2011-09-13 17:44:24 +00:00
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Remote filesystems</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Berkeley DB Programmer's Reference Guide" />
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<link rel="up" href="env.html" title="Chapter 9. The Berkeley DB Environment" />
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<link rel="prev" href="env_encrypt.html" title="Encryption" />
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<link rel="next" href="env_faq.html" title="Environment FAQ" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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2012-11-14 21:35:20 +00:00
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<p>Library Version 11.2.5.3</p>
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2011-09-13 17:44:24 +00:00
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Remote filesystems</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="env_encrypt.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 9.
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The Berkeley DB Environment
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</th>
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<td width="20%" align="right"> <a accesskey="n" href="env_faq.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="env_remote"></a>Remote filesystems</h2>
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</div>
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</div>
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</div>
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<p>When Berkeley DB database environment shared memory regions are backed by the
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filesystem, it is a common application error to create database
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environments backed by remote filesystems such as the Network File
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System (NFS), Windows network shares (SMB/CIFS) or the Andrew File
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System (AFS). Remote filesystems rarely support mapping files into
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process memory, and even more rarely support correct semantics for
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mutexes if the mapping succeeds. For this reason, we recommend database
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environment directories be created in a local filesystem.</p>
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<p>For remote filesystems that do allow remote files to be mapped into
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process memory, database environment directories accessed via remote
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filesystems cannot be used simultaneously from multiple clients (that
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is, from multiple computers). No commercial remote filesystem of which
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we're aware supports coherent, distributed shared memory for
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remote-mounted files. As a result, different machines will see
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different versions of these shared region files, and the behavior is
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undefined.</p>
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<p>Databases, log files, and temporary files may be placed on remote
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filesystems, as long as the remote filesystem fully supports standard
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POSIX filesystem semantics (although the application may incur a
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performance penalty for doing so). Further, read-only databases on
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remote filesystems can be accessed from multiple systems simultaneously.
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However, it is difficult (or impossible) for modifiable databases on
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remote filesystems to be accessed from multiple systems simultaneously.
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The reason is the Berkeley DB library caches modified database pages, and when
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those modified pages are written to the backing file is not entirely
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under application control. If two systems were to write database pages
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to the remote filesystem at the same time, database corruption could
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result. If a system were to write a database page back to the remote
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filesystem at the same time as another system read a page, a core dump
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in the reader could result.</p>
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<div class="variablelist">
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<dl>
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<dt>
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<span class="term">FreeBSD note:</span>
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</dt>
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<dd>Some historic FreeBSD releases will return ENOLCK from fsync and close
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calls on NFS-mounted filesystems, even though the call has succeeded.
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To support Berkeley DB on these releases, the Berkeley DB code should be modified
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to ignore ENOLCK errors, or no Berkeley DB files should be placed on
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NFS-mounted filesystems on these systems. Note that current FreeBSD
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releases do not suffer from this problem.</dd>
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<dt>
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<span class="term">Linux note:</span>
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</dt>
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<dd>Some historic Linux releases do not support complete semantics for the
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POSIX fsync call on NFS-mounted filesystems. No Berkeley DB files should be
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placed on NFS-mounted filesystems on these systems. Note that current
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Linux releases do not suffer from this problem.</dd>
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</dl>
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</div>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="env_encrypt.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="env.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="env_faq.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Encryption </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Environment FAQ</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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