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<li class="toctree-l1 current"><a class="current reference internal" href="#">Frequently Asked Questions</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#why-can-t-i-set-listen-address-to-listen-on-0-0-0-0-all-my-addresses">Why can’t I set <code class="docutils literal notranslate"><span class="pre">listen_address</span></code> to listen on 0.0.0.0 (all my addresses)?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#what-ports-does-cassandra-use">What ports does Cassandra use?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#what-happens-to-existing-data-in-my-cluster-when-i-add-new-nodes">What happens to existing data in my cluster when I add new nodes?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#i-delete-data-from-cassandra-but-disk-usage-stays-the-same-what-gives">I delete data from Cassandra, but disk usage stays the same. What gives?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#why-does-nodetool-ring-only-show-one-entry-even-though-my-nodes-logged-that-they-see-each-other-joining-the-ring">Why does nodetool ring only show one entry, even though my nodes logged that they see each other joining the ring?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#can-i-change-the-replication-factor-a-a-keyspace-on-a-live-cluster">Can I change the replication factor (a a keyspace) on a live cluster?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#can-i-store-large-blobs-in-cassandra">Can I Store (large) BLOBs in Cassandra?</a></li>
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<li class="toctree-l2"><a class="reference internal" href="#will-batching-my-operations-speed-up-my-bulk-load">Will batching my operations speed up my bulk load?</a></li>
<li class="toctree-l2"><a class="reference internal" href="#on-rhel-nodes-are-unable-to-join-the-ring">On RHEL nodes are unable to join the ring</a></li>
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<li class="toctree-l2"><a class="reference internal" href="#does-single-seed-mean-single-point-of-failure">Does single seed mean single point of failure?</a></li>
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<li class="toctree-l2"><a class="reference internal" href="#what-happens-if-two-updates-are-made-with-the-same-timestamp">What happens if two updates are made with the same timestamp?</a></li>
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  <div class="section" id="frequently-asked-questions">
<h1>Frequently Asked Questions<a class="headerlink" href="#frequently-asked-questions" title="Permalink to this headline">¶</a></h1>
<ul class="simple">
<li><a class="reference internal" href="#why-cant-list-all"><span class="std std-ref">Why can’t I set listen_address to listen on 0.0.0.0 (all my addresses)?</span></a></li>
<li><a class="reference internal" href="#what-ports"><span class="std std-ref">What ports does Cassandra use?</span></a></li>
<li><a class="reference internal" href="#what-happens-on-joins"><span class="std std-ref">What happens to existing data in my cluster when I add new nodes?</span></a></li>
<li><a class="reference internal" href="#asynch-deletes"><span class="std std-ref">I delete data from Cassandra, but disk usage stays the same. What gives?</span></a></li>
<li><a class="reference internal" href="#one-entry-ring"><span class="std std-ref">Why does nodetool ring only show one entry, even though my nodes logged that they see each other joining the ring?</span></a></li>
<li><a class="reference internal" href="#can-large-blob"><span class="std std-ref">Can I Store (large) BLOBs in Cassandra?</span></a></li>
<li><a class="reference internal" href="#nodetool-connection-refused"><span class="std std-ref">Nodetool says “Connection refused to host: 127.0.1.1” for any remote host. What gives?</span></a></li>
<li><a class="reference internal" href="#to-batch-or-not-to-batch"><span class="std std-ref">Will batching my operations speed up my bulk load?</span></a></li>
<li><a class="reference internal" href="#selinux"><span class="std std-ref">On RHEL nodes are unable to join the ring</span></a></li>
<li><a class="reference internal" href="#how-to-unsubscribe"><span class="std std-ref">How do I unsubscribe from the email list?</span></a></li>
<li><a class="reference internal" href="#cassandra-eats-all-my-memory"><span class="std std-ref">Why does top report that Cassandra is using a lot more memory than the Java heap max?</span></a></li>
<li><a class="reference internal" href="#what-are-seeds"><span class="std std-ref">What are seeds?</span></a></li>
