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.

       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.

       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 VG system ID when exporting the VG.

       vgimport sets the VG system ID to the system ID of the host doing the import.

   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.

SEE ALSO

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