[libvirt] [PATCH 2/2] docs: add page about virtlockd setup
Daniel P. Berrange
berrange at redhat.com
Mon Feb 16 11:59:37 UTC 2015
Introduce some basic docs describing the virtlockd setup.
Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
---
docs/locking-lockd.html.in | 160 +++++++++++++++++++++++++++++++++++++++++++++
docs/locking.html.in | 2 +-
docs/sitemap.html.in | 4 ++
3 files changed, 165 insertions(+), 1 deletion(-)
create mode 100644 docs/locking-lockd.html.in
diff --git a/docs/locking-lockd.html.in b/docs/locking-lockd.html.in
new file mode 100644
index 0000000..2b01ee5
--- /dev/null
+++ b/docs/locking-lockd.html.in
@@ -0,0 +1,160 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+ <body>
+ <h1>Virtual machine lock manager, virtlockd plugin</h1>
+
+ <ul id="toc"></ul>
+
+ <p>
+ This page describes use of the <code>virtlockd</code>
+ service as a <a href="locking.html">lock driver</a>
+ plugin for virtual machine disk mutual exclusion.
+ </p>
+
+ <h2><a name="background">virtlockd background</a></h2>
+
+ <p>
+ The virtlockd daemon is a single purpose binary which
+ focus exclusively on the task of acquiring and holding
+ locks on behalf of running virtual machines. It is
+ designed to offer a low overhead, portable locking
+ scheme can be used out of the box on virtualization
+ hosts with minimal configuration overheads. It makes
+ use of the POSIX fcntl advisory locking capability
+ to hold locks, which is supported by the majority of
+ commonly used filesystems
+ </p>
+
+ <h2><a name="sanlock">virtlockd daemon setup</a></h2>
+
+ <p>
+ In most OS, the virtlockd daemon itself will not require
+ any upfront configuration work. It is installed by default
+ when libvirtd is present, and a systemd socket unit is
+ registered such that the daemon will be automatically
+ started when first required. With OS that predate systemd
+ though, it will be neccessary to start it at boot time,
+ prior to libvirtd being started. On RHEL/Fedora distros,
+ this can be achieved as follows
+ </p>
+
+ <pre>
+ # chkconfig virtlockd on
+ # service virtlockd start
+ </pre>
+
+ <p>
+ The above instructions apply to the instance of virtlockd
+ that runs privileged, and is used by the libvirtd daemon
+ that runs privileged. If running libvirtd as an unprivileged
+ user, it will always automatically spawn an instance of
+ the virtlockd daemon unprivileged too. This requires no
+ setup at all.
+ </p>
+
+ <h2><a name="lockdplugin">libvirt lockd plugin configuration</a></h2>
+
+ <p>
+ Once the virtlockd daemon is running, or setup to autostart,
+ the next step is to configure the libvirt lockd plugin.
+ There is a separate configuration file for each libvirt
+ driver that is using virtlockd. For QEMU, we will edit
+ <code>/etc/libvirt/qemu-lockd.conf</code>
+ </p>
+
+ <p>
+ The default behaviour of the lockd plugin is to acquire locks
+ directly on the virtual disk images associated with the guest
+ <disk> elements. This ensures it can run out of the box
+ with not configuration, providing locking for disk images on
+ shared filesystems such as NFS. It does not provide any cross
+ host protection for storage that is backed by block devices,
+ since locks acquired on device nodes in /dev only apply within
+ the host. It may also be the case that the filesystem holding
+ the disk images is not capable of supporting fcntl locks.
+ </p>
+
+ <p>
+ To address these problems it is possible to tell lockd to
+ acquire locks on an indirect file. Essentially lockd will
+ calculate the SHA256 checksum of the fully qualified path,
+ and create a zero length file in a given directory whose
+ filename is the checksum. It will then acquire a lock on
+ that file. Assuming the block devices assigned to the guest
+ are using stable paths (eg /dev/disk/by-path/XXXXXXX) then
+ this will allow for locks to apply across hosts. This
+ feature can be enabled by setting a configuration setting
+ that specifies the directory in which to create the lock
+ files. The directory referred to should of course be
+ placed on a shared filesystem (eg NFS) that is accessible
+ to all hosts which can see the shared block devices.
