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Subject [3/3] accumulo git commit: Merge branch '1.6'
Date Mon, 08 Dec 2014 03:09:40 GMT
Merge branch '1.6'



Branch: refs/heads/master
Commit: 7580952a783a5c585a638c5abf811be3131d62d4
Parents: 53991fe 359f851
Author: Josh Elser <>
Authored: Sun Dec 7 22:08:17 2014 -0500
Committer: Josh Elser <>
Committed: Sun Dec 7 22:08:17 2014 -0500

 .../main/asciidoc/chapters/administration.txt   | 29 ++++++++++++++++++++
 1 file changed, 29 insertions(+)
diff --cc docs/src/main/asciidoc/chapters/administration.txt
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+++ b/docs/src/main/asciidoc/chapters/administration.txt
@@@ -1,602 -1,0 +1,631 @@@
 +// 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
 +// 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.
 +== Administration
 +=== Hardware
 +Because we are running essentially two or three systems simultaneously layered
 +across the cluster: HDFS, Accumulo and MapReduce, it is typical for hardware to
 +consist of 4 to 8 cores, and 8 to 32 GB RAM. This is so each running process can have
 +at least one core and 2 - 4 GB each.
 +One core running HDFS can typically keep 2 to 4 disks busy, so each machine may
 +typically have as little as 2 x 300GB disks and as much as 4 x 1TB or 2TB disks.
 +It is possible to do with less than this, such as with 1u servers with 2 cores and 4GB
 +each, but in this case it is recommended to only run up to two processes per
 +machine -- i.e. DataNode and TabletServer or DataNode and MapReduce worker but
 +not all three. The constraint here is having enough available heap space for all the
 +processes on a machine.
 +=== Network
 +Accumulo communicates via remote procedure calls over TCP/IP for both passing
 +data and control messages. In addition, Accumulo uses HDFS clients to
 +communicate with HDFS. To achieve good ingest and query performance, sufficient
 +network bandwidth must be available between any two machines.
 +In addition to needing access to ports associated with HDFS and ZooKeeper, Accumulo will
 +use the following default ports. Please make sure that they are open, or change
 +their value in conf/accumulo-site.xml.
 +.Accumulo default ports
 +|Port | Description | Property Name
 +|4445 | Shutdown Port (Accumulo MiniCluster) | n/a
 +|4560 | Accumulo monitor (for centralized log display) | monitor.port.log4j
 +|9997 | Tablet Server | tserver.port.client
 +|9999 | Master Server | master.port.client
 +|12234 | Accumulo Tracer | trace.port.client
 +|42424 | Accumulo Proxy Server | n/a
 +|50091 | Accumulo GC | gc.port.client
 +|50095 | Accumulo HTTP monitor | monitor.port.client
 +|10001 | Master Replication service | master.replication.coordinator.port
 +|10002 | TabletServer Replication service | replication.receipt.service.port
 +In addition, the user can provide +0+ and an ephemeral port will be chosen instead. This
 +ephemeral port is likely to be unique and not already bound. Thus, configuring ports to
 +use +0+ instead of an explicit value, should, in most cases, work around any issues of
 +running multiple distinct Accumulo instances (or any other process which tries to use the
 +same default ports) on the same hardware.
 +=== Installation
 +Choose a directory for the Accumulo installation. This directory will be referenced
 +by the environment variable +$ACCUMULO_HOME+. Run the following:
 +  $ tar xzf accumulo-1.6.0-bin.tar.gz    # unpack to subdirectory
 +  $ mv accumulo-1.6.0 $ACCUMULO_HOME # move to desired location
 +Repeat this step at each machine within the cluster. Usually all machines have the
 +same +$ACCUMULO_HOME+.
 +=== Dependencies
 +Accumulo requires HDFS and ZooKeeper to be configured and running
 +before starting. Password-less SSH should be configured between at least the
 +Accumulo master and TabletServer machines. It is also a good idea to run Network
 +Time Protocol (NTP) within the cluster to ensure nodes' clocks don't get too out of
 +sync, which can cause problems with automatically timestamped data.
