r.internal links raster maps between Mapsets inside a GRASS GIS data base so as to save space (and time).

r.internal will not remove a file or a directory that features a single hardlink (for files, or 2 hardlinks for directories).

Experimental or not, you decide. Under development, for sure.

Always run with -d first.

Quick examples

...

Linking

In a file system,

• a file is a link to an inode
• deleting a file, removes one link to the underlying inode
• an inode is only deleted when all (hard) links to the inode have been deleted

Types of Links

• hard: another file linking to the same underlying inode
• symbolic, a link to another file name in the file system

Hard links

• point to the same inode as the target file to link-to
• deleting, renaming or moving the original file, will not affect a hard link as it links to the underlying inode
• changes to the data on the inode are reflected in all files that refer to that inode

+

[+] share the same inode

[+] have the size of the content

[+] take less disk space as they only take up a directory entry

[+] require negligible amount of space (a few bytes), as there are no new inodes created while creating hard links.

[+] take less time to resolve

[+] performance will be slightly better while accessing directly the disk pointer instead of going through another file

[+] moving the source file to another location, on the same filesystem, will not break the link

[+] redundancy: data are not deleted until all links to the files are deleted

[+] useful when the original file is moved

[+] Hard links, on the same file system, are always resolved in a single look-up, and never involve network latency (if it's a hardlink on an NFS filesystem, the NFS server would do the resolution, and it would be invisible to the client system). Sometimes this is important. Not for me, but I can imagine high-performance systems where this might be important.

?

[?] "I also think things like mmap(2) and even open(2) use the same functionality as hardlinks to keep a file's inode active so that even if the file gets unlink(2)ed, the inode remains to allow the process continued access, and only once the process closes it does the file really go away. This allows for safer temporary files where you can read/write your data without anyone being able to access it."

-

[-] can't cross file systems: different inode indexing across difference file systems

[-] you need to explore the whole file system to find files sharing the same inode

[-] hard-links cannot point to directories.

[-] can "break" if the original file is recreated instead of modified in-place--a new inode will be created, but the hard link will still point to the old inode.

Symbolic links

• point to a file name
• a name of another file
• do not share the same inode

+

[+] show instantly the full path they point to

[+] can cross file systems: a soft link contains the full path pointing to another file

-

[-] need their own inode to store the name they point to

[-] have the file name size [?]

[-] consume space (usually 4KB, depending upon the filesystem)

[-] symlink to files that are/will be moved are/will be broken

[-] can point to other symlinks that are in symlinked directories

[-] symbolic links pointing to symbolic links that are on high-latency file systems, could result in network traffic to resolve

To Do

• unlinking is done via -u. Will be replaced with an operation=link[,unlink] option.
• overwriting existing links is not possible -- first unlink (-u) then (re-)link. Implement/use --o?

• Clean and summarise even further the Notes.
• Test for if target to link-to and "link" Mapsets are in the same Location (that is, they are defined under the same spatial reference system).

Sources

• http://man7.org/linux/man-pages/man7/inode.7.html
• http://man7.org/linux/man-pages/man2/link.2.html
• http://man7.org/linux/man-pages/man1/ln.1.html
• http://man7.org/linux/man-pages/man2/unlink.2.html
• http://www.geekride.com/understanding-unix-linux-filesystem-inodes/
• https://stackoverflow.com/a/28389692/1172302
• https://stackoverflow.com/a/185903/1172302
• https://stackoverflow.com/a/185997/1172302
• https://stackoverflow.com/a/23422478/1172302
• https://stackoverflow.com/a/20070006/1172302
• http://www.geekride.com/hard-link-vs-soft-link/
• (Personal discussion with a great System Administrator!)