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828 lines
26 KiB
HTML
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</head>
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<body>
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<div id="header">
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<h1>CSYNC User Guide</h1>
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<span id="author">Andreas Schneider</span><br />
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<span id="email"><tt><<a href="mailto:mail@cynapses.org">mail@cynapses.org</a>></tt></span><br />
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<div id="toc">
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<div id="toctitle">Table of Contents</div>
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<noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript>
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</div>
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</div>
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<div id="preamble">
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<div class="sectionbody">
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<div class="paragraph"><p>csync is a lightweight utility to synchronize files between two directories
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on a system or between multiple systems.</p></div>
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<div class="paragraph"><p>It synchronizes bidirectionally and allows the user to keep two copies of files
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and directories in sync. csync uses widely adopted protocols, such as smb or
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sftp, so that there is no need for a server component. It is a user-level
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program which means you don’t need to be a superuser or administrator.</p></div>
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<div class="paragraph"><p>Together with a Pluggable Authentication Module (PAM), the intent is to provide
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Roaming Home Directories for Linux (see <a href="#X80">The PAM Module</a>).</p></div>
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</div>
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</div>
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<h2 id="_introduction">1. Introduction</h2>
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<div class="sectionbody">
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<div class="paragraph"><p>It is often the case that we have multiple copies (called replicas) of a
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filesystem or part of a filesystem (for example on a notebook and desktop
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computer). Changes to each replica are often made independently, and as a
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result, they do not contain the same information. In that case, a file
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synchronizer is used to make them consistent again, without losing any
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information.</p></div>
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<div class="paragraph"><p>The goal is to detect conflicting updates (files which have been modified) and
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propagate non-conflicting updates to each replica. If there are no conflicts
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left, we are done, and the replicas are identical. To resolve or handle
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conflicts there are several algorithms available. They will be discussed
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one of the following sections.</p></div>
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</div>
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<h2 id="_basics">2. Basics</h2>
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<div class="sectionbody">
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<div class="paragraph"><p>This section describes some basics of file synchronization.</p></div>
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<h3 id="_paths">2.1. Paths</h3><div style="clear:left"></div>
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<div class="paragraph"><p>A path normally refers to a point which contains a set of files which should be
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synchronized. It is specified relative to the root of the replica locally, but
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has to be absolute if you use a protocol. The path is just a sequence of names
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separated by <em>/</em>.</p></div>
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<div class="admonitionblock">
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<table><tr>
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<td class="icon">
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<img src="./images/icons/note.png" alt="Note" />
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</td>
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<td class="content">The path separator is always a forward slash <em>/</em>, even for Windows.</td>
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</tr></table>
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</div>
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<div class="paragraph"><p>csync always uses the absolute path on remote replicas. This could
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<em>sftp://gladiac:secret@myserver/home/gladiac</em> for sftp.</p></div>
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<h3 id="_what_is_an_update">2.2. What is an update?</h3><div style="clear:left"></div>
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<div class="paragraph"><p>The contents of a path could be a file, a directory or a symbolic link
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(symbolic links are not supported yet). To be more precise, if the path refers
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to:</p></div>
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<div class="ulist"><ul>
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<li>
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<p>
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a regular file: the contents of the file are the byte stream and the
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metadata of the file.
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</p>
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</li>
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<li>
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<p>
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a directory: then the content is the metadata of the directory.
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</p>
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</li>
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<li>
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<p>
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a symbolic link: the content is the named file the link points to.
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</p>
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</li>
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</ul></div>
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<div class="paragraph"><p>csync keeps a record of each path which has been successfully synchronized. The
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path gets compared with the record and if it has changed since the last
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synchronization, we have an update. This is done by comparing the modification
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or change (modification time of the metadata) time. This is the way how updates
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are detected.</p></div>
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<h3 id="_what_is_a_conflict">2.3. What is a conflict?</h3><div style="clear:left"></div>
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<div class="paragraph"><p>A path is conflicting if it fulfills the following conditions:</p></div>
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<div class="olist arabic"><ol class="arabic">
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<li>
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<p>
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it has been updated in one replica,
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</p>
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</li>
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<li>
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<p>
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it or any of its descendants has been updated on the other replica too, and
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</p>
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</li>
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<li>
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<p>
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its contents in are not identical.
