3.5. The Metadata Daemon (lvmetad)
LVM can optionally use a central metadata cache, implemented through a daemon (
lvmetad) and a udev rule. The metadata daemon has two main purposes: it improves performance of LVM commands and it allows udev to automatically activate logical volumes or entire volume groups as they become available to the system.
LVM is configured to make use of the daemon when the
global/use_lvmetad variable is set to 1 in the lvm.conf configuration file. This is the default value. For information on the lvm.conf configuration file, see Appendix B, The LVM Configuration Files.
Note
The
lvmetad daemon is not currently supported across the nodes of a cluster, and requires that the locking type be local file-based locking. When you use the lvmconf --enable-cluster/--disable-cluster command, the lvm.conf file is configured appropriately, including the use_lvmetad setting (which should be 0 for locking_type=3). Note, however, that in a Pacemaker cluster, the ocf:heartbeat:clvm resource agent itself sets these parameters as part of the start procedure.
If you change the value of
use_lvmetad from 1 to 0, you must reboot or stop the lvmetad service manually with the following command.
# systemctl stop lvm2-lvmetad.service
Normally, each LVM command issues a disk scan to find all relevant physical volumes and to read volume group metadata. However, if the metadata daemon is running and enabled, this expensive scan can be skipped. Instead, the
lvmetad daemon scans each device only once, when it becomes available, using udev rules. This can save a significant amount of I/O and reduce the time required to complete LVM operations, particularly on systems with many disks.
When a new volume group is made available at runtime (for example, through hotplug or iSCSI), its logical volumes must be activated in order to be used. When the
lvmetad daemon is enabled, the activation/auto_activation_volume_list option in the lvm.conf configuration file can be used to configure a list of volume groups or logical volumes (or both) that should be automatically activated. Without the lvmetad daemon, a manual activation is necessary.
Note
When the
lvmetad daemon is running, the filter = setting in the /etc/lvm/lvm.conf file does not apply when you execute the pvscan --cache device command. To filter devices, you need to use the global_filter = setting. Devices that fail the global filter are not opened by LVM and are never scanned. You may need to use a global filter, for example, when you use LVM devices in VMs and you do not want the contents of the devices in the VMs to be scanned by the physical host.
