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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 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.

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
AUTHORS Marshall Kirk McKusick <mckusick@mckusick.com>
FreeBSD 14.0-RELEASE-p11 January 19, 2022 FreeBSD 14.0-RELEASE-p11