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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 |
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| BSD.debug.dist | ||
| BSD.include.dist | ||
| BSD.lib32.dist | ||
| BSD.release.dist | ||
| BSD.root.dist | ||
| BSD.sendmail.dist | ||
| BSD.tests.dist | ||
| BSD.usr.dist | ||
| BSD.var.dist | ||
| Makefile | ||
| README | ||
$FreeBSD$
Note: If you modify these files, please keep hier(7) updated!
These files are used to create empty file hierarchies for building the
system into. Some notes about working with them are placed here to try
and keep them in good working order.
a) The files use 4 space indentation, and other than in the header
comments, should not contain any tabs. An indentation of 4 is
preferable to the standard indentation of 8 because the indentation
of levels in these files can become quite deep causing the line to
overflow 80 characters.
This also matches with the files generated when using the
mtree -c option, which was implemented that way for the same reason.
b) Only directories should be listed here.
c) The listing should be kept in filename sorted order.
d) Sanity checking changes to these files can be done by following
this procedure (the sed -e is ugly, but fixing mtree -c to
not emit the trailing white space would be even uglier):
mkdir /tmp/MTREE
mtree -deU -f BSD.X.dist -p /tmp/MTREE
mtree -cdin -k uname,gname,mode -p /tmp/MTREE | \
sed -e 's/ *$//' >BSD.X.new
diff -u BSD.X.dist BSD.X.new
rm -r /tmp/MTREE
Note that you will get some differences about /set lines,
and uname= gname= on certain directory areas, mainly man page
sections. This is caused by mtree not having a look ahead
mechanism for making better selections for these as it
traverses the hierarchy.
The BSD.X.new file should NOT be committed, as it will be missing
the correct header, and important keywords like ``nochange''.
Simply use the diff for a sanity check to make sure things are in
the correct order and correctly indented.
e) Further sanity checking of the system builds with DESTDIR=/someplace
are more complicated, but can often catch missing entries in these
files. I tend to run this more complete sanity check shortly after
the target date for a new release is announced.
If you want details on it bug me about it via email to
rgrimes@FreeBSD.org.