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This is the unionfs translator for the GNU Hurd.
The unionfs translator was originally written by Moritz Schulte
<moritz@duesseldorf.ccc.de> and currently mantained by Gianluca Guida
<glguida@gmail.com>.
Introduction.
An unionfs is a filesystems that joins many filesystems into one, meaning
that you can see into an "unionfs" all files contained in the filesystems
that are part of the union.
When two or more directories with the same path are found in different
filesystems, their content is joined.
When two or more files with the same path are found in different filesystems,
unionfs has to solve this conflict. See below (Internals section) for
information on what this implementation does in such case.
Example:
To join "foo/" "bar/" and "baz/" in the directory "quux/", just do:
settrans -capfg quux/ /hurd/unionfs foo/ bar/ baz/
If you want to join even quux/ contents in the union itself, add -u as a
translator argument.
You can add filesystems at run-time with the fsysopts command.
Stowing feature.
This unionfs implements stowing feature, i.e. the translator will watch a
directory, called 'stow', that contains the directories to unite.
When a directory is added or removed in the stow, it will be added to or
removed from the unionfs.
Example:
To use "/stow" as the stow for the directory "foo/", do:
settrans -capfg foo/ /hurd/unionfs --stow=/stow
All directories contained in /stow/ will then be joined together in foo/;
you can delete or add directory at run-time and you will see unionfs adding
or removing files in foo/ automatically.
Another interesting feature of stow support of unionfs is the pattern matching
option.
For example, by using:
settrans -capfg foo/ /hurd/unionfs -m bar --stow=/stow
You will get joined in foo/ all sub-sub-directories of /stow matching "bar",
i.e. /stow/*/bar/; pattern matching will be done too in run-time added stow
subdirectories.
Furthermore, you can specify more complex matching pattern to the option,
like -m bar\* (to get all stow's sub-sub-directories beginning with "bar")
or specify multiple -m options.
Example:
This command
settrans -capfg /myfaketree/bin -m bin -m sbin --stow=/stow
will join in /myfaketree/bin all files that are in /stow/*/bin and
/stow/*/sbin. It is equivalent to:
settrans -capfg /myfaketree/bin -m [s]bin --stow=/stow
Internals.
This `unionfs' translator is simple, but it is definitely not a joke.
It works by keeping in memory a dynamically updated tree of nodes, each
node representing a directory in the unionfs. A node contains an array
of ports that give access to the corrisponding directory in the underlying
filesystems.
On lookup, the first entry found is chosen. Thus, it is very important the
underlying filesystems ordering, since the first underlying filesystem will
be the first one to be searched during lookups, and it is the filesystem
where new files and directories are written into.
At the moment, underlying filesystem ordering is set by option ordering.
See CAVEAT for other unexpected behaviour that could happen.
Please send all bug reports to Gianluca Guida <glguida@gmail.com>.
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