+ </p>
+
+ <pre>
+ $ su - root
+ # augtool -s set \
+ /files/etc/libvirt/qemu-lockd.conf/file_lockspace_dir \
+ "/var/lib/libvirt/lockd/files"
+ </pre>
+
+ <p>
+ If the guests are using either LVM and SCSI block devices
+ for their virtual disks, there is a unique identifier
+ associated with each device. It is possible to tell lockd
+ to use this UUID as the basis for acquiring locks, rather
+ than the SHA256 sum of the filename. The benefit of this
+ is that the locking protection will work even if the file
+ paths to the given block device are different on each
+ host.
+ </p>
+
+ <pre>
+ $ su - root
+ # augtool -s set \
+ /files/etc/libvirt/qemu-lockd.conf/scsi_lockspace_dir \
+ "/var/lib/libvirt/lockd/scsi"
+ # augtool -s set \
+ /files/etc/libvirt/qemu-lockd.conf/lvm_lockspace_dir \
+ "/var/lib/libvirt/lockd/lvm"
+ </pre>
+
+ <p>
+ It is important to remember that the changes made to the
+ <code>/etc/libvirt/qemu-lockd.conf</code> file must be
+ propagated to all hosts before any virtual machines are
+ launched on them. This ensures that all hosts are using
+ the same locking mechanism
+ </p>
+
+ <h2><a name="qemuconfig">QEMU/KVM driver configuration</a></h2>
+
+ <p>
+ The QEMU driver is capable of using the virtlockd plugin
+ since the release <span>1.0.2</span>.
+ The out of the box configuration, however, currently
+ uses the <strong>nop</strong> lock manager plugin.
+ To get protection for disks, it is thus necessary
+ to reconfigure QEMU to activate the <strong>lockd</strong>
+ driver. This is achieved by editing the QEMU driver
+ configuration file (<code>/etc/libvirt/qemu.conf</code>)
+ and changing the <code>lock_manager</code> configuration
+ tunable.
+ </p>
+
+ <pre>
+ $ su - root
+ # augtool -s set /files/etc/libvirt/qemu.conf/lock_manager lockd
+ # service libvirtd restart
+ </pre>
+
+ <p>
+ Every time you start a guest, the virtlockd daemon will acquire
+ locks on the disk files directly, or in one of the configured
+ lookaside directories based on SHA256 sum. To check that locks
+ are being acquired as expected, the <code>lslocks</code> tool
+ can be run.
+ </p>
+
+ </body>
+</html>
diff --git a/docs/locking.html.in b/docs/locking.html.in
index cf72eb0..c25ae85 100644
--- a/docs/locking.html.in
+++ b/docs/locking.html.in
@@ -28,7 +28,7 @@
nothing. This can be used if mutual exclusion between
virtual machines is not required, or if it is being
solved at another level in the management stack.</dd>
- <dt>lockd</dt>
+ <dt><a href="locking-lockd.html">lockd</a></dt>
<dd>This is the current preferred implementation shipped
with libvirt. It uses the <code>virtlockd</code> daemon
to manage locks using the POSIX fcntl() advisory locking
diff --git a/docs/sitemap.html.in b/docs/sitemap.html.in
index 5b911ce..a9fa945 100644
--- a/docs/sitemap.html.in
+++ b/docs/sitemap.html.in
@@ -103,6 +103,10 @@
<span>Ensuring exclusive guest access to disks</span>
<ul>
<li>
+ <a href="locking-lockd.html">virtlockd</a>
+ <span>virtlockd lock manager plugin</span>
+ </li>
+ <li>
<a href="locking-sanlock.html">Sanlock</a>
<span>Sanlock lock manager plugin</span>
</li>
--
2.1.0
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