Scanning with lvmetad¶

pvscan operates differently when used with the lvmetad(8) daemon.

Scanning disks is required to read LVM metadata and identify LVM PVs. Once read, lvmetad caches the metadata so that LVM commands can read it without repeatedly scanning disks. This is helpful because scanning disks is time consuming, and frequent scanning may interfere with the normal work of the system and disks.

When lvmetad is not used, LVM commands revert to scanning disks to read metadata. Any LVM command that needs metadata will scan disks for it; running the pvscan command is not necessary for the sake of other LVM commands.

When lvmetad is used, LVM commands avoid scanning disks by reading metadata from lvmetad. When new disks appear, they must be scanned so their metadata can be cached in lvmetad. This is done by the command pvscan --cache, which scans disks and passes the metadata to lvmetad.

The pvscan --cache command is typically run automatically by system services when a new device appears. Users do not generally need to run this command if the system and lvmetad are running properly.

Many scripts contain unnecessary pvscan (or vgscan) commands for historical reasons. To avoid disrupting the system with extraneous disk scanning, an ordinary pvscan (without --cache) will simply read metadata from lvmetad like other LVM commands. It does not do anything beyond displaying the current state of the cache.

Notes

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When given specific device name arguments, pvscan --cache will only read the named devices.

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LVM udev rules and systemd services are used to intiate automatic device scanning.

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To prevent devices from being scanned by pvscan --cache, add them to lvm.conf(5) devices/global_filter. The devices/filter setting does not apply to system level scanning. For more information, see:
lvmconfig --withcomments devices/global_filter

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If lvmetad is started or restarted after devices are visible, or if the global_filter has changed, then all devices must be rescanned for metadata with the command pvscan --cache.

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lvmetad ignores older metadata formats, e.g. lvm1, and should not be used if they exist.

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To notify lvmetad about a device that is no longer present, the major and minor numbers must be given, not the path.

Automatic activation¶

When event-driven system services detect a new LVM device, the first step is to automatically scan and cache the metadata from the device. This is done by pvscan --cache. A second step is to automatically activate LVs that are present on the new device. This auto-activation is done by the same pvscan --cache command when the option '-a|--activate ay' is included.

Auto-activation of VGs or LVs can be enabled/disabled using:
lvm.conf(5) activation/auto_activation_volume_list

For more information, see:
lvmconfig --withcomments activation/auto_activation_volume_list

When this setting is undefined, all LVs are auto-activated (when lvm is fully integrated with the event-driven system services.)

When a VG or LV is not auto-activated, traditional activation using vgchange or lvchange -a|--activate is needed.

Notes

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pvscan auto-activation can be only done in combination with --cache.

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Auto-activation is designated by the "a" argument in '-a|--activate ay'. This is meant to distinguish system generated commands from explicit user commands, although it can be used in any activation command. Whenever it is used, the auto_activation_volume_list is applied.

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Auto-activation is not yet supported for LVs that are part of partial or clustered volume groups.