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</p>
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</li>
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</ol></div>
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</div>
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<h2 id="_file_synchronization">3. File Synchronization</h2>
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<div class="sectionbody">
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<div class="paragraph"><p>The primary goal of the file synchronizer is correctness. It may change
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scattered or large parts of the filesystem. Since this in mostly not monitored
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by the user, and the file synchronizer is in a position to harm the system,
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csync must be safe, even in the case of unexpected errors (e.g. disk full).
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What was done to make csync safe is described in the following sections.</p></div>
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<div class="paragraph"><p>On problem concerning correctness is the handling of conflicts. Each file
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synchronizer tries to propagate conflicting changes to the other replica. At
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the end both replicas should be identical. There are different strategies to
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fulfill these goals.</p></div>
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<div class="paragraph"><p>csync is a three-phase file synchronizer. The decision for this design was that
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user interaction should be possible and it should be easy to understand the
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process. The three phases are update detection, reconciliation and propagation.
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These will be described in the following sections.</p></div>
|
|
<h3 id="_update_detection">3.1. Update detection</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>There are different strategies for update detection. csync uses a state-based
|
|
modtime-inode update detector. This means it uses the modification time to
|
|
detect updates. It doesn’t require many resources. A record of each file is
|
|
stored in a database (called statedb) and compared with the current
|
|
modification time during update detection. If the file has changed since the
|
|
last synchronization an instruction is set to evaluate it during the
|
|
reconciliation phase. If we don’t have a record for a file we investigate, it
|
|
is marked as new.</p></div>
|
|
<div class="paragraph"><p>It can be difficult to detect renaming of files. This problem is also solved
|
|
by the record we store in the statedb. If we don’t find the file by the name
|
|
in the database, we search for the inode number. If the inode number is found
|
|
then the file has been renamed.</p></div>
|
|
<h3 id="_reconciliation">3.2. Reconciliation</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>The most important component is the update detector, because the reconciler
|
|
depends on it. The correctness of reconciler is mandatory because it can damage
|
|
a filesystem. It decides which file:</p></div>
|
|
<div class="ulist"><ul>
|
|
<li>
|
|
<p>
|
|
Stays untouched
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
Has a conflict
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
Gets synchronized
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
or is <strong>deleted</strong>
|
|
</p>
|
|
</li>
|
|
</ul></div>
|
|
<div class="paragraph"><p>A wrong decision of the reconciler leads in most cases to a loss of data. So
|
|
there are several conditions which a file synchronizer has to follow.</p></div>
|
|
<h4 id="_algorithms">3.2.1. Algorithms</h4>
|
|
<div class="paragraph"><p>For conflict resolution several different algorithms could be implemented. The
|
|
most common algorithms are the merge and the conflict algorithm. The first
|
|
is a batch algorithm and the second is one which needs user interaction.</p></div>
|
|
<h5 id="_merge_algorithm">Merge algorithm</h5>
|
|
<div class="paragraph"><p>The merge algorithm is an algorithm which doesn’t need any user interaction. It
|
|
is simple and used for example by Microsoft for Roaming Profiles. If it detects
|
|
a conflict (the same file changed on both replicas) then it will use the most
|
|
recent file and overwrite the other. This means you can loose some data, but
|
|
normally you want the latest file.</p></div>
|
|
<h5 id="_conflict_algorithm">Conflict algorithm</h5>
|
|
<div class="paragraph"><p>This is not implemented yet.</p></div>
|
|
<div class="paragraph"><p>If a file has a conflict the user has to decide which file should be used.</p></div>
|
|
<h3 id="_propagation">3.3. Propagation</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>The next instance of the file synchronizer the propagator. It uses the
|
|
calculated records to apply them on the current replica.</p></div>
|
|
<div class="paragraph"><p>The propagator uses a two-phase-commit mechanism to simulate an atomic
|
|
filesystem operation.</p></div>
|
|
<div class="paragraph"><p>In the first phase we copy the file to a temporary file on the opposite
|
|
replica. This has the advantage that we can check if file which has been copied
|
|
to the opposite replica has been transfered successfully. If the connection
|
|
gets interrupted during the transfer we still have the original states of the
|
|
file. This means no data will be lost.</p></div>
|
|
<div class="paragraph"><p>In the second phase the file on the opposite replica will be overwritten by
|
|
the temporary file.</p></div>
|
|
<div class="paragraph"><p>After a successful propagation we have to merge the trees to reflect the
|
|
current state of the filesystem tree. This updated tree will be written as a
|
|
journal into the state database. It will be used during the update detection of
|
|
the next synchronization. See above for a description of the state database
|
|
during synchronization.</p></div>
|
|
<h3 id="_robustness">3.4. Robustness</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>This is a very important topic. The file synchronizer should not crash, and if
|
|
it has crashed, there should be no loss of data. To achieve this goal there are
|
|
several mechanisms which will be discussed in the following sections.</p></div>
|
|
<h4 id="_crash_resistance">3.4.1. Crash resistance</h4>
|
|
<div class="paragraph"><p>The synchronization process can be interrupted by different events, this can
|
|
be:</p></div>
|
|
<div class="ulist"><ul>
|
|
<li>
|
|
<p>
|
|
the system could be halted due to errors.
