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glabel.8: Warn against using generic labels on a shared device 

Also suggest against creating a generic label on a device which already
contains a filesystem.

PR:		264166
Reviewed by:	imp, delphij, Pau Amma <pauamma@gundo.com>
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D35326
This commit is contained in:
Mark Johnston 2022-05-26 09:48:17 -04:00
parent a3074ff212
commit 81f36fbc98
1 changed files with 21 additions and 1 deletions
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22
lib/geom/label/glabel.8
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@ -22,7 +22,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd March 12, 2017
.Dd September 27, 2023
.Dt GLABEL 8
.Os
.Sh NAME
@ -141,6 +141,26 @@ Generic labels created and managed solely by
are created in the
.Pa /dev/label/
directory.
Note that generic, automatic labels occupy some space on the device
and thus should not be added to a device already containing a file system.
In particular,
.Nm
reserves the last sector of the device to store the label information.
If the device already contains a file system,
.Nm
will overwrite the last sector, possibly damaging the file system, and the
file system may later overwrite the label sector.
Instead, create a label before initializing the file system, and initialize
that file system on the device created by
.Nm
under the
.Pa /dev/label/
directory.
Then the file system will correctly account for the space occupied by the
generic label,
since the
.Nm
device will be one sector smaller than the device from which it was created.
.Pp
Note that for all label types, nested GEOM classes will cause additional
device nodes to be created, with context-specific data appended to their