10.2. The LVM device filter
The Logical Volume Manager (LVM) device filter is a list of device name patterns. You can use it to specify a set of mandatory criteria by which the system can evaluate devices and consider them as valid for use with LVM. The LVM device filter enables you control over which devices LVM uses. This can help to prevent accidental data loss or unauthorized access to storage devices.
10.2.1. LVM device filter pattern characteristics
The patterns of LVM device filter are in the form of regular expression. A regular expression delimits with a character and precedes with either a for acceptance, or r for rejection. The first regular expression in the list that matches a device determines if LVM accepts or rejects (ignores) a specific device. Then, LVM looks for the initial regular expression in the list that matches the path of a device. LVM uses this regular expression to determine whether the device should be approved with an a outcome or rejected with an r outcome.
If a single device has multiple path names, LVM accesses these path names according to their order of listing. Before any r pattern, if at least one path name matches an a pattern, LVM approves the device. However, if all path names are consistent with an r pattern before an a pattern is found, the device is rejected.
Path names that do not match the pattern do not affect the approval status of the device. If no path names correspond to a pattern for a device, LVM still approves the device.
For each device on the system, the udev rules generate multiple symlinks. Directories contain symlinks, such as /dev/disk/by-id/, /dev/disk/by-uuid/, /dev/disk/by-path/ to ensure that each device on the system is accessible through multiple path names.
To reject a device in the filter, all of the path names associated with that particular device must match the corresponding reject r expressions. However, identifying all possible path names to reject can be challenging. This is why it is better to create filters that specifically accept certain paths and reject all others, using a series of specific a expressions followed by a single r|.*| expression that rejects everything else.
While defining a specific device in the filter, use a symlink name for that device instead of the kernel name. The kernel name for a device can change, such as /dev/sda while certain symlink names do not change such as /dev/disk/by-id/wwn-*.
The default device filter accepts all devices connected to the system. An ideal user configured device filter accepts one or more patterns and rejects everything else. For example, the pattern list ending with r|.*|.
You can find the LVM devices filter configuration in the devices/filter and devices/global_filter configuration fields in the lvm.conf file. The devices/filter and devices/global_filter configuration fields are equivalent.
In Red Hat Enterprise Linux 9, the /etc/lvm/devices/system.devices file is enabled by default. The system automatically enables the LVM devices filter, when the system.devices file is disabled.
Ressources supplémentaires
-
lvm.conf(5)man page
10.2.2. Examples of LVM device filter configurations
The following examples display the filter configurations to control the devices that LVM scans and uses later. To configure the device filter in the lvm.conf file, see
Some LVM filter configurations can cause duplicate Physical Volume (PV) warnings. See the example filter configurations in to avoid this issue.
To scan all the devices, enter:
filter = [ "|a.*|" ]
To remove the
cdromdevice to avoid delays if the drive contains no media, enter:filter = [ "r|^/dev/cdrom$|" ]
To add all loop devices and remove all other devices, enter:
filter = [ "a|loop|", "r|.*|" ]
To add all loop and Integrated Development Environment (IDE) devices and remove all other block devices, enter:
filter = [ "a|loop|", "a|/dev/hd.*|", "r|.*|" ]
To add only partition 8 on the first IDE drive and remove all other block devices, enter:
filter = [ "a|^/dev/hda8$|", "r|.*|" ]
Ressources supplémentaires
-
lvm.conf(5)man page
10.2.3. Applying an LVM device filter configuration
You can control which devices LVM scans by setting up filters in the lvm.conf configuration file.
Conditions préalables
-
You have disabled the
system.devicesfile feature. - You have prepared the device filter pattern that you want to use.
Procédure
Use the following command to test the device filter pattern, without actually modifying the
/etc/lvm/lvm.conffile. The following includes an example filter configuration.# lvs --config 'devices{ filter = [ "a|/dev/emcpower.|", "r|.|" ] }'
Add the device filter pattern in the configuration section
devicesof the/etc/lvm/lvm.conffile:filter = [ "a|/dev/emcpower.*|", "r|*.|" ]
Verify missing physical volumes and volume groups respectively:
# pvscan -v# vgscan -vScan only necessary devices on reboot:
# dracut --force --verboseThis command rebuilds the
initramfsfile system so that LVM scans only the necessary devices at the time of reboot.
