Limitations and warnings¶

To benefit fully from system ID, all hosts should have a system ID configured, and all VGs should have a system ID set. Without any method to restrict access, e.g. system ID or device filters, a VG that is visible to multiple hosts can be accidentally damaged or destroyed.

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A VG without a system ID can be used without restriction from any host where it is visible, even from hosts that have a system ID.

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Many VGs will not have a system ID set because LVM has not enabled it by default, and even when enabled, many VGs were created before the feature was added to LVM or enabled. A system ID can be assigned to these VGs by using vgchange --systemid (see below).

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Two hosts should not be assigned the same system ID. Doing so defeats the purpose of distinguishing different hosts with this value.

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Orphan PVs (or unused devices) on shared storage are unprotected by the system ID feature. Commands that use these PVs, such as vgcreate or vgextend, are not prevented from performing conflicting operations and corrupting the PVs. See the orphans section for more information.

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The system ID does not protect devices in a VG from programs other than LVM.

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A host using an old LVM version (without the system ID feature) will not recognize a system ID set in VGs. The old LVM can read a VG with a system ID, but is prevented from writing to the VG (or its LVs). The system ID feature changes the write mode of a VG, making it appear read-only to previous versions of LVM.

This also means that if a host downgrades to the old LVM version, it would lose access to any VGs it had created with a system ID. To avoid this, the system ID should be removed from local VGs before downgrading LVM to a version without the system ID feature.

Types of VG access¶

A local VG is meant to be used by a single host.

A shared or clustered VG is meant to be used by multiple hosts.

These can be further distinguished as:

Unrestricted: A local VG that has no system ID. This VG type is unprotected and accessible to any host.

Owned: A local VG that has a system ID set, as viewed from the host with a matching system ID (the owner). This VG type is acessible to the host.

Foreign: A local VG that has a system ID set, as viewed from any host with an unmatching system ID (or no system ID). It is owned by another host. This VG type is not accessible to the host.

Exported: A local VG that has been exported with vgexport and has no system ID. This VG type can only be accessed by vgimport which will change it to owned.

Shared: A shared or "lockd" VG has the lock_type set and has no system ID. A shared VG is meant to be used on shared storage from multiple hosts, and is only accessible to hosts using lvmlockd. Applicable only if LVM is compiled with lvmlockd support.

Clustered: A clustered or "clvm" VG has the clustered flag set and has no system ID. A clustered VG is meant to be used on shared storage from multiple hosts, and is only accessible to hosts using clvmd. Applicable only if LVM is compiled with clvm support.

Host system ID configuration¶

A host's own system ID can be defined in a number of ways. lvm.conf global/system_id_source defines the method LVM will use to find the local system ID:

none

LVM will not use a system ID. LVM is allowed to access VGs without a system ID, and will create new VGs without a system ID. An undefined system_id_source is equivalent to none.

lvm.conf

global {


    system_id_source = "none"
}

machineid

The content of /etc/machine-id is used as the system ID if available. See machine-id(5) and systemd-machine-id-setup(1) to check if machine-id is available on the host.

lvm.conf

global {


    system_id_source = "machineid"
}

uname

The string utsname.nodename from uname(2) is used as the system ID. A uname beginning with "localhost" is ignored and equivalent to none.

lvm.conf

global {


    system_id_source = "uname"
}

lvmlocal

The system ID is defined in lvmlocal.conf local/system_id.

lvm.conf

global {


    system_id_source = "lvmlocal"
}

lvmlocal.conf

local {


    system_id = "example_name"
}

file

The system ID is defined in a file specified by lvm.conf global/system_id_file.

lvm.conf

global {


    system_id_source = "file"


    system_id_file = "/path/to/file"
}

Changing system_id_source will likely cause the system ID of the host to change, which will prevent the host from using VGs that it previously used (see extra_system_ids below to handle this.)

If a system_id_source other than none fails to produce a system ID value, it is the equivalent of having none. The host will be allowed to access VGs with no system ID, but will not be allowed to access VGs with a system ID set.

