The LVM devices file lists devices that lvm can use.  The default file is
/etc/lvm/devices/system.devices, and the lvmdevices(8) command is used to
add or remove device entries.  If the file does not exist, or if lvm.conf
includes use_devicesfile=0, then lvm will not use a devices file.

To use a device with lvm, add it to the devices file with the command
lvmdevices --adddev, and to prevent lvm from seeing or using a device,
remove it from the devices file with lvmdevices --deldev.  The
vgimportdevices(8) command adds all PVs from a VG to the devices file,
and updates the VG metadata to include device IDs of the PVs.

Commands adding new devices to the devices file necessarily look outside
the existing devices file to find the devices to add.  pvcreate, vgcreate,
and vgextend also look outside the devices file to create new PVs and add
them to the devices file.

LVM records devices in the devices file using hardware-specific IDs, such
as the WWID, and attempts to use subsystem-specific IDs for virtual device
types (which also aim to be as unique and stable as possible.)
These device IDs are also written in the VG metadata.  When no hardware or
virtual ID is available, lvm falls back using the unstable device name as
the device ID.  When devnames are used, lvm performs extra scanning to
find devices if their devname changes, e.g. after reboot.

When proper device IDs are used, an lvm command will not look at devices
outside the devices file, but when devnames are used as a fallback, lvm
will scan devices outside the devices file to locate PVs on renamed
devices.  A config setting search_for_devnames can be used to control the
scanning for renamed devname entries.

Related to the devices file, the new command option --devices <devnames>
allows a list of devices to be specified for the command to use,
overriding the devices file.  The listed devices act as a sort of devices
file in terms of limiting which devices lvm will see and use.  Devices
that are not listed will appear to be missing to the lvm command.
    
Multiple devices files can be kept in /etc/lvm/devices, which allows lvm
to be used with different sets of devices, e.g. system devices do not need
to be exposed to a specific application, and the application can use lvm on
its own devices that are not exposed to the system.  The option
--devicesfile <filename> is used to select the devices file to use with the
command.  Without the option set, the default system devices file is used.

Setting --devicesfile "" causes lvm to not use a devices file.

With no devices file, lvm will use any device on the system, and applies
the filter to limit the full set of system devices.  With a devices file,
the regex filter is not used, and the filter settings in lvm.conf or the
command line are ignored.  The vgimportdevices command is one exception
which does apply the regex filter when looking for a VG to import.

If a devices file exists, lvm will use it, even if it's empty.  An empty
devices file means lvm will see no devices.

If the system devices file does not yet exist, the pvcreate or vgcreate
commands will create it if they see no existing VGs on the system.
lvmdevices --addev and vgimportdevices will always create a new devices file
if it does not yet exist.

It is recommended to use lvm commands to make changes to the devices file to
ensure proper updates.