NAME

       lvmsystemid — LVM system ID

DESCRIPTION

       The  lvm(8) system ID restricts Volume Group (VG) access to one host.  This is useful when a VG is placed
       on shared storage devices, or when local devices are visible to both host and  guest  operating  systems.
       In  cases  like  these,  a  VG  can be visible to multiple hosts at once, and some mechanism is needed to
       protect it from being used by more than one host at a time.

       A VG's system ID identifies one host as the VG owner.  The host with a matching system ID can use the  VG
       and  its LVs, while LVM on other hosts will ignore it.  This protects the VG from being accidentally used
       from other hosts.

       The system ID is a string that uniquely identifies a host.  It can be configured as a custom value, or it
       can  be  assigned  automatically  by LVM using some unique identifier already available on the host, e.g.
       machine-id or uname.

       When a new VG is created, the system ID of the local host is recorded in the VG metadata.   The  creating
       host  then  owns  the  new  VG,  and LVM on other hosts will ignore it.  When an existing, exported VG is
       imported (vgimport), the system ID of the local host is saved in the VG metadata, and the importing  host
       owns the VG.

       A  VG without a system ID can be used by LVM on any host where the VG's devices are visible.  When system
       IDs are not used, device filters should be configured on all hosts to exclude the VG's devices  from  all
       but one host.

       A  foreign  VG is a VG seen by a host with an unmatching system ID, i.e. the system ID in the VG metadata
       does not match the system ID configured on the host.  If the host has no system ID, and the VG does,  the
       VG  is  foreign  and LVM will ignore it.  If the VG has no system ID, access is unrestricted, and LVM can
       access it from any host, whether the host has a system ID or not.

       Changes to a host's system ID and a VG's system ID can be made in limited circumstances (see vgexport and
       vgimport).   Improper  changes  can  result in a host losing access to its VG, or a VG being accidentally
       damaged by access from an unintended host.  Even limited changes to the VG system ID may not be perfectly
       reflected  across hosts.  A more coherent view of shared storage requires an inter-host locking system to
       coordinate access and update caches.

       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.

       Print the system ID of a VG to check if it is set:

       vgs -o systemid VG

       Print the system ID of the local host to check if it is configured:

       lvm systemid

   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.

       • 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.

       • 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).

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

       • 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.

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

       • 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/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 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.

SEE ALSO

       vgcreate(8),  vgchange(8),  vgimport(8),  vgexport(8),  vgs(8),  lvmlockd(8), lvm.conf(5), machine-id(5),
       uname(2)