ${HBASE_CONF_DIR} $remote_cmd"
+args="--hosts ${HBASE_REGIONSERVERS} --config ${HBASE_CONF_DIR} $remote_cmd"
 
 command=$2
 case $command in

Added: hbase/trunk/bin/region_mover.rb
URL: http://svn.apache.org/viewvc/hbase/trunk/bin/region_mover.rb?rev=1088879&view=auto
==============================================================================
--- hbase/trunk/bin/region_mover.rb (added)
+++ hbase/trunk/bin/region_mover.rb Tue Apr  5 04:08:24 2011
@@ -0,0 +1,434 @@
+# Copyright 2011 The Apache Software Foundation
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+# Moves regions. Will confirm region access in current location and will
+# not move a new region until successful confirm of region loading in new
+# location. Presumes balancer is disabled when we run (not harmful if its
+# on but this script and balancer will end up fighting each other).
+# Does not work for case of multiple regionservers all running on the
+# one node.
+require 'optparse'
+include Java
+import org.apache.hadoop.hbase.HConstants
+import org.apache.hadoop.hbase.HBaseConfiguration
+import org.apache.hadoop.hbase.client.HBaseAdmin
+import org.apache.hadoop.hbase.client.Get
+import org.apache.hadoop.hbase.client.Scan
+import org.apache.hadoop.hbase.client.HTable
+import org.apache.hadoop.hbase.client.HConnectionManager
+import org.apache.hadoop.hbase.filter.FirstKeyOnlyFilter;
+import org.apache.hadoop.hbase.HServerAddress
+import org.apache.hadoop.hbase.util.Bytes
+import org.apache.hadoop.hbase.util.Writables
+import org.apache.hadoop.conf.Configuration
+import org.apache.commons.logging.Log
+import org.apache.commons.logging.LogFactory
+
+# Name of this script
+NAME = "region_mover"
+
+# Get root table reference
+def getRootTable(config)
+  # Keep meta reference in ruby global
+  if not $ROOT
+    $ROOT = HTable.new(config, HConstants::ROOT_TABLE_NAME)
+  end
+  return $ROOT
+end
+
+# Get meta table reference
+def getMetaTable(config)
+  # Keep meta reference in ruby global
+  if not $META
+    $META = HTable.new(config, HConstants::META_TABLE_NAME)
+  end
+  return $META
+end
+
+# Get table instance.
+# Maintains cache of table instances.
+def getTable(config, name)
+  # Keep dictionary of tables in ruby global
+  if not $TABLES
+    $TABLES = {}
+  end
+  key = Bytes.toString(name)
+  if not $TABLES[key]
+    $TABLES[key] = HTable.new(config, name)
+  end
+  return $TABLES[key]
+end
+
+
+# Returns true if passed region is still on 'original' when we look at .META.
+def isSameServer(admin, r, original)
+  server = getServerNameForRegion(admin, r)
+  return false unless server
+  return true unless original
+  return server == original
+end
+
+class RubyAbortable
+  include org.apache.hadoop.hbase.Abortable
+  def abort(why, e)
+    puts "ABORTED! why=" + why + ", e=" + e.to_s
+  end
+end
+
+# Get servername that is up in .META.; this is hostname + port + startcode comma-delimited.
+# Can return nil
+def getServerNameForRegion(admin, r)
+  if r.isRootRegion()
+    # Hack
+    tracker = org.apache.hadoop.hbase.zookeeper.RootRegionTracker.new(admin.getConnection().getZooKeeperWatcher(),
RubyAbortable.new())
+    tracker.start()
+    while not tracker.isLocationAvailable()
+      sleep 0.1
+    end
+    # Make a fake servername by appending ','
+    rootServer = tracker.getRootRegionLocation().toString() + ","
+    tracker.stop()
+    return rootServer
+  end
+  table = nil
+  if r.isMetaRegion()
+    table = getRootTable(admin.getConfiguration()) 
+  else
+    table = getMetaTable(admin.getConfiguration())
+  end
+  g = Get.new(r.getRegionName())
+  g.addColumn(HConstants::CATALOG_FAMILY, HConstants::SERVER_QUALIFIER)
+  g.addColumn(HConstants::CATALOG_FAMILY, HConstants::STARTCODE_QUALIFIER)
+  result = table.get(g)
+  server = result.getValue(HConstants::CATALOG_FAMILY, HConstants::SERVER_QUALIFIER)
+  startcode = result.getValue(HConstants::CATALOG_FAMILY, HConstants::STARTCODE_QUALIFIER)
+  return nil unless server
+  return java.lang.String.new(Bytes.toString(server)).replaceFirst(":", ",")  + "," + Bytes.toLong(startcode).to_s
+end
+
+# Trys to scan a row from passed region
+# Throws exception if can't
+def isSuccessfulScan(admin, r)
+  scan = Scan.new(r.getStartKey()) 
+  scan.setBatch(1)
+  scan.setCaching(1)
+  scan.setFilter(FirstKeyOnlyFilter.new()) 
+  table = getTable(admin.getConfiguration(), r.getTableDesc().getName()) 
+  scanner = table.getScanner(scan)
+  begin
+    results = scanner.next() 
+    # We might scan into next region, this might be an empty table.
