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4.6. 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.
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.
To take advantage of the daemon, you must do the following:
- Start the daemon through the
lvm2-lvmetadservice. To start the daemon automatically at boot time, use thechkconfig lvm2-lvmetad oncommand. To start the daemon manually, use theservice lvm2-lvmetad startcommand. - Configure LVM to make use of the daemon by setting the
global/use_lvmetadvariable to 1 in thelvm.confconfiguration file. For information on thelvm.confconfiguration file, see Appendix B, The LVM Configuration Files.
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, by means of 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. For information on the udev device manager and udev rules, see Section A.3, “Device Mapper Support for the udev Device Manager”.
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 and logical volumes that should be automatically activated. Without the lvmetad daemon, a manual activation is necessary. By default, this list is not defined, which means that all volumes are autoactivated once all of the physical volumes are in place. The autoactivation works recursively for LVM stacked on top of other devices, as it is event-based.
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.