 +=== Configuration
 +Accumulo is configured by editing several Shell and XML files found in
 ++$ACCUMULO_HOME/conf+. The structure closely resembles Hadoop's configuration
 +==== Edit conf/
 +Accumulo needs to know where to find the software it depends on. Edit
 +and specify the following:
 +. Enter the location of the installation directory of Accumulo for +$ACCUMULO_HOME+
 +. Enter your system's Java home for +$JAVA_HOME+
 +. Enter the location of Hadoop for +$HADOOP_PREFIX+
 +. Choose a location for Accumulo logs and enter it for +$ACCUMULO_LOG_DIR+
 +. Enter the location of ZooKeeper for +$ZOOKEEPER_HOME+
 +By default Accumulo TabletServers are set to use 1GB of memory. You may change
 +this by altering the value of +$ACCUMULO_TSERVER_OPTS+. Note the syntax is that of
 +the Java JVM command line options. This value should be less than the physical
 +memory of the machines running TabletServers.
 +There are similar options for the master's memory usage and the garbage collector
 +process. Reduce these if they exceed the physical RAM of your hardware and
 +increase them, within the bounds of the physical RAM, if a process fails because of
 +insufficient memory.
 +Note that you will be specifying the Java heap space in You should
 +make sure that the total heap space used for the Accumulo tserver and the Hadoop
 +DataNode and TaskTracker is less than the available memory on each slave node in
 +the cluster. On large clusters, it is recommended that the Accumulo master, Hadoop
 +NameNode, secondary NameNode, and Hadoop JobTracker all be run on separate
 +machines to allow them to use more heap space. If you are running these on the
 +same machine on a small cluster, likewise make sure their heap space settings fit
 +within the available memory.
 +==== Native Map
 +The tablet server uses a data structure called a MemTable to store sorted key/value
 +pairs in memory when they are first received from the client. When a minor compaction
 +occurs, this data structure is written to HDFS. The MemTable will default to using
 +memory in the JVM but a JNI version, called the native map, can be used to significantly
 +speed up performance by utilizing the memory space of the native operating system. The
 +native map also avoids the performance implications brought on by garbage collection
 +in the JVM by causing it to pause much less frequently.
++===== Building
 +32-bit and 64-bit Linux and Mac OS X versions of the native map can be built
 +from the Accumulo bin package by executing
 ++$ACCUMULO_HOME/bin/ If your system's
 +default compiler options are insufficient, you can add additional compiler
 +options to the command line, such as options for the architecture. These will be
 +passed to the Makefile in the environment variable +USERFLAGS+.
 +. +$ACCUMULO_HOME/bin/
 +. +$ACCUMULO_HOME/bin/ -m32+
 +After building the native map from the source, you will find the artifact in
 ++$ACCUMULO_HOME/lib/native+. Upon starting up, the tablet server will look
 +in this directory for the map library. If the file is renamed or moved from its
 +target directory, the tablet server may not be able to find it. The system can
 +also locate the native maps shared library by setting +LD_LIBRARY_PATH+
 +(or +DYLD_LIBRARY_PATH+ on Mac OS X) in +$ACCUMULO_HOME/conf/
++===== Configuration
++As mentioned, Accumulo will use the native libraries if they are found in the expected
++location and if it is not configured to ignore them. Using the native maps over JVM
++Maps nets a noticable improvement in ingest rates; however, certain configuration
++variables are important to modify when increasing the size of the native map.
++To adjust the size of the native map, increase the value of +tserver.memory.maps.max+.
++By default, the maximum size of the native map is 1GB. When increasing this value, it is
++also important to adjust the values of +table.compaction.minor.logs.threshold+ and
+++tserver.walog.max.size+. +table.compaction.minor.logs.threshold+ is the maximum
++number of write-ahead log files that a tablet can reference before they will be automatically
++minor compacted. +tserver.walog.max.size+ is the maximum size of a write-ahead log.
++The maximum size of the native maps for a server should be less than the product
++of the write-ahead log maximum size and minor compaction threshold for log files:
+++$table.compaction.minor.logs.threshold * $tserver.walog.max.size >= $tserver.memory.maps.max+
++This formula ensures that minor compactions won't be automatically triggered before the
++maps can be completely saturated.
++Subsequently, when increasing the size of the write-ahead logs, it can also be important
++to increase the HDFS block size that Accumulo uses when creating the files for the write-ahead
++This is controlled via +tserver.wal.blocksize+. A basic recommendation is that when
+++tserver.walog.max.size+ is larger than 2GB in size, set +tserver.wal.blocksize+ to 2GB.
 +==== Cluster Specification
 +On the machine that will serve as the Accumulo master:
 +. Write the IP address or domain name of the Accumulo Master to the +$ACCUMULO_HOME/conf/masters+
 +. Write the IP addresses or domain name of the machines that will be TabletServers in +$ACCUMULO_HOME/conf/slaves+,
one per line.
