DESCRIPTION

Local VGs may exist on shared storage where they are visible to multiple hosts. These VGs are intended to be used by only a single machine, even though they are visible to many. A system_id identifying a single host can be assigned to a VG to indicate the VGs owner. The VG owner can use the VG as usual, and all other hosts will ignore it. This protects the VG from accidental use by other hosts.

The system_id is not a dynamic property, and can only be changed in very limited circumstances (see vgexport and vgimport). Even limited changes to the VG system_id are not perfectly reflected across hosts. A more coherent view of shared storage requires using an inter-host locking system to coordinate access and update caches.

The system_id is a string uniquely identifying a host. It can be manually set to a custom value or it can be assigned automatically by lvm using a unique identifier already available on the host, e.g. machine-id or uname.

In vgcreate, the local system_id is saved in the new VG metadata. The local host owns the new VG, and other hosts cannot use it.

A VG without a system_id can be used by any host, and a VG with a system_id can only be used by a host with a matching system_id. A foreign VG is a VG with a system_id as viewed by a host with a system_id that does not match the VGs system_id. (Or from a host without a system_id.)

Valid system_id characters are the same as valid VG name characters. If a system_id contains invalid characters, those characters are omitted and remaining characters are used. If a system_id is longer than the maximum name length, the characters up to the maximum length are used. The maximum length of a system_id is 128 characters.

Limitations and warnings

To benefit fully from system_id, all hosts must have system_id set, and VGs must have system_id set. A VG on shared storage can be damaged or destroyed in some cases which the user must be careful to avoid.

• A VG without a system_id can be used without restriction from any host, even from hosts that have a system_id. Many VGs will not have a system_id and are unprotected. Verify that a VG has a system_id by running the command 'vgs -o+systemid'

  A VG will not have a system_id if it was created before this feature was added to lvm, or if it was created by a host that did not have a system_id defined. A system_id can be assigned to these VGs by using vgchange --systemid (see below).

• Two hosts should not be assigned the same system_id. Doing so defeats the purpose of the system_id which is to distinguish different hosts.

• Orphan PVs (or unused devices) on shared storage are completely 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.

• A host using an old version of lvm without the system_id feature will not recognize a new system_id in VGs from other hosts. Even though the old version of lvm is not blocked from reading a VG with a system_id, it is blocked from writing to the VG (or its LVs). The new system_id changes the write mode of a VG, making it appear read-only to previous lvm versions.

  This also means that if a host downgrades its version of lvm, it would lose access to any VGs it had created with a system_id. To avoid this, the system_id should be removed from VGs before downgrading to an lvm 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 one host with a matching system_id (the owner). This VG type is by definition acessible.

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 by definition not accessible.

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 lock_type set and 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 lockd support.

Clustered: A clustered or "clvm" VG has the clustered flag set and 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.

system_id_source

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 often cause the system_id to change, which may 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 resolve a system_id, the host will be allowed to access VGs with no system_id, but will not be allowed to access VGs with a defined system_id.

extra_system_ids

In some cases, it may be useful for a host to access VGs with different system_id's, 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_id's, those other system_id's can be listed in lvmlocal.conf local/extra_system_ids.

lvmlocal.conf

local {
    extra_system_ids = [ "my_other_name" ]
}

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 Devices

Overriding the 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 local system_id as determined by lvm.conf system_id_source. vgimport automatically scans storage for newly exported VGs.

After vgimport, the exporting host will 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 add the foreign system_id to its extra_system_ids list, change the system_id of the foreign VG to its own, and remove the foreign system_id from its extra_system_ids list.

shared VGs

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