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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>2.3. Replication modes</title><link rel="stylesheet" type="text/css" href="default.css" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="home" href="drbd-users-guide.html" title="The DRBD User’s Guide" /><link rel="up" href="ch-features.html" title="Chapter 2. DRBD Features" /><link rel="prev" href="s-dual-primary-mode.html" title="2.2. Dual-primary mode" /><link rel="next" href="s-replication-transports.html" title="2.4. Multiple replication transports" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">2.3. Replication modes</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="s-dual-primary-mode.html">Prev</a> </td><th width="60%" align="center">Chapter 2. DRBD Features</th><td width="20%" align="right"> <a accesskey="n" href="s-replication-transports.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="s-replication-protocols"></a>2.3. Replication modes</h2></div></div></div><p>DRBD supports three distinct replication modes, allowing three degrees
of replication synchronicity.</p><p><a id="fp-protocol-a"></a><strong>Protocol A. </strong>Asynchronous replication protocol. Local write operations on the
primary node are considered completed as soon as the local disk write
has finished, and the replication packet has been placed in the local
TCP send buffer. In the event of forced fail-over, data loss may
occur. The data on the standby node is consistent after fail-over,
however, the most recent updates performed prior to the crash could be
lost. Protocol A is most often used in long distance replication scenarios.
When used in combination with DRBD Proxy it makes an effective
disaster recovery solution. See <a class="xref" href="s-drbd-proxy.html" title="2.14. Long-distance replication with DRBD Proxy">Section 2.14, “Long-distance replication with DRBD Proxy”</a> for more information.</p><p><a id="fp-protocol-b"></a><strong>Protocol B. </strong>Memory synchronous (semi-synchronous) replication protocol. Local
write operations on the primary node are considered completed as soon
as the local disk write has occurred, and the replication packet has
reached the peer node. Normally, no writes are lost in case of forced
fail-over. However, in the event of simultaneous power failure on both
nodes and concurrent, irreversible destruction of the primary’s data
store, the most recent writes completed on the primary may be lost.</p><p><a id="fp-protocol-c"></a><strong>Protocol C. </strong>Synchronous replication protocol. Local write operations on the
primary node are considered completed only after both the local and
the remote disk write have been confirmed. As a result, loss of a
single node is guaranteed not to lead to any data loss. Data loss is,
of course, inevitable even with this replication protocol if both
nodes (or their storage subsystems) are irreversibly destroyed at the
same time.</p><p>By far, the most commonly used replication protocol in DRBD setups is
protocol C.</p><p>The choice of replication protocol influences two factors of your
deployment: <span class="emphasis"><em>protection</em></span> and <span class="emphasis"><em>latency</em></span>. <span class="emphasis"><em>Throughput</em></span>, by contrast, is
largely independent of the replication protocol selected.</p><p>See <a class="xref" href="s-configure-resource.html" title="5.3. Configuring your resource">Section 5.3, “Configuring your resource”</a> for an example resource configuration
which demonstrates replication protocol configuration.</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="s-dual-primary-mode.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch-features.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="s-replication-transports.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">2.2. Dual-primary mode </td><td width="20%" align="center"><a accesskey="h" href="drbd-users-guide.html">Home</a></td><td width="40%" align="right" valign="top"> 2.4. Multiple replication transports</td></tr></table></div></body></html>
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