event autoactivation¶

The most common form of autoactivation is "event based", in which complete VGs are activated in response to uevents which occur during system startup or at any time after the system has started. Another form of autoactivation is "static" in which complete VGs are activated at a fixed point during system startup by a systemd service, and not in response to events. This can be controlled with the lvm.conf setting event_activation.

Event based autoactivation is driven by udev, udev rules, and systemd. When a device is attached to a machine, a uevent is generated by the kernel to notify userspace of the new device. systemd-udev runs udev rules to process the new device. Udev rules use blkid to identify the device as an LVM PV and then execute the lvm-specific udev rule for the device, which triggers autoactivation.

There are two variations of event baed autoactivation that may be used on a system, depending on the LVM udev rule that is installed (found in /lib/udev/rules.d/.) The following summarizes the steps in each rule which lead to autoactivation:

69-dm-lvm-metad.rules

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device /dev/name with major:minor X:Y is attached to the machine

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systemd/udev runs blkid to identify /dev/name as an LVM PV

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udev rule 69-dm-lvm-metad.rules is run for /dev/name

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the lvm udev rule runs the systemd service lvm2-pvscan@X:Yservice

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the lvm2-pvscan service runs:
pvscan --cache -aay --major X --minor Y

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pvscan reads the device, records that the PV is online (see pvs_online), and checks if the VG is complete.

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if the VG is complete, pvscan creates the vgs_online temp file, and activates the VG.

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the activation command output can be seen from systemctl status lvm2-pvscan*

69-dm-lvm.rules

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device /dev/name with major:minor X:Y is attached to the machine

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systemd/udev runs blkid to identify /dev/name as an LVM PV

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udev rule 69-dm-lvm.rules is run for /dev/name

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the lvm udev rule runs:
pvscan --cache --listvg --checkcomplete --vgonline
--autoactivation event --udevoutput --journal=output /dev/name

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pvscan reads the device, records that the PV is online (see pvs_online), and checks if the VG is complete.

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if the VG is complete, pvscan creates the vgs_online temp file, and prints the name of the VG for the udev rule to import: LVM_VG_NAME_COMPLETE='vgname'

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if the lvm udev rule sees LVM_VG_NAME_COMPLETE from pvscan, it activates the VG using a transient systemd service named lvm-activate-<vgname>.

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the lvm-activate-<vgname> service runs
vgchange -aay --autoactivation event <vgname>

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the activation command output can be seen from journalctl -u lvm-activate-<vgname>

pvscan options¶

--cache
Read the <device> arg (and only that device), and record that the PV is online by creating the /run/lvm/pvs_online/<pvid> file containing the name of the VG and the device for the PV.

-aay
Activate the VG from the pvscan command (includes implicit --checkcomplete and --vgonline.)

--checkcomplete
Check if the VG is complete, i.e. all PVs are present on the system, by checking /run/lvm/pvs_online/<pvid> files.

--vgonline
Create /run/lvm/vgs_online/<vgname> if the VG is complete (used to ensure only one command performs activation.)

--autoactivation event
Inform the command it is used for event based autoactivation.

--listvg
Print the name of the VG using the device.

--udevoutput
Only print output that can be imported to the udev rule, using the udev environment key format, i.e. NAME='value'.

--journal=output
Send standard command output to the journal (when stdout is reserved for udev output.)

run files¶

Autoactivation commands use a number of temp files in /run/lvm (with the expectation that /run is cleared between boots.)

pvs_online
pvscan --cache creates a file here for each PV that is attached. The file is named with the PVID and contains the VG name and device information. The existence of the file is used to determine when all PVs for a given VG are present. The device information in these files is also used to optimize locating devices for a VG when the VG is activated.

pvs_lookup
pvscan --cache creates a file here named for a VG (if one doesn't already exist.) The file contains a list of PVIDs in the VG. This is needed when a PV is processed which has no VG metadata, in which case the list of PVIDs from the lookup file is used to check if the VG is complete.

vgs_online
The first activation command (pvscan or vgchange) to create a file here, named for the VG, will activate the VG. This resolves a race when concurrent commands attempt to activate a VG at once.

static autoactivation¶

When event autoactivation is disabled by setting lvm.conf event_activation=0, autoactivation is performed at one or more static points during system startup. At these points, a vgchange -aay command is run to activate complete VGs from devices that are present on the system at that time. pvscan commands (and lvm2-pvscan services) do not perform autoactivation in this mode. pvscan commands may still be run from uevents but will do nothing when they read the event_activation=0 setting.

The static vgchange -aay commands are run by three systemd services at three points during startup: lvm2-activation-early, lvm2-activation, and lvm2-activation-net. These static activation services are "generated services", so the service files are created at run time by the lvm2-activation-generator command (run by systemd). lvm2-activation-generator creates the services if lvm.conf event_activation=0.

The limitation of this method is that devices may not be attached to the system (or set up) at a reliable point in time during startup, and they may not be present when the services run vgchange. In this case, the VGs will not be autoactivated. So, the timing of device attachment/setup determines whether static autoactivation will produce the same results as event autoactivation. For this reason, static autoactivation is not recommended.

Sometimes, static autoactivation is mistakenly expected to disable all autoactivation of particular VGs. This may appear to be effective if those VGs are slow to be attached or set up. But, the only correct and reliable way to disable autoactivation is using vgchange/lvchange --setautoactivation n, or lvm.conf auto_activation_volume_list.