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Chapter 12. Grouping LVM objects with tags

You can assign tags to Logical Volume Manager (LVM) objects to group them. With this feature, you can automate the control of LVM behavior, such as activation, by a group. You can also use tags instead of LVM objects arguments.

12.1. LVM object tags
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A Logical Volume Manager (LVM) tag groups LVM objects of the same type. You can attach tags to objects such as physical volumes, volume groups, and logical volumes , as well as to hosts in a cluster configuration .

To avoid ambiguity, prefix each tag with @. Each tag is expanded by replacing it with all the objects that possess that tag and that are of the type expected by its position on the command line.

LVM tags are strings of up to 1024 characters. LVM tags cannot start with a hyphen.

A valid tag consists of a limited range of characters only. The allowed characters are A-Z a-z 0-9 _ + . - / = ! : # &.

Only objects in a volume group can be tagged. Physical volumes lose their tags if they are removed from a volume group; this is because tags are stored as part of the volume group metadata and that is deleted when a physical volume is removed.

You can apply some commands to all volume groups (VG), logical volumes (LV), or physical volumes (PV) that have the same tag. The man page of the given command shows the syntax, such as VG|Tag, LV|Tag, or PV|Tag when you can substitute a tag name for a VG, LV, or PV name.

12.2. Listing LVM tags
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The following example shows how to list LVM tags.

Procedure

  • Use the following command to list all the logical volumes with the database tag: 

    # lvs @database
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  • Use the following command to list the currently active host tags: 

    # lvm tags
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12.3. Adding tags to LVM objects
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You can add tags to LVM objects to group them by using the --addtag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.

Procedure

  • To add a tag to an existing PV, use: 

    # pvchange --addtag <@tag> <PV>
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  • To add a tag to an existing VG, use: 

    # vgchange --addtag <@tag> <VG>
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  • To add a tag to a VG during creation, use: 

    # vgcreate --addtag <@tag> <VG>
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  • To add a tag to an existing LV, use: 

    # lvchange --addtag <@tag> <LV>
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  • To add a tag to a LV during creation, use: 

    # lvcreate --addtag <@tag> ...
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12.4. Removing tags from LVM objects
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If you no longer want to keep your LVM objects grouped, you can remove tags from the objects by using the --deltag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.
  • You have created tags on physical volumes (PV), volume groups (VG), or logical volumes (LV).

Procedure

  • To remove a tag from an existing PV, use: 

    # pvchange --deltag @tag PV
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  • To remove a tag from an existing VG, use: 

    # vgchange --deltag @tag VG
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  • To remove a tag from an existing LV, use: 

    # lvchange --deltag @tag LV
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12.5. Defining LVM host tags
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This procedure describes how to define LVM host tags in a cluster configuration. You can define host tags in the configuration files.

Procedure

  • Set hosttags = 1 in the tags section to automatically define host tag using the machine’s host name.

    This allows you to use a common configuration file which can be replicated on all your machines so they hold identical copies of the file, but the behavior can differ between machines according to the host name.

For each host tag, an extra configuration file is read if it exists: lvm_hosttag.conf. If that file defines new tags, then further configuration files will be appended to the list of files to read in.

For example, the following entry in the configuration file always defines tag1, and defines tag2 if the host name is host1: 

tags { tag1 { }  tag2 { host_list = ["host1"] } }
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12.6. Controlling logical volume activation with tags
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This procedure describes how to specify in the configuration file that only certain logical volumes should be activated on that host.

Procedure

For example, the following entry acts as a filter for activation requests (such as vgchange -ay) and only activates vg1/lvol0 and any logical volumes or volume groups with the database tag in the metadata on that host: 

activation { volume_list = ["vg1/lvol0", "@database" ] }
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The special match @* that causes a match only if any metadata tag matches any host tag on that machine.

As another example, consider a situation where every machine in the cluster has the following entry in the configuration file: 

tags { hosttags = 1 }
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If you want to activate vg1/lvol2 only on host db2, do the following:

  1. Run lvchange --addtag @db2 vg1/lvol2 from any host in the cluster.
  2. Run lvchange -ay vg1/lvol2.

This solution involves storing host names inside the volume group metadata.

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