Depending on the Instance your using, the client may hit HDFS first to get some instance information. Then either instance will hit Zookeeper to get the root tablet location. It will then scan that to find the appropriate !METADATA tablet, but will aggressively cache everything. The client will then scan (with heavy caching again) the !METADATA tablets necessary to find the tablet(s) for the range of writes. And then the writes will occur to appropriate tablets discovered in the !METADATA scan.

I built 1.5 from source last night and wanted to try it out on my existing Hadoop cluster without overwriting my current 1.4 set. Is there a way to specify the /accumulo directory in HDFS such that you can run multiple instances?

From the monitoring user interface, see the Documentation link, then Configuration, see the first property:

instance.dfs.dir

You'll also change all the port numbers from the defaults. And there's a port number in conf/generic_logger.xml that points to the logging port on the monitor.

For example, here are some entries from my conf/accumulo-site.xml file:

<property>
<name>master.port.client</name>
<value>10010</value>
</property>

<property>
<name>tserver.port.client</name>
<value>10011</value>
</property>

<property>
<name>gc.port.client</name>
<value>10101</value>
</property>

<property>
<name>trace.port.client</name>
<value>10111</value>
</property>

<property>
<name>monitor.port.client</name>
<value>11111</value>
</property>

<property>
<name>monitor.port.log4j</name>
<value>1560</value>
</property>

And conf generic_logger.xml:

<!-- Send all logging data to a centralized logger -->
<appender name="N1" class="org.apache.log4j.net.SocketAppender">
<param name="remoteHost" value="${org.apache.accumulo.core.host.log}"/>
<param name="port" value="1560"/>
<param name="application" value="${org.apache.accumulo.core.application}:${org.apache.accumulo.core.ip.localhost.hostname}"/>
<param name="Threshold" value="WARN"/>
</appender>