<!DOCTYPE html>
<html lang="en" data-content_root="../../">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Reference counting in pnfs — The Linux Kernel documentation</title>
<link rel="stylesheet" type="text/css" href="../../_static/pygments.css?v=fa44fd50" />
<link rel="stylesheet" type="text/css" href="../../_static/alabaster.css?v=3918102e" />
<script src="../../_static/documentation_options.js?v=5929fcd5"></script>
<script src="../../_static/doctools.js?v=9bcbadda"></script>
<script src="../../_static/sphinx_highlight.js?v=dc90522c"></script>
<link rel="index" title="Index" href="../../genindex.html" />
<link rel="search" title="Search" href="../../search.html" />
<link rel="next" title="RPC Cache" href="rpc-cache.html" />
<link rel="prev" title="NFS LOCALIO" href="localio.html" />
<link rel="stylesheet" href="../../_static/custom.css" type="text/css" />
</head><body>
<div class="document">
<div class="sphinxsidebar" role="navigation" aria-label="Main">
<div class="sphinxsidebarwrapper">
<p class="logo"><a href="../../index.html">
<img class="logo" src="../../_static/logo.svg" alt="Logo of The Linux Kernel"/>
</a></p>
<h1 class="logo"><a href="../../index.html">The Linux Kernel</a></h1>
<p class="blurb">6.18.50</p>
<search id="searchbox" style="display: none" role="search">
<h3 id="searchlabel">Quick search</h3>
<div class="searchformwrapper">
<form class="search" action="../../search.html" method="get">
<input type="text" name="q" aria-labelledby="searchlabel" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
<input type="submit" value="Go" />
</form>
</div>
</search>
<script>document.getElementById('searchbox').style.display = "block"</script>
<p>
<h3 class="kernel-toc-contents">Contents</h3>
<input type="checkbox" class="kernel-toc-toggle" id = "kernel-toc-toggle" checked>
<label class="kernel-toc-title" for="kernel-toc-toggle"></label>
<div class="kerneltoc" id="kerneltoc">
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../process/development-process.html">Development process</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../process/submitting-patches.html">Submitting patches</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../process/code-of-conduct.html">Code of conduct</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../maintainer/index.html">Maintainer handbook</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../process/index.html">All development-process docs</a></li>
</ul>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="../../core-api/index.html">Core API</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../driver-api/index.html">Driver APIs</a></li>
<li class="toctree-l1 current"><a class="reference internal" href="../../subsystem-apis.html">Subsystems</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="../../subsystem-apis.html#core-subsystems">Core subsystems</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../subsystem-apis.html#human-interfaces">Human interfaces</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../subsystem-apis.html#networking-interfaces">Networking interfaces</a></li>
<li class="toctree-l2 current"><a class="reference internal" href="../../subsystem-apis.html#storage-interfaces">Storage interfaces</a><ul class="current">
<li class="toctree-l3 current"><a class="reference internal" href="../index.html">Filesystems in the Linux kernel</a></li>
<li class="toctree-l3"><a class="reference internal" href="../../block/index.html">Block</a></li>
<li class="toctree-l3"><a class="reference internal" href="../../cdrom/index.html">CD-ROM</a></li>
<li class="toctree-l3"><a class="reference internal" href="../../scsi/index.html">SCSI Subsystem</a></li>
<li class="toctree-l3"><a class="reference internal" href="../../target/index.html">TCM Virtual Device</a></li>
<li class="toctree-l3"><a class="reference internal" href="../../nvme/index.html">NVMe Subsystem</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="../../subsystem-apis.html#other-subsystems">Other subsystems</a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="../../locking/index.html">Locking</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../process/license-rules.html">Licensing rules</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../doc-guide/index.html">Writing documentation</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../dev-tools/index.html">Development tools</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../dev-tools/testing-overview.html">Testing guide</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../kernel-hacking/index.html">Hacking guide</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../trace/index.html">Tracing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../fault-injection/index.html">Fault injection</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../livepatch/index.html">Livepatching</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../rust/index.html">Rust</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../admin-guide/index.html">Administration</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../kbuild/index.html">Build system</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../admin-guide/reporting-issues.html">Reporting issues</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../tools/index.html">Userspace tools</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../userspace-api/index.html">Userspace API</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../firmware-guide/index.html">Firmware</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../devicetree/index.html">Firmware and Devicetree</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../arch/index.html">CPU architectures</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../staging/index.html">Unsorted documentation</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../translations/index.html">Translations</a></li>
</ul>
</div>
<script type="text/javascript"> <!--
var sbar = document.getElementsByClassName("sphinxsidebar")[0];
let currents = document.getElementsByClassName("current")
if (currents.length) {
sbar.scrollTop = currents[currents.length - 1].offsetTop;
}
--> </script>
<div role="note" aria-label="source link">
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../../_sources/filesystems/nfs/pnfs.rst.txt"
rel="nofollow">Show Source</a></li>
</ul>
</div>
</div>
</div>
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body" role="main">
<section id="reference-counting-in-pnfs">
<h1>Reference counting in pnfs<a class="headerlink" href="#reference-counting-in-pnfs" title="Link to this heading">¶</a></h1>
<p>The are several inter-related caches. We have layouts which can
reference multiple devices, each of which can reference multiple data servers.