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
the disk could be full or the quota exceeded.
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
the network or power cable could be pulled out.
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
the user could force a stop of the synchronization process.
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
various communication errors could occur.
|
|
</p>
|
|
</li>
|
|
</ul></div>
|
|
<div class="paragraph"><p>That no data will be lost due to an event we enforce the following invariant:</p></div>
|
|
<div class="admonitionblock">
|
|
<table><tr>
|
|
<td class="icon">
|
|
<img src="./images/icons/important.png" alt="Important" />
|
|
</td>
|
|
<td class="content">At every moment of the synchronization each file, has either its
|
|
original content or its correct final content.</td>
|
|
</tr></table>
|
|
</div>
|
|
<div class="paragraph"><p>This means that the original content can not be incorrect, no data can be lost
|
|
until we overwrite it after a successful synchronization. Therefore, each
|
|
interrupted synchronization process is a partial sync and can be continued and
|
|
completed by simply running csync again. The only problem could be an error of
|
|
the filesystem, so we reach this invariant only approximately.</p></div>
|
|
<h4 id="_transfer_errors">3.4.2. Transfer errors</h4>
|
|
<div class="paragraph"><p>With the Two-Phase-Commit we check the file size after the file has transferred
|
|
and we are able to detect transfer errors. A more robust approach would be a
|
|
transfer protocol with checksums, but this is not doable at the moment. We may
|
|
add this in the future.</p></div>
|
|
<div class="paragraph"><p>Future filesystems, like btrfs, will help to compare checksums instead of the
|
|
filesize. This will make the synchronization safer. This does not imply that it
|
|
is unsafe now, but checksums are safer than simple filesize checks.</p></div>
|
|
<h4 id="_database_loss">3.4.3. Database loss</h4>
|
|
<div class="paragraph"><p>It is possible that the state database could get corrupted. If this happens,
|
|
all files get evaluated. In this case the file synchronizer wont delete any
|
|
file, but it could occur that deleted files will be restored from the other
|
|
replica.</p></div>
|
|
<div class="paragraph"><p>To prevent a corruption or loss of the database if an error occurs or the user
|
|
forces an abort, the synchronizer is working on a copy of the database and will
|
|
use a Two-Phase-Commit to save it at the end.</p></div>
|
|
</div>
|
|
<h2 id="_getting_started">4. Getting started</h2>
|
|
<div class="sectionbody">
|
|
<h3 id="_installing_csync">4.1. Installing csync</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>See the <tt>README</tt> and <tt>INSTALL</tt> files for install prerequisites and
|
|
procedures. Packagers should take a look at <a href="#X90">Appendix A: Packager Notes</a>.</p></div>
|
|
<h3 id="_using_the_commandline_client">4.2. Using the commandline client</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>The synopsis of the commandline client is</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync [OPTION...] SOURCE DESTINATION</tt></pre>
|
|
</div></div>
|
|
<div class="paragraph"><p>It synchronizes the content of SOURCE with DESTINATION and vice versa. The
|
|
DESTINATION can be a local directory or a remote file server.</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync /home/csync scheme://user:password@server:port/full/path</tt></pre>
|
|
</div></div>
|
|
<h4 id="_examples">4.2.1. Examples</h4>
|
|
<div class="paragraph"><p>To synchronize two local directories:</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync /home/csync/replica1 /home/csync/relplica2</tt></pre>
|
|
</div></div>
|
|
<div class="paragraph"><p>Two synchronizer a local directory with an smb server, use</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync /home/csync smb://rupert.galaxy.site/Users/csync</tt></pre>
|
|
</div></div>
|
|
