идентификатор системы LVM (LVM system ID)
Описание (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 accessible 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.