 +Note that if using domain names rather than IP addresses, DNS must be configured
 +properly for all machines participating in the cluster. DNS can be a confusing source
 +of errors.
 +==== Accumulo Settings
 +Specify appropriate values for the following settings in
 ++$ACCUMULO_HOME/conf/accumulo-site.xml+ :
 +    <name></name>
 +    <value>zooserver-one:2181,zooserver-two:2181</value>
 +    <description>list of zookeeper servers</description>
 +This enables Accumulo to find ZooKeeper. Accumulo uses ZooKeeper to coordinate
 +settings between processes and helps finalize TabletServer failure.
 +    <name>instance.secret</name>
 +    <value>DEFAULT</value>
 +The instance needs a secret to enable secure communication between servers. Configure your
 +secret and make sure that the +accumulo-site.xml+ file is not readable to other users.
 +For alternatives to storing the +instance.secret+ in plaintext, please read the
 ++Sensitive Configuration Values+ section.
 +Some settings can be modified via the Accumulo shell and take effect immediately, but
 +some settings require a process restart to take effect. See the configuration documentation
 +(available in the docs directory of the tarball and in <<configuration>>) for
 +==== Deploy Configuration
 +Copy the masters, slaves,, and if necessary, accumulo-site.xml
 +from the +$ACCUMULO_HOME/conf/+ directory on the master to all the machines
 +specified in the slaves file.
 +==== Sensitive Configuration Values
 +Accumulo has a number of properties that can be specified via the accumulo-site.xml
 +file which are sensitive in nature, instance.secret and
 +are two common examples. Both of these properties, if compromised, have the ability
 +to result in data being leaked to users who should not have access to that data.
 +In Hadoop-2.6.0, a new CredentialProvider class was introduced which serves as a common
 +implementation to abstract away the storage and retrieval of passwords from plaintext
 +storage in configuration files. Any Property marked with the +Sensitive+ annotation
 +is a candidate for use with these CredentialProviders. For version of Hadoop which lack
 +these classes, the feature will just be unavailable for use.
 +A comma separated list of CredentialProviders can be configured using the Accumulo Property Each configured URL will be consulted
 +when the Configuration object for accumulo-site.xml is accessed.
 +==== Using a JavaKeyStoreCredentialProvider for storage
 +One of the implementations provided in Hadoop-2.6.0 is a Java KeyStore CredentialProvider.
 +Each entry in the KeyStore is the Accumulo Property key name. For example, to store the
 +\texttt{instance.secret}, the following command can be used:
 +  hadoop credential create instance.secret --provider jceks://file/etc/accumulo/conf/accumulo.jceks
 +The command will then prompt you to enter the secret to use and create a keystore in: 
 +  /etc/accumulo/conf/accumulo.jceks
 +Then, accumulo-site.xml must be configured to use this KeyStore as a CredentialProvider:
 +    <name></name>
 +    <value>jceks://file/etc/accumulo/conf/accumulo.jceks</value>
 +This configuration will then transparently extract the +instance.secret+ from
 +the configured KeyStore and alleviates a human readable storage of the sensitive
 +A KeyStore can also be stored in HDFS, which will make the KeyStore readily available to
 +all Accumulo servers. If the local filesystem is used, be aware that each Accumulo server
 +will expect the KeyStore in the same location.
 +==== Custom Table Tags
 +Accumulo has the ability for users to add custom tags to tables.  This allows
 +applications to set application-level metadata about a table.  These tags can be
 +anything from a table description, administrator notes, date created, etc.
 +This is done by naming and setting a property with a prefix +table.custom.*+.
 +=== Initialization
 +Accumulo must be initialized to create the structures it uses internally to locate
 +data across the cluster. HDFS is required to be configured and running before
 +Accumulo can be initialized.
 +Once HDFS is started, initialization can be performed by executing
 ++$ACCUMULO_HOME/bin/accumulo init+ . This script will prompt for a name
 +for this instance of Accumulo. The instance name is used to identify a set of tables
 +and instance-specific settings. The script will then write some information into
 +HDFS so Accumulo can start properly.
 +The initialization script will prompt you to set a root password. Once Accumulo is
 +initialized it can be started.
 +=== Running
 +==== Starting Accumulo
 +Make sure Hadoop is configured on all of the machines in the cluster, including
 +access to a shared HDFS instance. Make sure HDFS and ZooKeeper are running.
