<?xml version="1.0"?> <html> <body> <h1>Virtual machine disk locking</h1> <ul id="toc"></ul> <p> This page describes how to ensure a single disk cannot be used by more than one running VM at a time, across any host in a network. This is critical to avoid data corruption of guest files systems that are not cluster aware. </p> <h2><a name="plugins">Lock manager plugins</a></h2> <p> libvirt includes a pluggable framework for lock managers, which hypervisor drivers can use to ensure safety for guest domain disks, and potentially other resources. At this time there are only two plugin implementations, a "no op" implementation which does absolutely nothing, and a <a href="https://fedorahosted.org/sanlock/">sanlock</a> implementation which uses the Disk Paxos algorithm to ensure safety. </p> <h2><a name="sanlock">Sanlock daemon setup</a></h2> <p> On many operating systems, the <strong>sanlock</strong> plugin is distributed in a sub-package which needs to be installed separately from the main libvirt RPM. On a Fedora/RHEL host this can be done with the <code>yum</code> command </p> <pre> $ su - root # yum install libvirt-lock-sanlock </pre> <p> The next step is to start the sanlock daemon. For maximum safety sanlock prefers to have a connection to a watchdog daemon. This will cause the entire host to be rebooted in the event that sanlock crashes / terminates abnormally. To start the watchdog daemon on a Fedora/RHEL host the following commands can be run: </p> <pre> $ su - root # chkconfig wdmd on # service wdmd start </pre> <p> Once the watchdog is running, sanlock can be started as follows </p> <pre> # chkconfig sanlock on # service sanlock start </pre> <p> <em>Note:</em> if you wish to avoid the use of the watchdog, add the following line to <code>/etc/sysconfig/sanlock</code> before starting it </p> <pre> SANLOCKOPTS="-w 0" </pre> <p> The sanlock daemon must be started on every single host that will be running virtual machines. So repeat these steps as necessary. </p> <h2><a name="sanlockplugin">libvirt sanlock plugin configuration</a></h2> <p> Once the sanlock daemon is running, the next step is to configure the libvirt sanlock plugin. There is a separate configuration file for each libvirt driver that is using sanlock. For QEMU, we will edit <code>/etc/libvirt/qemu-sanlock.conf</code> There is one mandatory parameter that needs to be set, the <code>host_id</code>. This is a integer between 1 and 2000, which must be set to a <strong>unique</strong> value on each host running virtual machines. </p> <pre> $ su - root # augtool -s set /files/etc/libvirt/qemu-sanlock.conf/host_id 1 </pre> <p> Repeat this on every host, changing <strong>1</strong> to a unique value for the host. </p> <h2><a name="sanlockstorage">libvirt sanlock storage configuration</a></h2> <p> The sanlock plugin needs to create leases in a directory that is on a filesystem shared between all hosts running virtual machines. Obvious choices for this include NFS or GFS2. The libvirt sanlock plugin expects its lease directory be at <code>/var/lib/libvirt/sanlock</code> so update the host's <code>/etc/fstab</code> to mount a suitable shared/cluster filesystem at that location </p> <pre> $ su - root # echo "some.nfs.server:/export/sanlock /var/lib/libvirt/sanlock nfs hard,nointr 0 0" >> /etc/fstab # mount /var/lib/libvirt/sanlock </pre> <p> In terms of storage requirements, if the filesystem uses 512 byte sectors, you need to allow for <code>1MB</code> of storage for each guest disk. So if you have a network with 20 virtualization hosts, each running 50 virtual machines and an average of 2 disks per guest, you will need <code>20*50*2 == 2000 MB</code> of storage for sanlock. </p> <p> On one of the hosts on the network is it wise to setup a cron job which runs the <code>virt-sanlock-cleanup</code> script periodically. This scripts deletes any lease files which are not currently in use by running virtual machines, freeing up disk space on the shared filesystem. Unless VM disks are very frequently created + deleted it should be sufficient to run the cleanup once a week. </p> <h2><a name="qemuconfig">QEMU/KVM driver configuration</a></h2> <p> The QEMU/KVM driver is fully integrated with the lock manager framework as of release <span>0.9.3</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>sanlock</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 sanlock # service libvirtd restart </pre> <p> If all went well, libvirtd will have talked to sanlock and created the basic lockspace. This can be checked by looking for existence of the following file </p> <pre> # ls /var/lib/libvirt/sanlock/ __LIBVIRT__DISKS__ </pre> <p> Every time you start a guest, additional lease files will appear in this directory, one for each virtual disk. The lease files are named based on the MD5 checksum of the fully qualified path of the virtual disk backing file. So if the guest is given a disk backed by <code>/var/lib/libvirt/images/demo.img</code> expect to see a lease <code>/var/lib/libvirt/sanlock/bfa0240911bc17753e0b473688822159</code> </p> <p> It should be obvious that for locking to work correctly, every host running virtual machines should have storage configured in the same way. The easiest way to do this is to use the libvirt storage pool capability to configure any NFS volumes, iSCSI targets, or SCSI HBAs used for guest storage. Simply replicate the same storage pool XML across every host. It is important that any storage pools exposing block devices are configured to create volume paths under <code>/dev/disks/by-path</code> to ensure stable paths across hosts. An example iSCSI configuration which ensures this is: </p> <pre> <pool type='iscsi'> <name>myiscsipool</name> <source> <host name='192.168.254.8'/> <device path='your-iscsi-target-iqn'/> </source> <target> <path>/dev/disk/by-path</path> </target> </pool> </pre> </body> </html>