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LVMSYSTEMID(7)                                                  LVMSYSTEMID(7)



NAME
       lvmsystemid -- LVM system ID


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  charac-
       ters,  those characters are omitted and remaining characters are used.  If a system_id is longer than the maxi-
       mum 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  sys-
         tem_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.  Com-
         mands that use these PVs, such as vgcreate or vgextend, are not prevented from performing conflicting  opera-
         tions 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 sys-
       tem_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 clus-
       tered 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 can-
       not be changed, and is not used to control access.  When system_id_source is "uname", the  system_id  and  cre-
       ation_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  sys-
       tem_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), lvm.conf(5), machine-id(5), uname(2), vgs(8)




Red Hat, Inc       LVM TOOLS 2.02.143(2)-RHEL6 (2016-12-13)     LVMSYSTEMID(7)