 +Make sure ZooKeeper is configured and running on at least one machine in the
 +Start Accumulo using the +bin/ script.
 +To verify that Accumulo is running, check the Status page as described in
 +<<monitoring>>. In addition, the Shell can provide some information about the
status of
 +tables via reading the metadata tables.
 +==== Stopping Accumulo
 +To shutdown cleanly, run +bin/ and the master will orchestrate the
 +shutdown of all the tablet servers. Shutdown waits for all minor compactions to finish,
so it may
 +take some time for particular configurations.
 +==== Adding a Node
 +Update your +$ACCUMULO_HOME/conf/slaves+ (or +$ACCUMULO_CONF_DIR/slaves+) file to account
for the addition.
 +  $ACCUMULO_HOME/bin/accumulo admin start <host(s)> {<host> ...}
 +Alternatively, you can ssh to each of the hosts you want to add and run:
 +Make sure the host in question has the new configuration, or else the tablet
 +server won't start; at a minimum this needs to be on the host(s) being added,
 +but in practice it's good to ensure consistent configuration across all nodes.
 +==== Decomissioning a Node
 +If you need to take a node out of operation, you can trigger a graceful shutdown of a tablet
 +server. Accumulo will automatically rebalance the tablets across the available tablet servers.
 +  $ACCUMULO_HOME/bin/accumulo admin stop <host(s)> {<host> ...}
 +Alternatively, you can ssh to each of the hosts you want to remove and run:
 +Be sure to update your +$ACCUMULO_HOME/conf/slaves+ (or +$ACCUMULO_CONF_DIR/slaves+) file
 +account for the removal of these hosts. Bear in mind that the monitor will not re-read the
 +slaves file automatically, so it will report the decomissioned servers as down; it's
 +recommended that you restart the monitor so that the node list is up to date.
 +=== Monitoring
 +==== Accumulo Monitor
 +The Accumulo Monitor provides an interface for monitoring the status and health of
 +Accumulo components. The Accumulo Monitor provides a web UI for accessing this information
 +Things highlighted in yellow may be in need of attention.
 +If anything is highlighted in red on the monitor page, it is something that definitely needs
 +The Overview page contains some summary information about the Accumulo instance, including
the version, instance name, and instance ID.
 +There is a table labeled Accumulo Master with current status, a table listing the active
Zookeeper servers, and graphs displaying various metrics over time.
 +These include ingest and scan performance and other useful measurements.
 +The Master Server, Tablet Servers, and Tables pages display metrics grouped in different
ways (e.g. by tablet server or by table).
 +Metrics typically include number of entries (key/value pairs), ingest and query rates.
 +The number of running scans, major and minor compactions are in the form _number_running_
 +Another important metric is hold time, which is the amount of time a tablet has been waiting
but unable to flush its memory in a minor compaction.
 +The Server Activity page graphically displays tablet server status, with each server represented
as a circle or square.
 +Different metrics may be assigned to the nodes' color and speed of oscillation.
 +The Overall Avg metric is only used on the Server Activity page, and represents the average
of all the other metrics (after normalization).
 +Similarly, the Overall Max metric picks the metric with the maximum normalized value.
 +The Garbage Collector page displays a list of garbage collection cycles, the number of files
found of each type (including deletion candidates in use and files actually deleted), and
the length of the deletion cycle.
 +The Traces page displays data for recent traces performed (see the following section for
information on <<tracing>>).
 +The Recent Logs page displays warning and error logs forwarded to the monitor from all Accumulo
 +Also, the XML and JSON links provide metrics in XML and JSON formats, respectively.
 +==== SSL
 +SSL may be enabled for the monitor page by setting the following properties in the +accumulo-site.xml+
 +  monitor.ssl.keyStore
 +  monitor.ssl.keyStorePassword
 +  monitor.ssl.trustStore
 +  monitor.ssl.trustStorePassword
 +If the Accumulo conf directory has been configured (in particular the
file must be set up), the script in the Accumulo +bin+ directory
can be used to create the keystore and truststore files with random passwords.
 +The script will print out the properties that need to be added to the +accumulo-site.xml+
 +The stores can also be generated manually with the Java +keytool+ command, whose usage can
be seen in the script.
 +If desired, the SSL ciphers allowed for connections can be controlled via the following
properties in +accumulo-site.xml+:
 +  monitor.ssl.include.ciphers
 +  monitor.ssl.exclude.ciphers
 +If SSL is enabled, the monitor URL can only be accessed via https.