Overriding system ID¶

In some cases, it may be necessary for a host to access VGs with different system IDs, e.g. if a host's system ID changes, and it wants to use VGs that it created with its old system ID. To allow a host to access VGs with other system IDs, those other system IDs can be listed in lvmlocal.conf local/extra_system_ids.

lvmlocal.conf

local {


    extra_system_ids = [ "my_other_name" ]
}

A safer option may be configuring the extra values as needed on the command line as:
--config 'local/extra_system_ids=["id"]'

vgcreate¶

In vgcreate, the host running the command assigns its own system ID to the new VG. To override this and set another system ID:

vgcreate --systemid SystemID VG PVs

Overriding the host's system ID makes it possible for a host to create a VG that it may not be able to use. Another host with a system ID matching the one specified may not recognize the new VG without manually rescanning devices.

If the --systemid argument is an empty string (""), the VG is created with no system ID, making it accessible to other hosts (see warnings above.)

report/display¶

The system ID of a VG is displayed with the "systemid" reporting option.

Report/display commands ignore foreign VGs by default. To report foreign VGs, the --foreign option can be used. This causes the VGs to be read from disk. Because lvmetad caching is not used, this option can cause poor performance.

vgs --foreign -o +systemid

When a host with no system ID sees foreign VGs, it warns about them as they are skipped. The host should be assigned a system ID, after which standard reporting commands will silently ignore foreign VGs.

vgexport/vgimport¶

vgexport clears the system ID.

Other hosts will continue to see a newly exported VG as foreign because of local caching (when lvmetad is used). Manually updating the local lvmetad cache with pvscan --cache will allow a host to recognize the newly exported VG.

vgimport sets the VG system ID to the system ID of the host doing the import. vgimport automatically scans storage for newly exported VGs.

After vgimport, the exporting host may continue to see the VG as exported, and not owned by the new host. Manually updating the local cache with pvscan --cache will allow a host to recognize the newly imported VG as foreign.

vgchange¶

A host can change the system ID of its own VGs, but the command requires confirmation because the host may lose access to the VG being changed:

vgchange --systemid SystemID VG

The system ID can be removed from a VG by specifying an empty string ("") as the new system ID. This makes the VG accessible to other hosts (see warnings above.)

A host cannot directly change the system ID of a foreign VG.

To move a VG from one host to another, vgexport and vgimport should be used.

To forcibly gain ownership of a foreign VG, a host can temporarily add the foreign system ID to its extra_system_ids list, and change the system ID of the foreign VG to its own. See Overriding system ID above.

shared VGs¶

A shared VG has no system ID set, allowing multiple hosts to use it via lvmlockd. Changing a VG to shared will clear the existing system ID. Applicable only if LVM is compiled with lvmlockd support.

clustered VGs¶

A clustered/clvm VG has no system ID set, allowing multiple hosts to use it via clvmd. Changing a VG to clustered will clear the existing system ID. Changing a VG to not clustered will set the system ID to the host running the vgchange command.

creation_host¶

In vgcreate, the VG metadata field creation_host is set by default to the host's uname. The creation_host cannot be changed, and is not used to control access. When system_id_source is "uname", the system_id and creation_host fields will be the same.

orphans¶

Orphan PVs are unused devices; they are not currently used in any VG. Because of this, they are not protected by a system ID, and any host can use them. Coordination of changes to orphan PVs is beyond the scope of system ID. The same is true of any block device that is not a PV.

The effects of this are especially evident when LVM uses lvmetad caching. For example, if multiple hosts see an orphan PV, and one host creates a VG using the orphan, the other hosts will continue to report the PV as an orphan. Nothing would automatically prevent the other hosts from using the newly allocated PV and corrupting it. If the other hosts run a command to rescan devices, and update lvmetad, they would then recognize that the PV has been used by another host. A command that rescans devices could be pvscan --cache, or vgs --foreign.