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man lvmsystemid: expanded limitations and warnings

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David Teigland 2015-02-20 12:21:23 -06:00
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1 changed files with 44 additions and 11 deletions
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man/lvmsystemid.7.in
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@ -23,23 +23,56 @@ 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 will ignore it.
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
.B foreign VG
is a VG with a system_id as viewed by a host with a system_id
that does not match the VG's system_id. (Or from a host without a
system_id.)
To benefit fully from system_id, all hosts must have system_id set, and
VGs must have system_id set. Two hosts should not be assigned the same
system_id. Doing so defeats the purpose of the 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.
maximum length of a system_id is 127 characters.
.SS 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 corrupted or
destroyed in the following cases which the user must be careful to avoid:
.IP \[bu] 2
A host using an old version of lvm without the system_id feature will not
recognize the system_id of other hosts' VGs and will not be stopped from
using them.
.IP \[bu] 2
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).
.IP \[bu] 2
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.
.IP \[bu] 2
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
.B orphans
section for more information.
.P
.SS Types of VG access
A "local VG" is meant to be used by a single host.
@ -267,10 +300,10 @@ 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. If the other hosts run a command to rescan
devices, and update lvmetad, they would then recognize the PV has become
used by another host. A command that rescans devices could be pvscan
--cache, or vgs --foreign.
the newly allocated PV and corrupting it. If the other hosts run a
command to rescan devices, and update lvmetad, they would then recognize
the PV has been used by another host. A command that rescans devices
could be pvscan --cache, or vgs --foreign.
.SH SEE ALSO
.BR vgcreate (8),