 +This also allows you to access the Accumulo shell through the monitor page.
 +The left navigation bar will have a new link to Shell.
 +An Accumulo user name and password must be entered for access to the shell.
 +=== Metrics
 +Accumulo is capable of using the Hadoop Metrics2 library and is configured by default to
use it. Metrics2 is a library
 +which allows for routing of metrics generated by registered MetricsSources to configured
MetricsSinks. Examples of sinks
 +that are implemented by Hadoop include file-based logging, Graphite and Ganglia. All metric
sources are exposed via JMX
 +when using Metrics2.
 +Previous to Accumulo 1.7.0, JMX endpoints could be exposed in addition to file-based logging
of those metrics configured via
 +the +accumulo-metrics.xml+ file. This mechanism can still be used by setting +general.legacy.metrics+
to +true+ in +accumulo-site.xml+.
 +==== Metrics2 Configuration
 +Metrics2 is configured by examining the classpath for a file that matches +hadoop-metrics2*.properties+.
The example configuration
 +files that Accumulo provides for use include as a
template which can be used to enable 
 +file, Graphite or Ganglia sinks (some minimal configuration required for Graphite and Ganglia).
Because the Hadoop configuration is
 +also on the Accumulo classpath, be sure that you do not have multiple Metrics2 configuration
files. It is recommended to consolidate
 +metrics in a single properties file in a central location to remove ambiguity. The contents
 +can be added to a central in +$HADOOP_CONF_DIR+.
 +As a note for configuring the file sink, the provided path should be absolute. A relative
path or file name will be created relative
 +to the directory in which the Accumulo process was started. External tools, such as logrotate,
can be used to prevent these files
 +from growing without bound.
 +Each server process should have log messages from the Metrics2 library about the sinks that
were created. Be sure to check
 +the Accumulo processes log files when debugging missing metrics output.
 +For additional information on configuring Metrics2, visit the
page for Metrics2].
 +=== Tracing
 +It can be difficult to determine why some operations are taking longer
 +than expected. For example, you may be looking up items with very low
 +latency, but sometimes the lookups take much longer. Determining the
 +cause of the delay is difficult because the system is distributed, and
 +the typical lookup is fast.
 +Accumulo has been instrumented to record the time that various
 +operations take when tracing is turned on. The fact that tracing is
 +enabled follows all the requests made on behalf of the user throughout
 +the distributed infrastructure of accumulo, and across all threads of
 +These time spans will be inserted into the +trace+ table in
 +Accumulo. You can browse recent traces from the Accumulo monitor
 +page. You can also read the +trace+ table directly like any
 +other table.
 +The design of Accumulo's distributed tracing follows that of
 +[Google's Dapper].
 +==== Tracers
 +To collect traces, Accumulo needs at least one server listed in
 + +$ACCUMULO_HOME/conf/tracers+. The server collects traces
 +from clients and writes them to the +trace+ table. The Accumulo
 +user that the tracer connects to Accumulo with can be configured with
 +the following properties
 +  trace.user
 +Other tracer configuration properties include
 +  trace.port.client
 +  trace.table
 +==== Configuring Tracing
 +Traces are collected via SpanReceivers. The default SpanReceiver
 +configured is org.apache.accumulo.core.trace.ZooTraceClient, which
 +sends spans to an Accumulo Tracer process, as discussed in the
 +previous section. This default can be changed to a different span
 +receiver, or additional span receivers can be added in a
 +comma-separated list, by modifying the property
 +  trace.span.receivers
 +Individual span receivers may require their own configuration
 +parameters, which are grouped under the trace.span.receiver.*
 +prefix.  The ZooTraceClient requires the following property that
 +indicates where the tracer servers will register themselves in
 +  trace.span.receiver.zookeeper.path
 +This is configured to /tracers by default.  If multiple Accumulo
 +instances are sharing the same ZooKeeper quorum, take care to
 +configure Accumulo with unique values for this property.
 +Hadoop can also be configured to send traces to Accumulo, as of
 +Hadoop 2.6.0, by setting the following properties in Hadoop's
 +core-site.xml file (the path property is optional if left as the
 +  <property>
 +    <name>hadoop.htrace.spanreceiver.classes</name>
 +    <value>org.apache.accumulo.core.trace.ZooTraceClient</value>
 +  </property>
 +  <property>
 +    <name></name>
 +    <value>zookeeperHost:2181</value>
 +  </property>
 +  <property>
 +    <name>hadoop.tracer.zookeeper.path</name>
 +    <value>/tracers</value>
 +  </property>
 +The accumulo-core, accumulo-trace, and libthrift jars must also
 +be placed on Hadoop's classpath.
