The gunion(8) 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(8) 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(8) 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 commit
all the writes recorded on the top disk to the lower disk.
Another use of the gunion(8) 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.
Further details can be found in the gunion(8) manual page.
Reviewed by: Chuck Silvers, kib (earlier version)
tested by: Peter Holm
Differential Revision: https://reviews.freebsd.org/D32697
Sample output:
% geom -t
Geom Class Provider
da0 DISK da0
da0 PART da0s1
da0s1 PART da0s1a
ffs.da0s1a VFS
da0s1a DEV
da0s1 DEV
da0 DEV
da1 DISK da1
swap SWAP
da1 DEV
cd0 DISK cd0
cd0 DEV
Reviewed by: oshogbo
Approved by: re (kib)
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D17151
Make `geom XXX list` and `geom XXX status` outputs more consistent:
Add -a options to print all geoms, not only ones with providers.
Add -g option for `status` to report geom's names, not provider's.
Make `status` by default report provider's status (if present), not geom's.
Make `status` report consumer's statuses, not only "synchronized" field.
Note that due to e.g. write throttling ('wdrain'), it can stall all the disk
I/O instead of just the device it's configured for. Using it for removable
media is therefore not a good idea.
Reviewed by: pjd (earlier version)
providers with limited physical storage and add physical storage as
needed.
Submitted by: Ivan Voras
Sponsored by: Google Summer of Code 2006
Approved by: re (kensmith)