+    # But if no exception, presume scanning is working.
+  ensure
+    scanner.close()
+    table.close()
+  end
+end
+
+# Check region has moved successful and is indeed hosted on another server
+# Wait until that is the case.
+def move(admin, r, newServer, original)
+  # Now move it. Do it in a loop so can retry if fail.  Have seen issue where
+  # we tried move region but failed and retry put it back on old location;
+  # retry in this case.
+  retries = admin.getConfiguration.getInt("hbase.move.retries.max", 5)
+  count = 0
+  same = true
+  while count < retries and same
+    if count > 0
+      $LOG.info("Retry " + count.to_s + " of maximum " + retries.to_s)
+    end
+    count = count + 1
+    begin
+      admin.move(Bytes.toBytes(r.getEncodedName()), Bytes.toBytes(newServer))
+    rescue java.lang.reflect.UndeclaredThrowableException => e
+      $LOG.info("Exception moving "  + r.getEncodedName() +
+        "; split/moved? Continuing: " + e)
+      return
+    end
+    # Wait till its up on new server before moving on
+    maxWaitInSeconds = admin.getConfiguration.getInt("hbase.move.wait.max", 60)
+    maxWait = Time.now + maxWaitInSeconds
+    while Time.now < maxWait
+      same = isSameServer(admin, r, original)
+      break unless same
+      sleep 0.1
+    end
+  end
+  raise RuntimeError, "Region stuck on #{original}, newserver=#{newServer}" if same
+  # Assert can Scan from new location.
+  isSuccessfulScan(admin, r)
+end
+
+# Return the hostname portion of a servername (all up to first ',')
+def getHostnamePortFromServerName(serverName)
+  parts = serverName.split(',')
+  return parts[0] + ":" + parts[1]
+end
+
+# Return the hostname:port out of a servername (all up to first ',')
+def getHostnameFromServerName(serverName)
+  return serverName.split(',')[0]
+end
+
+# Return array of servernames where servername is hostname+port+startcode
+# comma-delimited
+def getServers(admin)
+  serverInfos = admin.getClusterStatus().getServerInfo()
+  servers = []
+  for server in serverInfos
+    servers << server.getServerName()
+  end
+  return servers
+end
+
+# Remove the servername whose hostname portion matches from the passed
+# array of servers.  Returns as side-effect the servername removed.
+def stripServer(servers, hostname)
+  count = servers.length
+  servername = nil
+  for server in servers
+    if getHostnameFromServerName(server) == hostname
+      servername = servers.delete(server)
+    end
+  end
+  # Check server to exclude is actually present
+  raise RuntimeError, "Server %s not online" % hostname unless servers.length < count
+  return servername
+end
+
+# Return servername that matches passed hostname
+def getServerName(servers, hostname)
+  servername = nil
+  for server in servers
+    if getHostnameFromServerName(server) == hostname
+      servername = server
+      break
+    end
+  end
+  raise ArgumentError, "Server %s not online" % hostname unless servername
+  return servername
+end
+
+# Create a logger and disable the DEBUG-level annoying client logging
+def configureLogging(options)
+  apacheLogger = LogFactory.getLog(NAME)
+  # Configure log4j to not spew so much
+  unless (options[:debug]) 
+    logger = org.apache.log4j.Logger.getLogger("org.apache.hadoop.hbase.client")
+    logger.setLevel(org.apache.log4j.Level::INFO)
+  end
+  return apacheLogger
+end
+
+# Get configuration instance
+def getConfiguration()
+  config = HBaseConfiguration.create()
+  # No prefetching on .META.
+  config.setInt("hbase.client.prefetch.limit", 1)
+  # Make a config that retries at short intervals many times
+  config.setInt("hbase.client.pause", 500)
+  config.setInt("hbase.client.retries.number", 100)
+  return config
+end
+
+# Now get list of regions on targetServer
+def getRegions(config, servername)
+  connection = HConnectionManager::getConnection(config)
+  hsa = HServerAddress.new(getHostnamePortFromServerName(servername))
+  rs = connection.getHRegionConnection(hsa)
+  return rs.getOnlineRegions()
+end
+
+def deleteFile(filename)
+  f = java.io.File.new(filename)
+  f.delete() if f.exists()
+end
+
+# Write HRegionInfo to file
+# Need to serialize in case non-printable characters.
