GUNION(8) FreeBSD System Manager's Manual GUNION(8)
NAME
gunion - control utility for UNION GEOM class
SYNOPSIS
gunion create [-v] [-o offset] [-s size] [-S secsize] [-Z gunionname]
upperdev lowerdev
gunion destroy [-fv] prov ...
gunion reset [-v] prov ...
gunion revert [-v] prov ...
gunion commit [-frv] prov ...
gunion list
gunion status
gunion load
gunion unload
DESCRIPTION
The gunion utility is used to track changes to a read-only disk on a
writable disk. Logically, a writable disk is placed over a read-only
disk. Write requests are intercepted and stored on the writable disk.
Read requests are first checked to see if they have been written on the
top (writable disk) and if found are returned. If they have not been
written on the top disk, then they are read from the lower disk.
The gunion utility can be especially useful if you have a large disk with
a corrupted filesystem that you are unsure of how to repair. You can use
gunion to place another disk over the corrupted disk and then attempt to
repair the filesystem. If the repair fails, you can revert all the
changes in the upper disk and be back to the unchanged state of the lower
disk thus allowing you to try another approach to repairing it. If the
repair is successful you can request that all the writes recorded on the
top disk be written to the lower disk.
Another use of the gunion utility is to try out upgrades to your system.
Place the upper disk over the disk holding your filesystem that is to be
upgraded and then run the upgrade on it. If it works, commit it; if it
fails, revert the upgrade. An example is given below.
The upper disk must be at least the size of the disk that it covers. The
union metadata exists only for the period of time that the union is
instantiated, so it is important to commit the updates before destroying
the union. If the top disk is about 2.5 percent larger for 512 byte
sector disks (or 0.5 percent larger for 4K sector disks) than the disk
that it covers, it is posible (thought not currently implemented) to save
the union metadata between instantiations of the union device.
If you do not have physical media available to use for the upper layer,
the md(4) disk can be used instead. When used in swap mode the changes
are all held in buffer memory. Pages get pushed out to the swap when the
system is under memory pressure, otherwise they stay in the operating
memory. If long-term persistance is desired, vnode mode can be used in
which a regular file is used as backing store. The disk space used by
the file is based on the amount of data that is written to the top
device.
The first argument to gunion indicates an action to be performed:
create Set up a union provider on the two given devices. The first
device given is used as the top device and must be writable.
The second device given is used as the bottom device and need
only be readable. The second device may be mounted read-only
but it is recommended that it be unmounted and accessed only
through a mount of the union device. If the operation succeeds,
the new provider should appear with name
/dev/<upperdev>-<lowerdev>.union. An alternate name can be
specified with the -Z flag. The kernel module geom_union.ko
will be loaded if it is not loaded already.
Additional options include:
-o offset Where to begin on the original provider. The
default is to start at the beginning of the disk
(i.e., at offset 0). This option may be used to
skip over partitioning information stored at the
beginning of a disk. The offset must be a
multiple of the sector size.
-s size Size of the transparent provider. The default is
to be the same size as the lower disk. Any extra
space at the end of the upper disk may be used to
store union metadata.
-S secsize Sector size of the transparent provider. The
default is to be the same sector size as the
lower disk.
-v Be more verbose.
-Z gunionname The name of the new provider. The suffix
".union" will be appended to the provider name.
destroy Turn off the given union providers.
Additional options include:
-f Force the removal of the specified provider.
-v Be more verbose.
revert Discard all the changes made in the top layer thus reverting to
the original state of the lower device. The union device may
not be mounted or otherwise in use when a revert operation is
being done.
commit Write all the changes made in the top device to the lower device
thus committing the lower device to have the same data as the
union.
Additional options include:
-f The commit command will not allow the lower device to be
mounted or otherwise in use while the commit operation is
being done. However, the -f flag may be specified to allow
the lower device to be mounted read-only. To prevent a
filesystem panic on the mounted lower-device filesystem,
immediately after the commit operation finishes the lower-
device filesystem should be unmounted and then remounted to
update its metadata state. If the lower-device filesystem
is currently being used as the root filesystem then the -r
flag should be specified to reboot the system at the
completion of the commit operation.
-r Reboot the system at the completion of the commit operation.
-v Be more verbose.
reset Reset statistics for the given union providers.
list See geom(8).
status See geom(8).
load See geom(8).
unload See geom(8).
EXIT STATUS
Exit status is 0 on success, and 1 if the command fails.
EXAMPLES
The following example shows how to create and destroy a union provider
with disks /dev/da0p1 as the read-only disk on the bottom and /dev/md0 as
the wriable disk on the top.
gunion create -v md0 da0p1
mount /dev/md0-da0p1.union /mnt
Proceed to make changes in /mnt filesystem. If they are successful and
you want to keep them.
umount /mnt
gunion commit -v md0-da0p1.union
If they are unsuccessful and you want to roll back.
umount /mnt
gunion revert -v md0-da0p1.union
When done eliminate the union.
umount /mnt
gunion destroy -v md0-da0p1.union
All uncommitted changes will be discarded when the union is destroyed.
If you use the name of the full disk, for example da0 and it is labelled,
then a union name will appear for the disk as md0-da0.union as well as
for each partition on the disk as md0-da0p1.union, md0-da0p2.union, etc.
A commit operation can be done only on md0-da0.union and will commit
changes to all the partitions. If partition level commits are desired,
then a union must be created for each partition.
The traffic statistics for the given union providers can be obtained with
the list command. The example below shows the number of bytes written
with newfs(8):
gunion create md0 da0p1
newfs /dev/md0-da0p1.union
gunion list
SYSCTL VARIABLES
The following sysctl(8) variables can be used to control the behavior of
the UNION GEOM class. The default value is shown next to each variable.
kern.geom.union.debug: 0
Debug level of the UNION GEOM class. This can be set to a number
between 0 and 4 inclusive. If set to 0, no debug information is
printed. If set to 1, all the verbose messages are logged. If
set to 2, addition error-related information is logged. If set
to 3, mapping operations are logged. If set to 4, the maximum
amount of debug information is printed.
SEE ALSO
geom(4), geom(8)
HISTORY
The gunion utility appeared in FreeBSD 14.0.
AUTHORS
Marshall Kirk McKusick <mckusick@mckusick.com>
FreeBSD 15.1-STABLE-HBSD January 19, 2022 GUNION(8)