Re: ext2/3 subdirectory limit [WAS: Choosing and tuning Linux file systems]

[Theodore Tso <tytso@mit.edu>]

On Mon, Jun 26, 2006 at 02:36:35PM +0200, Tomas Hruby wrote:
> I was always wondering why it increases link_count of the parent
> directory when creating a subdirectory. It is clear that .. points
> to the parent, but the subdirectory cannot exist without its parent
> and you cannot delete the parent if it is not empty. Correct me if I
> am wrong.
> 
> What is the link_count (incremented by subdirectories) used for? Is
> it ext2/3 specific or should it be implemented in such a way by
> other FS too? I am asking becuse I see no reason why to do so in our
> FS.

No, it's not ext2/3 specific; it's an old Unix tradition.  I've heard
reports of programs that attempt to optimize recursive descents by
counting the number of directories found so far and comparing it with
st_nlinks.  This is dangerous since it's possible for directories to
be created during the readdir(), and in any case not all filesystems
do follow this behavior.  So preserving it just to keep application
programs from breaking is probably a bad idea; in the case of ext2/3
we've been preserving it mainly because to do otherwise would cause
e2fsck to complain.  

We could create an rocompat feature which disabled this, or which
causes st_nlink above 32000 to mean "infinity"; such patches have
existed, but in practice it's relatively rare that people actually
find this to be a limitation, so none of the patches have managed to
achieve the necessary activation energy to actually get integrated
into both the kernel and e2fsprogs.

						- Ted