lvm2/man/lvmdevices.8_des

The LVM devices file lists devices that lvm can use.  The default file is
\fI#DEFAULT_SYS_DIR#/devices/system.devices\fP, and the \fBlvmdevices\fP(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.
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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.
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Commands that add new devices to the devices file necessarily look outside
the existing devices file to find the devices being added.  pvcreate,
vgcreate, and vgextend also look outside the devices file to create new
PVs and add those PVs to the devices file.
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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 as IDs, lvm performs extra scanning
to find devices if their devname changes, e.g. after reboot.
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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.
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Related to the devices file, the 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.
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Multiple devices files can be kept in \fI#DEFAULT_SYS_DIR#/devices\fP, which
allows lvm to be used with different sets of devices.  For example, 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.
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Setting --devicesfile "" causes lvm to not use a devices file.
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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.
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If a devices file exists, lvm will use it, even if it's empty.  An empty
devices file means lvm will see no devices.
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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.
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It is recommended to use lvm commands to make changes to the devices file to
ensure proper updates.
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The device ID and device ID type are included in the VG metadata and can
be reported with pvs -o deviceid,deviceidtype.  (Note that the lvmdevices
command does not update VG metadata, but subsequent lvm commands modifying
the metadata will include the device ID.)
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Possible device ID types are:
.br
.IP \[bu] 2
.B sys_wwid
uses the wwid reported by the wwid sysfs file. This is the first choice.
.IP \[bu] 2
.B wwid_naa
uses the naa wwid decoded from the vpd_pg83 sysfs file.
.IP \[bu] 2
.B wwid_eui
uses the eui wwid decoded from the vpd_pg83 sysfs file.
.IP \[bu] 2
.B wwid_t10
uses the t10 wwid decoded from the vpd_pg83 sysfs file.
.IP \[bu] 2
.B sys_serial
uses the serial number reported by the serial sysfs file or the vpd_pg80
file. A serial number is used if no wwid is available.
.IP \[bu] 2
.B mpath_uuid
is used for dm multipath devices, reported by sysfs.
.IP \[bu] 2
.B crypt_uuid
is used for dm crypt devices, reported by sysfs.
.IP \[bu] 2
.B md_uuid
is used for md devices, reported by sysfs.
.IP \[bu] 2
.B lvmlv_uuid
is used if a PV is placed on top of an lvm LV, reported by sysfs.
.IP \[bu] 2
.B loop_file
is used for loop devices, the backing file name repored by sysfs.
.IP \[bu] 2
.B devname
the device name is used if no other type applies.
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The default choice for device ID type can be overridden using lvmdevices
--addev --deviceidtype <type>.  If the specified type is available for the
device it will be used, otherwise the device will be added using the type
that would otherwise be chosen.

LVM commands run by dmeventd will use the devices file
\fI#DEFAULT_SYS_DIR#/devices/dmeventd.devices\fP if it exists,
otherwise system.devices is used.  VGs that require the dmeventd
service should be included in system.devices, even if they are
included in dmeventd.devices.

.SS Device ID refresh
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A machine identifier is saved in the devices file, and is used to detect
when the devices file has been created by a different machine. If the
devices file was created by a different machine, it indicates that PVs may
have been copied or restored onto new devices on a new machine. In this
case, lvm will search for the PVs listed in system.devices on new devices.
If found, the device IDs will be updated in system.devices for the
existing PVIDs (assuming the original device IDs are also no longer
found.)
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The machine identifier used in system.devices will be either the DMI
product_uuid from /sys/devices/virtual/dmi/id/product_uuid, or the
hostname from uname(2). See lvm.conf device_ids_refresh_checks to
configure this.