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+ <p><a href="/docs/unreleased/">Accumulo unreleased docs</a>
>> Administration >> Multi-Volume
Installations</p>
+
+
+ <h1>Multi-Volume Installations</h1>
+
+ <p>This is an advanced configuration setting for very large clusters
+under a lot of write pressure.</p>
+
+<p>The HDFS NameNode holds all of the metadata about the files in
+HDFS. For fast performance, all of this information needs to be stored
+in memory. A single NameNode with 64G of memory can store the
+metadata for tens of millions of files.However, when scaling beyond a
+thousand nodes, an active Accumulo system can generate lots of updates
+to the file system, especially when data is being ingested. The large
+number of write transactions to the NameNode, and the speed of a
+single edit log, can become the limiting factor for large scale
+Accumulo installations.</p>
+
+<p>You can see the effect of slow write transactions when the Accumulo
+Garbage Collector takes a long time (more than 5 minutes) to delete
+the files Accumulo no longer needs. If your Garbage Collector
+routinely runs in less than a minute, the NameNode is performing well.</p>
+
+<p>However, if you do begin to experience slow-down and poor GC
+performance, Accumulo can be configured to use multiple NameNode
+servers. The configuration <code
class="highlighter-rouge">instance.volumes</code> should be set to a
+comma-separated list, using full URI references to different NameNode
+servers:</p>
+
+<div class="language-xml highlighter-rouge"><pre class="highlight"><code><span
class="nt"><property></span>
+ <span class="nt"><name></span>instance.volumes<span
class="nt"></name></span>
+ <span class="nt"><value></span>hdfs://ns1:9001,hdfs://ns2:9001<span
class="nt"></value></span>
+<span class="nt"></property></span>
+</code></pre>
+</div>
+
+<p>The introduction of multiple volume support in 1.6 changed the way Accumulo
+stores pointers to files. It now stores fully qualified URI references to
+files. Before 1.6, Accumulo stored paths that were relative to a table
+directory. After an upgrade these relative paths will still exist and are
+resolved using instance.dfs.dir, instance.dfs.uri, and Hadoop configuration in
+the same way they were before 1.6.</p>
+
+<p>If the URI for a namenode changes (e.g. namenode was running on host1 and
its
+moved to host2), then Accumulo will no longer function. Even if Hadoop and
+Accumulo configurations are changed, the fully qualified URIs stored in
+Accumulo will still contain the old URI. To handle this Accumulo has the
+following configuration property for replacing URI stored in its metadata. The
+example configuration below will replace ns1 with nsA and ns2 with nsB in
+Accumulo metadata. For this property to take affect, Accumulo will need to be
+restarted.</p>
+
+<div class="language-xml highlighter-rouge"><pre class="highlight"><code><span
class="nt"><property></span>
+ <span class="nt"><name></span>instance.volumes.replacements<span
class="nt"></name></span>
+ <span class="nt"><value></span>hdfs://ns1:9001 hdfs://nsA:9001,
hdfs://ns2:9001 hdfs://nsB:9001<span class="nt"></value></span>
+<span class="nt"></property></span>
+</code></pre>
+</div>
+
+<p>Using viewfs or HA namenode, introduced in Hadoop 2, offers another option
for
+managing the fully qualified URIs stored in Accumulo. Viewfs and HA namenode
+both introduce a level of indirection in the Hadoop configuration. For
+example assume viewfs:///nn1 maps to hdfs://nn1 in the Hadoop configuration.
+If viewfs://nn1 is used by Accumulo, then its easy to map viewfs://nn1 to
+hdfs://nnA by changing the Hadoop configuration w/o doing anything to Accumulo.
+A production system should probably use a HA namenode. Viewfs may be useful on
+a test system with a single non HA namenode.</p>
+
+<p>You may also want to configure your cluster to use Federation,
+available in Hadoop 2.0, which allows DataNodes to respond to multiple
+NameNode servers, so you do not have to partition your DataNodes by
+NameNode.</p>
+
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