<li><a class="reference internal" href="#are-seeds-spof"><span class="std std-ref">Does single seed mean single point of failure?</span></a></li>
<li><a class="reference internal" href="#why-message-dropped"><span class="std std-ref">Why do I see “… messages dropped …” in the logs?</span></a></li>
<li><a class="reference internal" href="#oom-map-failed"><span class="std std-ref">Cassandra dies with java.lang.OutOfMemoryError: Map failed</span></a></li>
<li><a class="reference internal" href="#what-on-same-timestamp-update"><span class="std std-ref">What happens if two updates are made with the same timestamp?</span></a></li>
<li><a class="reference internal" href="#why-bootstrapping-stream-error"><span class="std std-ref">Why bootstrapping a new node fails with a “Stream failed” error?</span></a></li>
</ul>
<div class="section" id="why-can-t-i-set-listen-address-to-listen-on-0-0-0-0-all-my-addresses">
<span id="why-cant-list-all"></span><h2>Why can’t I set <code class="docutils literal notranslate"><span class="pre">listen_address</span></code> to listen on 0.0.0.0 (all my addresses)?<a class="headerlink" href="#why-can-t-i-set-listen-address-to-listen-on-0-0-0-0-all-my-addresses" title="Permalink to this headline">¶</a></h2>
<p>Cassandra is a gossip-based distributed system and <code class="docutils literal notranslate"><span class="pre">listen_address</span></code> is the address a node tells other nodes to reach
it at. Telling other nodes “contact me on any of my addresses” is a bad idea; if different nodes in the cluster pick
different addresses for you, Bad Things happen.</p>
<p>If you don’t want to manually specify an IP to <code class="docutils literal notranslate"><span class="pre">listen_address</span></code> for each node in your cluster (understandable!), leave
it blank and Cassandra will use <code class="docutils literal notranslate"><span class="pre">InetAddress.getLocalHost()</span></code> to pick an address. Then it’s up to you or your ops team
to make things resolve correctly (<code class="docutils literal notranslate"><span class="pre">/etc/hosts/</span></code>, dns, etc).</p>
<p>One exception to this process is JMX, which by default binds to 0.0.0.0 (Java bug 6425769).</p>
<p>See <a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-256">CASSANDRA-256</a> and <a class="reference external" href="https://issues.apache.org/jira/browse/CASSANDRA-43">CASSANDRA-43</a> for more gory details.</p>
</div>
<div class="section" id="what-ports-does-cassandra-use">
<span id="what-ports"></span><h2>What ports does Cassandra use?<a class="headerlink" href="#what-ports-does-cassandra-use" title="Permalink to this headline">¶</a></h2>
<p>By default, Cassandra uses 7000 for cluster communication (7001 if SSL is enabled),  9042 for native protocol clients,
and 7199 for JMX. The internode communication and native protocol ports
are configurable in the <a class="reference internal" href="../configuration/cassandra_config_file.html#cassandra-yaml"><span class="std std-ref">Cassandra Configuration File</span></a>. The JMX port is configurable in <code class="docutils literal notranslate"><span class="pre">cassandra-env.sh</span></code> (through JVM
options). All ports are TCP.</p>
</div>
<div class="section" id="what-happens-to-existing-data-in-my-cluster-when-i-add-new-nodes">
<span id="what-happens-on-joins"></span><h2>What happens to existing data in my cluster when I add new nodes?<a class="headerlink" href="#what-happens-to-existing-data-in-my-cluster-when-i-add-new-nodes" title="Permalink to this headline">¶</a></h2>
<p>When a new nodes joins a cluster, it will automatically contact the other nodes in the cluster and copy the right data
to itself. See <a class="reference internal" href="../operating/topo_changes.html#topology-changes"><span class="std std-ref">Adding, replacing, moving and removing nodes</span></a>.</p>
</div>
<div class="section" id="i-delete-data-from-cassandra-but-disk-usage-stays-the-same-what-gives">
<span id="asynch-deletes"></span><h2>I delete data from Cassandra, but disk usage stays the same. What gives?<a class="headerlink" href="#i-delete-data-from-cassandra-but-disk-usage-stays-the-same-what-gives" title="Permalink to this headline">¶</a></h2>
<p>Data you write to Cassandra gets persisted to SSTables. Since SSTables are immutable, the data can’t actually be removed
when you perform a delete, instead, a marker (also called a “tombstone”) is written to indicate the value’s new status.
