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<h1>Resource Lock Manager</h1>
<ul><li>
<a href="#goals">Goals</a>
</li><li>
<a href="#requirement">Requirements</a>
</li><li>
<a href="#design">Design</a>
</li><li>
<a href="#impl">Plugin Implementations</a>
</li><li>
<a href="#qemuIntegrate">QEMU Driver integration</a>
</li><li>
<a href="#usagePatterns">Lock usage patterns</a>
<ul><li>
<a href="#usageLockAcquire">Lock acquisition</a>
</li><li>
<a href="#usageLockAttach">Lock release</a>
</li></ul>
</li></ul>
<p>
This page describes the design of the resource lock manager
that is used for locking disk images, to ensure exclusive
access to content.
</p>
<h2>
<a name="goals" id="goals">Goals</a>
</h2>
<p>
The high level goal is to prevent the same disk image being
used by more than one QEMU instance at a time (unless the
disk is marked as shareable, or readonly). The scenarios
to be prevented are thus:
</p>
<ol><li>
Two different guests running configured to point at the
same disk image.
</li><li>
One guest being started more than once on two different
machines due to admin mistake
</li><li>
One guest being started more than once on a single machine
due to libvirt driver bug on a single machine.
</li></ol>
<h2>
<a name="requirement" id="requirement">Requirements</a>
</h2>
<p>
The high level goal leads to a set of requirements
for the lock manager design
</p>
<ol><li>
A lock must be held on a disk whenever a QEMU process
has the disk open
</li><li>
The lock scheme must allow QEMU to be configured with
readonly, shared write, or exclusive writable disks
</li><li>
A lock handover must be performed during the migration
process where 2 QEMU processes will have the same disk
open concurrently.
</li><li>
The lock manager must be able to identify and kill the
process accessing the resource if the lock is revoked.
</li><li>
Locks can be acquired for arbitrary VM related resources,
as determined by the management application.
</li></ol>
<h2>
<a name="design" id="design">Design</a>
</h2>
<p>
Within a lock manager the following series of operations
will need to be supported.
</p>
<ul><li>
<strong>Register object</strong>
Register the identity of an object against which
locks will be acquired
</li><li>
<strong>Add resource</strong>
Associate a resource with an object for future
lock acquisition / release
</li><li>
<strong>Acquire locks</strong>
Acquire the locks for all resources associated
with the object
</li><li>
<strong>Release locks</strong>
Release the locks for all resources associated
with the object
</li><li>
<strong>Inquire locks</strong>
Get a representation of the state of the locks
for all resources associated with the object
</li></ul>
<h2>
<a name="impl" id="impl">Plugin Implementations</a>
</h2>
<p>
Lock manager implementations are provided as LGPLv2+
licensed, dlopen()able library modules. The plugins
will be loadable from the following location:
</p>
<pre>
/usr/{lib,lib64}/libvirt/lock_manager/$NAME.so
</pre>
<p>
The lock manager plugin must export a single ELF
symbol named <code>virLockDriverImpl</code>, which is
a static instance of the <code>virLockDriver</code>
struct. The struct is defined in the header file
</p>
<pre>
#include <libvirt/plugins/lock_manager.h>
</pre>
<p>
All callbacks in the struct must be initialized
to non-NULL pointers. The semantics of each
callback are defined in the API docs embedded
in the previously mentioned header file
</p>
<h2>
<a name="qemuIntegrate" id="qemuIntegrate">QEMU Driver integration</a>
</h2>
<p>
With the QEMU driver, the lock plugin will be set
in the <code>/etc/libvirt/qemu.conf</code> configuration
file by specifying the lock manager name.
</p>
<pre>
lockManager="sanlock"
</pre>
<p>
By default the lock manager will be a 'no op' implementation
for backwards compatibility
</p>
<h2>
<a name="usagePatterns" id="usagePatterns">Lock usage patterns</a>
</h2>
<p>
The following pseudo code illustrates the common
patterns of operations invoked on the lock
manager plugin callbacks.
</p>
<h3>
<a name="usageLockAcquire" id="usageLockAcquire">Lock acquisition</a>
</h3>
<p>
Initial lock acquisition will be performed from the
process that is to own the lock. This is typically
the QEMU child process, in between the fork+exec
pairing. When adding further resources on the fly,
to an existing object holding locks, this will be
done from the libvirtd process.
</p>
<pre>
virLockManagerParam params[] = {
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UUID,
.key = "uuid",
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_STRING,
.key = "name",
.value = { .str = dom->def->name },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UINT,
.key = "id",
.value = { .i = dom->def->id },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UINT,
.key = "pid",
.value = { .i = dom->pid },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_CSTRING,
.key = "uri",
.value = { .cstr = driver->uri },
},
};
mgr = virLockManagerNew(lockPlugin,
VIR_LOCK_MANAGER_TYPE_DOMAIN,
ARRAY_CARDINALITY(params),
params,
0)));
foreach (initial disks)
virLockManagerAddResource(mgr,
VIR_LOCK_MANAGER_RESOURCE_TYPE_DISK,
$path, 0, NULL, $flags);
if (virLockManagerAcquire(lock, NULL, 0) < 0);
...abort...
</pre>
<h3>
<a name="usageLockAttach" id="usageLockAttach">Lock release</a>
</h3>
<p>
The locks are all implicitly released when the process
that acquired them exits, however, a process may
voluntarily give up the lock by running
</p>
<pre>
char *state = NULL;
virLockManagerParam params[] = {
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UUID,
.key = "uuid",
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_STRING,
.key = "name",
.value = { .str = dom->def->name },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UINT,
.key = "id",
.value = { .i = dom->def->id },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_UINT,
.key = "pid",
.value = { .i = dom->pid },
},
{ .type = VIR_LOCK_MANAGER_PARAM_TYPE_CSTRING,
.key = "uri",
.value = { .cstr = driver->uri },
},
};
mgr = virLockManagerNew(lockPlugin,
VIR_LOCK_MANAGER_TYPE_DOMAIN,
ARRAY_CARDINALITY(params),
params,
0)));
foreach (initial disks)
virLockManagerAddResource(mgr,
VIR_LOCK_MANAGER_RESOURCE_TYPE_DISK,
$path, 0, NULL, $flags);
virLockManagerRelease(mgr, & state, 0);
</pre>
<p>
The returned state string can be passed to the
<code>virLockManagerAcquire</code> method to
later re-acquire the exact same locks. This
state transfer is commonly used when performing
live migration of virtual machines. By validating
the state the lock manager can ensure no other
VM has re-acquire the same locks on a different
host. The state can also be obtained without
releasing the locks, by calling the
<code>virLockManagerInquire</code> method.
</p>
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