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Chapter 10. Limiting LVM device visibility and usage

You can limit the devices that are visible and usable to Logical Volume Manager (LVM) by controlling the devices that LVM can scan.

To adjust the configuration of LVM device scanning, edit the LVM device filter settings in the /etc/lvm/lvm.conf file. The filters in the lvm.conf file consist of a series of simple regular expressions. The system applies these expressions to each device name in the /dev directory to decide whether to accept or reject each detected block device.

10.1. Persistent identifiers for LVM filtering
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Traditional Linux device names, such as /dev/sda, are subject to changes during system modifications and reboots. Persistent Naming Attributes (PNAs) like World Wide Identifier (WWID), Universally Unique Identifier (UUID), and path names are based on unique characteristics of the storage devices and are resilient to changes in hardware configurations. This makes them more stable and predictable across system reboots.

Implementation of persistent device identifiers in LVM filtering enhances the stability and reliability of LVM configurations. It also reduces the risk of system boot failures associated with the dynamic nature of device names.

10.2. The LVM device filter
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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
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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.

10.2.2. Examples of LVM device filter configurations
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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 Applying an LVM device filter configuration

Note

You might encounter duplicate Physical Volume (PV) warnings when dealing with copied or cloned PVs. You can set up filters to resolve this. See the example filter configurations in Example LVM device filters that prevent duplicate PV warnings.

  • To scan all the devices, enter: 

    filter = [ "a|.*|" ]
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  • To remove the cdrom device to avoid delays if the drive contains no media, enter: 

    filter = [ "r|^/dev/cdrom$|" ]
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  • To add all loop devices and remove all other devices, enter: 

    filter = [ "a|loop|", "r|.*|" ]
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  • To add all loop and SCSI devices and remove all other block devices, enter: 

    filter = [ "a|loop|", "a|/dev/sd.*|", "r|.*|" ]
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  • To add only partition 8 on the first SCSI drive and remove all other block devices, enter: 

    filter = [ "a|^/dev/sda8$|", "r|.*|" ]
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  • To add all partitions from a specific device identified by WWID along with all multipath devices, enter: 

    filter = [ "a|/dev/disk/by-id/<disk-id>.|", "a|/dev/mapper/mpath.|", "r|.*|" ]
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    The command also removes any other block devices.

10.2.3. Applying an LVM device filter configuration
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You can control which devices LVM scans by setting up filters in the lvm.conf configuration file.

Prerequisites

  • You have prepared the device filter pattern that you want to use.

Procedure

  1. Use the following command to test the device filter pattern, without actually modifying the /etc/lvm/lvm.conf file. The following includes an example filter configuration. 

    # lvs --config 'devices{ filter = [ "a|/dev/emcpower.|", "r|.|" ] }'
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  2. Add the device filter pattern in the configuration section devices of the /etc/lvm/lvm.conf file: 

    filter = [ "a|/dev/emcpower.*|", "r|*.|" ]
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  3. Scan only necessary devices on reboot: 

    # dracut --force --verbose
    Copy to Clipboard Toggle word wrap

    This command rebuilds the initramfs file system so that LVM scans only the necessary devices at the time of reboot.

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