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 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.
SEE ALSO
vgcreate(8), vgchange(8), vgimport(8), vgexport(8), lvm.conf(5), machine-id(5), uname(2), vgs(8)
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