<div class="paragraph"><p>If you use kerberos, you don’t have to specify a username or a password. If you
|
|
don’t use kerberos, the commandline client will ask about the user and the
|
|
password. If you don’t want to be prompted, you can specify it on the
|
|
commandline:</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync /home/csync smb://csync:secret@rupert.galaxy.site/Users/csync</tt></pre>
|
|
</div></div>
|
|
<div class="paragraph"><p>If you use the sftp protocol and want to specify a port, you do it the
|
|
following way:</p></div>
|
|
<div class="literalblock">
|
|
<div class="content">
|
|
<pre><tt>csync /home/csync sftp://csync@krikkit.galaxy.site:2222/home/csync</tt></pre>
|
|
</div></div>
|
|
<div class="paragraph"><p>The remote destination is supported by plugins. By default csync ships with smb
|
|
and sftp support. For more information, see the manpage of <tt>csync(1)</tt>.</p></div>
|
|
<h3 id="_exclude_lists">4.3. Exclude lists</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>csync provides exclude lists with simple shell wildcard patterns. There is a
|
|
global exclude list, which is normally located in
|
|
<em>/etc/csync/csync_exclude.conf</em> and it has already some sane defaults. If you
|
|
run csync the first time, it will create an empty exclude list for the user.
|
|
This file will be <em>~/.csync/csync_exclude.conf</em>. If you run both files are used
|
|
and maybe an additional one if you specify it.</p></div>
|
|
<div class="paragraph"><p>The entries in the file are newline separated. Use
|
|
<em>/etc/csync/csync_exclude.conf</em> as an example.</p></div>
|
|
<h3 id="_debug_messages_and_dry_run">4.4. Debug messages and dry run</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>For log messages csync uses log4c. It is a logging mechanism which uses debug
|
|
levels and categories. There is a config file where you can specify the debug
|
|
level for each component. It is located at <em>~/.csync/csync_log.conf</em>.</p></div>
|
|
<div class="paragraph"><p>Available debug priorities are:</p></div>
|
|
<div class="ulist"><ul>
|
|
<li>
|
|
<p>
|
|
trace
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
debug
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
info
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
warn
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
error
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
fatal
|
|
</p>
|
|
</li>
|
|
<li>
|
|
<p>
|
|
none
|
|
</p>
|
|
</li>
|
|
</ul></div>
|
|
<div class="paragraph"><p>A more detailed description can be found at the
|
|
<a href="http://log4c.sourceforge.net/">log4c homepage</a>. A good introduction can
|
|
be found <a href="http://logging.apache.org/log4j/1.2/manual.html">here</a>.</p></div>
|
|
<div class="paragraph"><p>To simulate a run of the file synchronizer, you should set the priority to
|
|
<em>debug</em> for the categories <em>csync.updater</em> and <em>csync.reconciler</em> in the config
|
|
file <em>~/.csync/csync_log.conf</em>. Then run csync with the <em>--dry-run</em> option.
|
|
This will only run update detection and reconciliation.</p></div>
|
|
<h3 id="X80">4.5. The PAM module</h3><div style="clear:left"></div>
|
|
<div class="paragraph"><p>pam_csync is a PAM module to provide roaming home directories for a user
|
|
session. This module is aimed at environments with central file servers where a
|
|
user wishes to store his home directory. The Authentication Module verifies the
|
|
identity of a user and triggers a synchronization with the server on the first
|
|
login and the last logout. More information can be found in the manpage of the
|
|
module pam_csync(8) or pam itself pam(8).</p></div>
|
|
</div>
|
|
<h2 id="X90">5. Appendix A: Packager Notes</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph"><p>Read the <tt>README</tt>, <tt>INSTALL</tt> and <tt>FAQ</tt> files (in the distribution root
|
|
directory).</p></div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2009-02-10 12:48:31 CEST
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|