Never fear though, on the first compaction that occurs between the data and the tombstone, the data will be expunged
completely and the corresponding disk space recovered. See <a class="reference internal" href="../operating/compaction.html#compaction"><span class="std std-ref">Compaction</span></a> for more detail.</p>
</div>
<div class="section" id="why-does-nodetool-ring-only-show-one-entry-even-though-my-nodes-logged-that-they-see-each-other-joining-the-ring">
<span id="one-entry-ring"></span><h2>Why does nodetool ring only show one entry, even though my nodes logged that they see each other joining the ring?<a class="headerlink" href="#why-does-nodetool-ring-only-show-one-entry-even-though-my-nodes-logged-that-they-see-each-other-joining-the-ring" title="Permalink to this headline">¶</a></h2>
<p>This happens when you have the same token assigned to each node. Don’t do that.</p>
<p>Most often this bites people who deploy by installing Cassandra on a VM (especially when using the Debian package, which
auto-starts Cassandra after installation, thus generating and saving a token), then cloning that VM to other nodes.</p>
<p>The easiest fix is to wipe the data and commitlog directories, thus making sure that each node will generate a random
token on the next restart.</p>
</div>
<div class="section" id="can-i-change-the-replication-factor-a-a-keyspace-on-a-live-cluster">
<span id="change-replication-factor"></span><h2>Can I change the replication factor (a a keyspace) on a live cluster?<a class="headerlink" href="#can-i-change-the-replication-factor-a-a-keyspace-on-a-live-cluster" title="Permalink to this headline">¶</a></h2>
<p>Yes, but it will require running a full repair (or cleanup) to change the replica count of existing data:</p>
<ul class="simple">
<li><a class="reference internal" href="../cql/ddl.html#alter-keyspace-statement"><span class="std std-ref">Alter</span></a> the replication factor for desired keyspace (using cqlsh for instance).</li>
<li>If you’re reducing the replication factor, run <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">cleanup</span></code> on the cluster to remove surplus replicated data.
Cleanup runs on a per-node basis.</li>
<li>If you’re increasing the replication factor, run <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">repair</span> <span class="pre">-full</span></code> to ensure data is replicated according to the new
configuration. Repair runs on a per-replica set basis. This is an intensive process that may result in adverse cluster
performance. It’s highly recommended to do rolling repairs, as an attempt to repair the entire cluster at once will
most likely swamp it. Note that you will need to run a full repair (<code class="docutils literal notranslate"><span class="pre">-full</span></code>) to make sure that already repaired
sstables are not skipped.</li>
</ul>
</div>
<div class="section" id="can-i-store-large-blobs-in-cassandra">
<span id="can-large-blob"></span><h2>Can I Store (large) BLOBs in Cassandra?<a class="headerlink" href="#can-i-store-large-blobs-in-cassandra" title="Permalink to this headline">¶</a></h2>
<p>Cassandra isn’t optimized for large file or BLOB storage and a single <code class="docutils literal notranslate"><span class="pre">blob</span></code> value is always read and send to the
client entirely. As such, storing small blobs (less than single digit MB) should not be a problem, but it is advised to
manually split large blobs into smaller chunks.</p>
<p>Please note in particular that by default, any value greater than 16MB will be rejected by Cassandra due the
<code class="docutils literal notranslate"><span class="pre">max_mutation_size_in_kb</span></code> configuration of the <a class="reference internal" href="../configuration/cassandra_config_file.html#cassandra-yaml"><span class="std std-ref">Cassandra Configuration File</span></a> file (which default to half of
<code class="docutils literal notranslate"><span class="pre">commitlog_segment_size_in_mb</span></code>, which itself default to 32MB).</p>
</div>
