[aklug] Re: Linux/BSD: Why do directories need to be empty?

On Sat, May 14, 2011 at 3:51 PM, Christopher Howard
<christopher.howard@frigidcode.com> wrote:
> Why is it that on Linux and BSDs that the system requires directories to
> be empty before they are deleted? A directory is fundamentally just a
> file, right? Why shouldn't I be able to just unlink it and let the child
> files disappear by virtue of having no more references to them in the
> file system tree? Instead of that simple approach, I have to recursively
> unlink every child file and rmdir every subdirectory (or, of course, let
> "rm -rf" do it for me.) Seems lame.
>
> This was kind of problematic for me the other day because I had a script
> that was supposed to "rm -rf" a certain backup directory, but it
> actually wasn't doing it completely because there were some
> subdirectories that didn't have their write permission bits set.
>

I'm not sure if you're familiar with how databases are indexed but
most filesystems are indexed to a similar degree. In order to list
files in a directory the operating system needs to know where to look
first and then it can iterate through files that match that path.
Files have parent's for the most part and those parents are indexed in
a way that allows children to be reallocated to different parents
quickly. Directories are in many cases simply nodes in the filesystem
that don't contain data blocks.

Removing small files has a comparable speed to removing directory
entries.. large files take more time as individual blocks are
reallocated for use. Removing a directory would abandon child data
and blocks would have to be reallocated for use. All of this could be
done in the background and the file nodes could be temporarily
re-parented to a different directory within the same filesystem. I
believe there are some tools that do exactly this out there.

There are quite a few reasons not to remove files in the background
that would complicate a majority of the common tools we use. The
biggest one I can think of is that removing a file is something you
mean to do and you do so knowing you are reclaiming space and a little
acknowledgement by way of an error code or a clean exit goes a long
way. If files go missing and that space isn't immediately reclaimed
then your process may be expecting something it isn't getting. Why
bother making the deletion asynchronous (rhetorical)?

In the end.. reallocating + deleting through a forked process
(~/Trash, C:\Recycle Bin) works pretty durn well.

If you want speed and acknowledement, check out the reallocation
(deletion) rates of other filesystems than what you are using.

- Shane

> --
> frigidcode.com
> theologia.indicium.us
> ---------
> To unsubscribe, send email to <aklug-request@aklug.org>
> with 'unsubscribe' in the message body.
>
>
---------
To unsubscribe, send email to <aklug-request@aklug.org>
with 'unsubscribe' in the message body.
Received on Sun May 15 16:32:42 2011