 +==== Instrumenting a Client
 +Tracing can be used to measure a client operation, such as a scan, as
 +the operation traverses the distributed system. To enable tracing for
 +your application call
 +import org.apache.accumulo.core.trace.DistributedTrace;
 +DistributedTrace.enable(hostname, "myApplication");
 +// do some tracing
 +Once tracing has been enabled, a client can wrap an operation in a trace.
 +import org.htrace.Sampler;
 +import org.htrace.Trace;
 +import org.htrace.TraceScope;
 +TraceScope scope = Trace.startSpan("Client Scan", Sampler.ALWAYS);
 +BatchScanner scanner = conn.createBatchScanner(...);
 +// Configure your scanner
 +for (Entry entry : scanner) {
 +Additionally, the user can create additional Spans within a Trace.
 +The sampler for the trace should only be specified with the first span, and subsequent spans
will be collected depending on whether that first span was sampled.
 +TraceScope scope = Trace.startSpan("Client Update", Sampler.ALWAYS);
 +TraceScope readScope = Trace.startSpan("Read");
 +TraceScope writeScope = Trace.startSpan("Write");
 +Like Dapper, Accumulo tracing supports user defined annotations to associate additional
data with a Trace.
 +Checking whether currently tracing is necessary when using a sampler other than Sampler.ALWAYS.
 +int numberOfEntriesRead = 0;
 +TraceScope readScope = Trace.startSpan("Read");
 +// Do the read, update the counter
 +if (Trace.isTracing)
 +  readScope.getSpan().addKVAnnotation("Number of Entries Read".getBytes(StandardCharsets.UTF_8),
 +      String.valueOf(numberOfEntriesRead).getBytes(StandardCharsets.UTF_8));
 +It is also possible to add timeline annotations to your spans.
 +This associates a string with a given timestamp between the start and stop times for a span.
 +writeScope.getSpan().addTimelineAnnotation("Initiating Flush");
 +Some client operations may have a high volume within your
 +application. As such, you may wish to only sample a percentage of
 +operations for tracing. As seen below, the CountSampler can be used to
 +help enable tracing for 1-in-1000 operations
 +import org.htrace.impl.CountSampler;
 +Sampler sampler = new CountSampler(1000);
 +TraceScope readScope = Trace.startSpan("Read", sampler);
 +Remember to close all spans and disable tracing when finished.
 +==== Viewing Collected Traces
 +To view collected traces, use the "Recent Traces" link on the Monitor
 +UI. You can also programmatically access and print traces using the
 ++TraceDump+ class.
 +==== Tracing from the Shell
 +You can enable tracing for operations run from the shell by using the
 ++trace on+ and +trace off+ commands.
 +root@test test> trace on
 +root@test test> scan
 +a b:c []    d
 +root@test test> trace off
 +Waiting for trace information
 +Waiting for trace information
 +Trace started at 2013/08/26 13:24:08.332
 +Time  Start  Service@Location       Name
 + 3628+0      shell@localhost shell:root
 +    8+1690     shell@localhost scan
 +    7+1691       shell@localhost scan:location
 +    6+1692         tserver@localhost startScan
 +    5+1692           tserver@localhost tablet read ahead 6
 +=== Logging
 +Accumulo processes each write to a set of log files. By default these are found under
 +=== Recovery
 +In the event of TabletServer failure or error on shutting Accumulo down, some
 +mutations may not have been minor compacted to HDFS properly. In this case,
 +Accumulo will automatically reapply such mutations from the write-ahead log
 +either when the tablets from the failed server are reassigned by the Master (in the
 +case of a single TabletServer failure) or the next time Accumulo starts (in the event of
 +failure during shutdown).
 +Recovery is performed by asking a tablet server to sort the logs so that tablets can easily
find their missing
 +updates. The sort status of each file is displayed on
 +Accumulo monitor status page. Once the recovery is complete any
 +tablets involved should return to an ``online'' state. Until then those tablets will be
 +unavailable to clients.
 +The Accumulo client library is configured to retry failed mutations and in many
 +cases clients will be able to continue processing after the recovery process without
 +throwing an exception.

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