<div class="section" id="nodetool-says-connection-refused-to-host-127-0-1-1-for-any-remote-host-what-gives">
<span id="nodetool-connection-refused"></span><h2>Nodetool says “Connection refused to host: 127.0.1.1” for any remote host. What gives?<a class="headerlink" href="#nodetool-says-connection-refused-to-host-127-0-1-1-for-any-remote-host-what-gives" title="Permalink to this headline">¶</a></h2>
<p>Nodetool relies on JMX, which in turn relies on RMI, which in turn sets up its own listeners and connectors as needed on
each end of the exchange. Normally all of this happens behind the scenes transparently, but incorrect name resolution
for either the host connecting, or the one being connected to, can result in crossed wires and confusing exceptions.</p>
<p>If you are not using DNS, then make sure that your <code class="docutils literal notranslate"><span class="pre">/etc/hosts</span></code> files are accurate on both ends. If that fails, try
setting the <code class="docutils literal notranslate"><span class="pre">-Djava.rmi.server.hostname=&lt;public</span> <span class="pre">name&gt;</span></code> JVM option near the bottom of <code class="docutils literal notranslate"><span class="pre">cassandra-env.sh</span></code> to an
interface that you can reach from the remote machine.</p>
</div>
<div class="section" id="will-batching-my-operations-speed-up-my-bulk-load">
<span id="to-batch-or-not-to-batch"></span><h2>Will batching my operations speed up my bulk load?<a class="headerlink" href="#will-batching-my-operations-speed-up-my-bulk-load" title="Permalink to this headline">¶</a></h2>
<p>No. Using batches to load data will generally just add “spikes” of latency. Use asynchronous INSERTs instead, or use
true <a class="reference internal" href="../operating/bulk_loading.html#bulk-loading"><span class="std std-ref">Bulk Loading</span></a>.</p>
<p>An exception is batching updates to a single partition, which can be a Good Thing (as long as the size of a single batch
stay reasonable). But never ever blindly batch everything!</p>
</div>
<div class="section" id="on-rhel-nodes-are-unable-to-join-the-ring">
<span id="selinux"></span><h2>On RHEL nodes are unable to join the ring<a class="headerlink" href="#on-rhel-nodes-are-unable-to-join-the-ring" title="Permalink to this headline">¶</a></h2>
<p>Check if <a class="reference external" href="https://en.wikipedia.org/wiki/Security-Enhanced_Linux">SELinux</a> is on; if it is, turn it off.</p>
</div>
<div class="section" id="how-do-i-unsubscribe-from-the-email-list">
<span id="how-to-unsubscribe"></span><h2>How do I unsubscribe from the email list?<a class="headerlink" href="#how-do-i-unsubscribe-from-the-email-list" title="Permalink to this headline">¶</a></h2>
<p>Send an email to <code class="docutils literal notranslate"><span class="pre">user-unsubscribe&#64;cassandra.apache.org</span></code>.</p>
</div>
<div class="section" id="why-does-top-report-that-cassandra-is-using-a-lot-more-memory-than-the-java-heap-max">
<span id="cassandra-eats-all-my-memory"></span><h2>Why does top report that Cassandra is using a lot more memory than the Java heap max?<a class="headerlink" href="#why-does-top-report-that-cassandra-is-using-a-lot-more-memory-than-the-java-heap-max" title="Permalink to this headline">¶</a></h2>
<p>Cassandra uses <a class="reference external" href="https://en.wikipedia.org/wiki/Memory-mapped_file">Memory Mapped Files</a> (mmap) internally. That is, we
use the operating system’s virtual memory system to map a number of on-disk files into the Cassandra process’ address
space. This will “use” virtual memory; i.e. address space, and will be reported by tools like top accordingly, but on 64
bit systems virtual address space is effectively unlimited so you should not worry about that.</p>
<p>What matters from the perspective of “memory use” in the sense as it is normally meant, is the amount of data allocated
on brk() or mmap’d /dev/zero, which represent real memory used. The key issue is that for a mmap’d file, there is never
a need to retain the data resident in physical memory. Thus, whatever you do keep resident in physical memory is