Each data server can be referenced by multiple devices. Each device
can be referenced by multiple layouts. To keep all of this straight,
we need to reference count.</p>
<section id="struct-pnfs-layout-hdr">
<h2>struct pnfs_layout_hdr<a class="headerlink" href="#struct-pnfs-layout-hdr" title="Link to this heading">¶</a></h2>
<p>The on-the-wire command LAYOUTGET corresponds to <code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span>
<span class="pre">pnfs_layout_segment</span></code>, usually referred to by the variable name lseg.
Each nfs_inode may hold a pointer to a cache of these layout
segments in nfsi->layout, of type <code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">pnfs_layout_hdr</span></code>.</p>
<p>We reference the header for the inode pointing to it, across each
outstanding RPC call that references it (LAYOUTGET, LAYOUTRETURN,
LAYOUTCOMMIT), and for each lseg held within.</p>
<p>Each header is also (when non-empty) put on a list associated with
<code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">nfs_client</span></code> (cl_layouts). Being put on this list does not bump
the reference count, as the layout is kept around by the lseg that
keeps it in the list.</p>
</section>
<section id="deviceid-cache">
<h2>deviceid_cache<a class="headerlink" href="#deviceid-cache" title="Link to this heading">¶</a></h2>
<p>lsegs reference device ids, which are resolved per nfs_client and
layout driver type. The device ids are held in a RCU cache (<code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span>
<span class="pre">nfs4_deviceid_cache</span></code>). The cache itself is referenced across each
mount. The entries (<code class="xref c c-struct broken_xref docutils literal notranslate"><span class="pre">struct</span> <span class="pre">nfs4_deviceid</span></code>) themselves are held across
the lifetime of each lseg referencing them.</p>
<p>RCU is used because the deviceid is basically a write once, read many
data structure. The hlist size of 32 buckets needs better
justification, but seems reasonable given that we can have multiple
deviceid’s per filesystem, and multiple filesystems per nfs_client.</p>
<p>The hash code is copied from the nfsd code base. A discussion of
hashing and variations of this algorithm can be found <a class="reference external" href="http://groups.google.com/group/comp.lang.c/browse_thread/thread/9522965e2b8d3809">here.</a></p>
</section>
<section id="data-server-cache">
<h2>data server cache<a class="headerlink" href="#data-server-cache" title="Link to this heading">¶</a></h2>
<p>file driver devices refer to data servers, which are kept in a module
level cache. Its reference is held over the lifetime of the deviceid
pointing to it.</p>
</section>
<section id="lseg">
<h2>lseg<a class="headerlink" href="#lseg" title="Link to this heading">¶</a></h2>
<p>lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
bit which holds it in the pnfs_layout_hdr’s list. When the final lseg
is removed from the pnfs_layout_hdr’s list, the NFS_LAYOUT_DESTROYED
bit is set, preventing any new lsegs from being added.</p>
</section>
<section id="layout-drivers">
<h2>layout drivers<a class="headerlink" href="#layout-drivers" title="Link to this heading">¶</a></h2>
<p>PNFS utilizes what is called layout drivers. The STD defines 4 basic
layout types: “files”, “objects”, “blocks”, and “flexfiles”. For each
of these types there is a layout-driver with a common function-vectors
table which are called by the nfs-client pnfs-core to implement the
different layout types.</p>
<p>Files-layout-driver code is in: fs/nfs/filelayout/.. directory
Blocks-layout-driver code is in: fs/nfs/blocklayout/.. directory
Flexfiles-layout-driver code is in: fs/nfs/flexfilelayout/.. directory</p>
</section>
<section id="blocks-layout-setup">
<h2>blocks-layout setup<a class="headerlink" href="#blocks-layout-setup" title="Link to this heading">¶</a></h2>
<p>TODO: Document the setup needs of the blocks layout driver</p>
</section>
</section>
</div>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="footer">
©The kernel development community.
|
Powered by <a href="https://www.sphinx-doc.org/">Sphinx 8.1.3</a>
& <a href="https://alabaster.readthedocs.io">Alabaster 0.7.16</a>
|
<a href="../../_sources/filesystems/nfs/pnfs.rst.txt"
rel="nofollow">Page source</a>
</div>
</body>
</html>