+# Format is count of regionnames followed by serialized regionnames.
+def writeFile(filename, regions)
+  fos = java.io.FileOutputStream.new(filename)
+  dos = java.io.DataOutputStream.new(fos)
+  # Write out a count of region names
+  dos.writeInt(regions.size())
+  # Write actual region names.
+  for r in regions
+    bytes = Writables.getBytes(r)
+    Bytes.writeByteArray(dos, bytes)
+  end
+  dos.close()
+end
+
+# See writeFile above.
+# Returns array of HRegionInfos
+def readFile(filename)
+  f = java.io.File.new(filename)
+  return java.util.ArrayList.new() unless f.exists()
+  fis = java.io.FileInputStream.new(f)
+  dis = java.io.DataInputStream.new(fis)
+  # Read count of regions
+  count = dis.readInt()
+  regions = java.util.ArrayList.new(count)
+  index = 0
+  while index < count
+    regions.add(Writables.getHRegionInfo(Bytes.readByteArray(dis)))
+    index = index + 1
+  end
+  dis.close()
+  return regions
+end
+
+# Move regions off the passed hostname
+def unloadRegions(options, hostname)
+  # Get configuration
+  config = getConfiguration()
+  # Clean up any old files.
+  filename = getFilename(options, hostname)
+  deleteFile(filename)
+  # Get an admin instance
+  admin = HBaseAdmin.new(config) 
+  servers = getServers(admin)
+  # Remove the server we are unloading from from list of servers.
+  # Side-effect is the servername that matches this hostname 
+  servername = stripServer(servers, hostname)
+  movedRegions = java.util.ArrayList.new()
+  while true
+    rs = getRegions(config, servername)
+    break if rs.length == 0
+    count = 0
+    $LOG.info("Moving " + rs.length.to_s + " region(s) from " + servername +
+      " during this cycle");
+    for r in rs
+      # Get a random server to move the region to.
+      server = servers[rand(servers.length)]
+      $LOG.info("Moving region " + r.getEncodedName() + " (" + count.to_s +
+        " of " + rs.length.to_s + ") to server=" + server);
+      count = count + 1
+      # Assert we can scan region in its current location
+      isSuccessfulScan(admin, r)
+      # Now move it.
+      move(admin, r, server, servername)
+      movedRegions.add(r)
+    end
+  end
+  if movedRegions.size() > 0 
+    # Write out file of regions moved
+    writeFile(filename, movedRegions)
+    $LOG.info("Wrote list of moved regions to " + filename)
+  end
+end
+
+# Move regions to the passed hostname
+def loadRegions(options, hostname)
+  # Get configuration
+  config = getConfiguration()
+  # Get an admin instance
+  admin = HBaseAdmin.new(config) 
+  filename = getFilename(options, hostname) 
+  regions = readFile(filename)
+  return if regions.isEmpty()
+  servername = nil
+  # Wait till server is up
+  maxWaitInSeconds = admin.getConfiguration.getInt("hbase.serverstart.wait.max", 180)
+  maxWait = Time.now + maxWaitInSeconds
+  while Time.now < maxWait
+    servers = getServers(admin)
+    begin
+      servername = getServerName(servers, hostname)
+    rescue ArgumentError => e
+      $LOG.info("hostname=" + hostname.to_s + " is not up yet, waiting");
+    end
+    break if servername
+    sleep 0.5
+  end
+  $LOG.info("Moving " + regions.size().to_s + " regions to " + servername)
+  count = 0
+  for r in regions
+    exists = false
+    begin
+      exists = isSuccessfulScan(admin, r)
+    rescue org.apache.hadoop.hbase.NotServingRegionException => e
+      $LOG.info("Failed scan of " + e.message)
+    end
+    count = count + 1
+    next unless exists
+    currentServer = getServerNameForRegion(admin, r)
+    if currentServer and currentServer == servername
+      $LOG.info("Region " + r.getRegionNameAsString() + " (" + count.to_s +
+        " of " + regions.length.to_s + ") already on target server=" + servername) 
+      next
+    end
+    $LOG.info("Moving region " + r.getEncodedName() + " (" + count.to_s +
+      " of " + regions.length.to_s + ") to server=" + servername);
+    move(admin, r, servername, currentServer)
+  end
+end
+
+def getFilename(options, targetServer)
+  filename = options[:file]
+  if not filename
+    filename = "/tmp/" + targetServer
+  end
+  return filename
+end
+
+
+# Do command-line parsing
+options = {}
+optparse = OptionParser.new do |opts|
+  opts.banner = "Usage: #{NAME}.rb [options] load|unload <hostname>"
+  opts.separator 'Load or unload regions by moving one at a time'
+  options[:file] = nil
+  opts.on('-f', '--filename=FILE', 'File to save regions list into unloading, or read from
loading; default /tmp/<hostname>') do |file|
+    options[:file] = file
+  end
+  opts.on('-h', '--help', 'Display usage information') do
+    puts opts
+    exit
+  end
+  options[:debug] = false
+  opts.on('-d', '--debug', 'Display extra debug logging') do
+    options[:debug] = true
+  end
+end
+optparse.parse!