essentially just there as a cache, in the same way as normal I/O will cause the kernel page cache to retain data that
you read/write.</p>
<p>The difference between normal I/O and mmap() is that in the mmap() case the memory is actually mapped to the process,
thus affecting the virtual size as reported by top. The main argument for using mmap() instead of standard I/O is the
fact that reading entails just touching memory - in the case of the memory being resident, you just read it - you don’t
even take a page fault (so no overhead in entering the kernel and doing a semi-context switch). This is covered in more
detail <a class="reference external" href="http://www.varnish-cache.org/trac/wiki/ArchitectNotes">here</a>.</p>
</div>
<div class="section" id="what-are-seeds">
<span id="id1"></span><h2>What are seeds?<a class="headerlink" href="#what-are-seeds" title="Permalink to this headline">¶</a></h2>
<p>Seeds are used during startup to discover the cluster.</p>
<p>If you configure your nodes to refer some node as seed, nodes in your ring tend to send Gossip message to seeds more
often (also see the <a class="reference internal" href="../architecture/dynamo.html#gossip"><span class="std std-ref">section on gossip</span></a>) than to non-seeds. In other words, seeds are worked as hubs of
Gossip network. With seeds, each node can detect status changes of other nodes quickly.</p>
<p>Seeds are also referred by new nodes on bootstrap to learn other nodes in ring. When you add a new node to ring, you
need to specify at least one live seed to contact. Once a node join the ring, it learns about the other nodes, so it
doesn’t need seed on subsequent boot.</p>
<p>You can make a seed a node at any time. There is nothing special about seed nodes. If you list the node in seed list it
is a seed</p>
<p>Seeds do not auto bootstrap (i.e. if a node has itself in its seed list it will not automatically transfer data to itself)
If you want a node to do that, bootstrap it first and then add it to seeds later. If you have no data (new install) you
do not have to worry about bootstrap at all.</p>
<p>Recommended usage of seeds:</p>
<ul class="simple">
<li>pick two (or more) nodes per data center as seed nodes.</li>
<li>sync the seed list to all your nodes</li>
</ul>
</div>
<div class="section" id="does-single-seed-mean-single-point-of-failure">
<span id="are-seeds-spof"></span><h2>Does single seed mean single point of failure?<a class="headerlink" href="#does-single-seed-mean-single-point-of-failure" title="Permalink to this headline">¶</a></h2>
<p>The ring can operate or boot without a seed; however, you will not be able to add new nodes to the cluster. It is
recommended to configure multiple seeds in production system.</p>
</div>
<div class="section" id="why-can-t-i-call-jmx-method-x-on-jconsole">
<span id="cant-call-jmx-method"></span><h2>Why can’t I call jmx method X on jconsole?<a class="headerlink" href="#why-can-t-i-call-jmx-method-x-on-jconsole" title="Permalink to this headline">¶</a></h2>
<p>Some of JMX operations use array argument and as jconsole doesn’t support array argument, those operations can’t be
called with jconsole (the buttons are inactive for them). You need to write a JMX client to call such operations or need
array-capable JMX monitoring tool.</p>
</div>
<div class="section" id="why-do-i-see-messages-dropped-in-the-logs">
<span id="why-message-dropped"></span><h2>Why do I see “… messages dropped …” in the logs?<a class="headerlink" href="#why-do-i-see-messages-dropped-in-the-logs" title="Permalink to this headline">¶</a></h2>
<p>This is a symptom of load shedding – Cassandra defending itself against more requests than it can handle.</p>
<p>Internode messages which are received by a node, but do not get not to be processed within their proper timeout (see
<code class="docutils literal notranslate"><span class="pre">read_request_timeout</span></code>, <code class="docutils literal notranslate"><span class="pre">write_request_timeout</span></code>, … in the <a class="reference internal" href="../configuration/cassandra_config_file.html#cassandra-yaml"><span class="std std-ref">Cassandra Configuration File</span></a>), are dropped rather than
