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.

To take advantage of the daemon, you must do the following:

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.