+
+# Check ARGVs
+if ARGV.length < 2
+  puts optparse
+  exit 1
+end
+hostname = ARGV[1]
+if not hostname
+  opts optparse
+  exit 2
+end
+# Create a logger and save it to ruby global
+$LOG = configureLogging(options) 
+case ARGV[0]
+  when 'load'
+    loadRegions(options, hostname)
+  when 'unload'
+    unloadRegions(options, hostname)
+  else
+    puts optparse
+    exit 3
+end

Modified: hbase/trunk/src/docbkx/book.xml
URL: http://svn.apache.org/viewvc/hbase/trunk/src/docbkx/book.xml?rev=1088879&r1=1088878&r2=1088879&view=diff
==============================================================================
--- hbase/trunk/src/docbkx/book.xml (original)
+++ hbase/trunk/src/docbkx/book.xml Tue Apr  5 04:08:24 2011
@@ -1076,6 +1076,56 @@ public static byte[][] getHexSplits(Stri
     <section xml:id="compression.tool"><title>Compression Tool</title>
         <para>See <link linkend="compression.tool" >Compression Tool</link>.</para>
     </section>
+    <section xml:id="decommission"><title>Node Decommission</title>
+        <para>You can have a node gradually shed its load and then shutdown using the
+            <command>graceful_restart.sh</command> script.  Here is its usage:
+            <computeroutput>$ ./bin/graceful_stop.sh 
+Usage: graceful_stop.sh [--config &amp;conf-dir>] [--restart] [--reload] &amp;hostname>
+  restart     If we should restart after graceful stop
+  reload      Move offloaded regions back on to the stopped server
+  debug       Move offloaded regions back on to the stopped server
+  hostname    Hostname of server we are to stop</computeroutput>
+        </para>
+        <para>
+            To decommission a loaded regionserver, run the following:
+            <programlisting>$  ./bin/graceful_stop.sh HOSTNAME</programlisting>
+            where <varname>HOSTNAME</varname> is the host carrying the RegionServer
+            you would decommission.  The script will move the regions off the
+            decommissioned regionserver one at a time to minimize region churn.
+            It will verify the region deployed in the new location before it
+            will moves the next region and so on until the decommissioned server
+            is carrying zero regions.  At this point, the <command>graceful_stop</command>
+            tells the RegionServer stop.  The master will at this point notice the
+            RegionServer gone but all regions will have already been redeployed
+            and because the RegionServer went down cleanly, there will be no
+            WAL logs to split.
+            <note><title>Load Balancer</title>
+            <para> 
+                It is assumed that the Region Load Balancer is disabled while the
+                <command>graceful_stop</command> script runs (otherwise the balancer
+                and the decommission script will end up fighting over region deployments).
+                Use the shell to disable the balancer:
+                <programlisting>hbase(main):001:0> balance_switch false
+true
+0 row(s) in 0.3590 seconds</programlisting>
+This turns the balancer OFF.  To reenable, do:
+                <programlisting>hbase(main):001:0> balance_switch true
+false
+0 row(s) in 0.3590 seconds</programlisting>
+            </para> 
+        </note>
+        </para>
+        <para>
+            You can also ask this script to restart a RegionServer after the shutdown
+            AND move its old regions back into place.  The latter you might do to
+            retain data locality.  A primitive rolling restart might be effected by
+            running something like the following:
+            <programlisting>$ for i in `cat conf/regionservers|sort`; do ./bin/graceful_stop.sh
--restart --reload --debug $i; done &amp;> /tmp/log.txt &amp;
+            </programlisting>
+            Tail the output of <filename>/tmp/log.txt</filename> to follow the
scripts
+            progress.
+        </para>
+    </section>
   </appendix>
 
   <appendix xml:id="compression">



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