Hard links: an example case
Posted by isilanes on November 29, 2007
One argument I tend to hear from Windows users is that in Windows you can do as much as you can with Linux, and that the technical advantages of Linux only show up if you are really an utter geek. This is one of (I hope) a series of entries in my blog, illustrating some cases where this doesn’t hold: I took advantage of tools provided by Linux in a way that anyone could have, not just geeks.
The moral of it all is that Windows encourages a lack of choice and flexibility that makes users tend not to be creative, and think the cage Windows keeps them in is actually a shelter from the storm, when it’s not. They think that what can’t be done with Windows, needs not be done. I think otherwise…
Today I will try to provide an example in which hard links can be useful. Under Windows XP hard links can be created, using the fsutil utility, but only for NTFS file systems, and only by the Administrator account (and only from the command line). If you want to learn more about links and specially Windows links, read this interesting sell-shocked.org article.
I download a lot of music from Jamendo, using the BitTorrent p2p protocol. After having downloaded a given album, I tend to leave the torrent open, so that people can continue uploading from my computer.
However, I also want to have my music collection tidy and ordered, so I immediately organize the newly-dowloaded songs moving them to a neat directory tree I have, will all my music.
So, there is a conflict between keeping the files in the bittorrent download/upload dir, and properly organizing them. I don’t want to have to wait until I decide to stop sharing a file to organize it, and I don’t want to risk deleting the files if I remove them from the bittorrent client before saving them elsewhere. I could get over all this by simply making a copy of the files… but then I would be filling twice as much disk space, and with GBs of shared files, this is not neat at all.
What I do is hardlink all the downloaded files to their final location. If I download all torrents to /scratch/ktorrent/, a downloaded album will look like that:
% ls /scratch/ktorrent/album1/
song1.ogg song2.ogg song3.ogg [...]
If I want to save the album under my artist1 directory, I do the following:
% mkdir /scratch/music/artist1/album1
% ln /scratch/ktorrent/album1/* /scratch/music/artist1/album1/
This way all the “song*.ogg” files will appear to be in both /scratch/music/artist1/album1/ and /scratch/ktorrent/album1/ at the same time.
1 – I can keep sharing the files in /scratch/ktorrent/album1/, while listening to and/or manipulating the /scratch/music/artist1/album1/ files as if I had 2 copies of each.
2 – The total size is not affected. The hard links do not “occupy” space (only a few bytes each).
3 – I can delete the files in the shared directory without any fear. Only the “copy” in /scratch/ktorrent/ disappears, while the other “copy” in /scratch/music/artist1/album1/ becomes the only copy (just as if it had always been a “normal” file, and the only one).
Recall that all files are hard links. Normally a given file is the only hard link to a given piece of data in the hard disk, but there can be more “links” pointing to that data. When we remove files, we only remove the “link” pointing to the data.