Unionfs - a unification file system for Linux

Unionfs is not a command, but rather a file system. This manual page describes additional mount options that Unionfs supports.

mount -t unionfs -o dirs=/branch1[=ro|=rw]:/branch2[=ro|=rw]:...:/branchN[=ro|=rw],imap=forwardmap:reversemap1:reversemap2:...:reversemapN,[copyup=preserve,currentuser,mounter] none /mnt/unionfs

dirs specifies a colon separated list of which directories compose the union. Additionally, read-only or read-write permissions of the branch can be specified by appending "=ro" or "=rw" to each directory. Directories that come earlier in the list have a higher precedence than those which come later.

debug specifies the FiST debugging level. 1 through 8 specify individual log-levels. If debug is set to 11 through 18, the output will include debug-10 and all lower levels. If you set this to a non-zero value lots of output will be sent to the kernel ring buffer, some of which may be sensitive. The default value for this option is 0 which will produce no output.

delete controls how Unionfs deletes and renames objects. Possible values are "all", and "whiteout" with the default behavior being all.

imap specifies a colon separated list of files that are used as the inode map files. You should specify the forward map first, followed by the reverse maps. The order of the reverse maps does not matter. The default behavior for the imap option if left blank is to not use persistent inode mappings.

copyup specifies which copy-up mode is used to provide copy-on-write semantics in Unionfs. Possible values are preserve, currentuser, and mounter. Preserve keeps the permissions in the higher-precedence directory the same as in the lower-precedence directory. If currentuser is selected, then the copied-up file will be owned by the user whose actions initiated the copy-up operation. Finally, mounter uses an owner and permission specified at mount time. The default behavior for copyup is preserve.

copyupuid specifies the uid of the copied-up files. This has no default behavior and must be set if copyup is set to the mounter option.

copyupgid specifies the gid of the copied-up files. This has no default behavior and must be set if copyup is set to the mounter option.

copyupmode specifies the mode of the copied-up files. This has no default behavior and must be set if copyup is set to the mounter option.

Charles Wright <cwright@cs.sunysb.edu>, Mohammad Zubair <mzubair@ic.sunysb.edu>, Erez Zadok <ezk@cs.sunysb.edu>

unionctl(1), uniondbg(1), http://www.fsl.cs.sunysb.edu/