processed (since the as the coordinator node will no longer be waiting for a response).</p>
<p>For writes, this means that the mutation was not applied to all replicas it was sent to. The inconsistency will be
repaired by read repair, hints or a manual repair. The write operation may also have timeouted as a result.</p>
<p>For reads, this means a read request may not have completed.</p>
<p>Load shedding is part of the Cassandra architecture, if this is a persistent issue it is generally a sign of an
overloaded node or cluster.</p>
</div>
<div class="section" id="cassandra-dies-with-java-lang-outofmemoryerror-map-failed">
<span id="oom-map-failed"></span><h2>Cassandra dies with <code class="docutils literal notranslate"><span class="pre">java.lang.OutOfMemoryError:</span> <span class="pre">Map</span> <span class="pre">failed</span></code><a class="headerlink" href="#cassandra-dies-with-java-lang-outofmemoryerror-map-failed" title="Permalink to this headline">¶</a></h2>
<p>If Cassandra is dying <strong>specifically</strong> with the “Map failed” message, it means the OS is denying java the ability to
lock more memory. In linux, this typically means memlock is limited. Check <code class="docutils literal notranslate"><span class="pre">/proc/&lt;pid</span> <span class="pre">of</span> <span class="pre">cassandra&gt;/limits</span></code> to verify
this and raise it (eg, via ulimit in bash). You may also need to increase <code class="docutils literal notranslate"><span class="pre">vm.max_map_count.</span></code> Note that the debian
package handles this for you automatically.</p>
</div>
<div class="section" id="what-happens-if-two-updates-are-made-with-the-same-timestamp">
<span id="what-on-same-timestamp-update"></span><h2>What happens if two updates are made with the same timestamp?<a class="headerlink" href="#what-happens-if-two-updates-are-made-with-the-same-timestamp" title="Permalink to this headline">¶</a></h2>
<p>Updates must be commutative, since they may arrive in different orders on different replicas. As long as Cassandra has a
deterministic way to pick the winner (in a timestamp tie), the one selected is as valid as any other, and the specifics
should be treated as an implementation detail. That said, in the case of a timestamp tie, Cassandra follows two rules:
first, deletes take precedence over inserts/updates. Second, if there are two updates, the one with the lexically larger
value is selected.</p>
</div>
<div class="section" id="why-bootstrapping-a-new-node-fails-with-a-stream-failed-error">
<span id="why-bootstrapping-stream-error"></span><h2>Why bootstrapping a new node fails with a “Stream failed” error?<a class="headerlink" href="#why-bootstrapping-a-new-node-fails-with-a-stream-failed-error" title="Permalink to this headline">¶</a></h2>
<p>Two main possibilities:</p>
<ol class="arabic simple">
<li>the GC may be creating long pauses disrupting the streaming process</li>
<li>compactions happening in the background hold streaming long enough that the TCP connection fails</li>
</ol>
<p>In the first case, regular GC tuning advices apply. In the second case, you need to set TCP keepalive to a lower value
(default is very high on Linux). Try to just run the following:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ sudo /sbin/sysctl -w net.ipv4.tcp_keepalive_time=60 net.ipv4.tcp_keepalive_intvl=60 net.ipv4.tcp_keepalive_probes=5
</pre></div>
</div>
<p>To make those settings permanent, add them to your <code class="docutils literal notranslate"><span class="pre">/etc/sysctl.conf</span></code> file.</p>
<p>Note: <a class="reference external" href="https://cloud.google.com/compute/">GCE</a>’s firewall will always interrupt TCP connections that are inactive for
more than 10 min. Running the above command is highly recommended in that environment.</p>
</div>
</div>


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