Btrfs st_nlink for directories

Btrfs st_nlink for directories

[Neil Schemenauer]

Hi,

It looks like Btrfs does not follow Unix traditions for st_nlink
attribute of directories. It seems to be always one, no matter the
number of sub-directories.

Is this intentional? I couldn't find it discussed anywhere. I
gather the Mac OS HFS+ doesn't follow traditional st_nlink behavior
as well. The 'find' man page has this note:

   -noleaf
          Do  not  optimize  by  assuming that directories contain 2 fewer
          subdirectories than their  hard  link  count.   This  option  is
          needed  when  searching  filesystems that do not follow the Unix
          directory-link convention, such as CD-ROM or MS-DOS  filesystems
          or  AFS  volume  mount  points.  Each directory on a normal Unix
          filesystem has at least 2 hard  links:  its  name  and  its  `.'
          entry.   Additionally,  its  subdirectories (if any) each have a
          `..'  entry linked to that directory.  When find is examining  a
          directory,  after it has statted 2 fewer subdirectories than the
          directory's link count, it knows that the rest of the entries in
          the directory are non-directories (`leaf' files in the directory
          tree).  If only the files' names need to be examined,  there  is
          no  need  to  stat  them;  this  gives a significant increase in
          search speed.

Regards,

  Neil

Re: Btrfs st_nlink for directories

[Aneesh Kumar K. V]

On Fri, 22 Jan 2010 20:28:12 -0600, Neil Schemenauer <nas@arctrix.com> wrote:
> Hi,
> 
> It looks like Btrfs does not follow Unix traditions for st_nlink
> attribute of directories. It seems to be always one, no matter the
> number of sub-directories.
> 
> Is this intentional? I couldn't find it discussed anywhere. I
> gather the Mac OS HFS+ doesn't follow traditional st_nlink behavior
> as well. The 'find' man page has this note:

I have sent patches with message-id
1264279089-14913-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
to the list. Let me know if they works for your

-aneesh

Re: Btrfs st_nlink for directories

[Chris Mason]

On Sun, Jan 24, 2010 at 02:12:59AM +0530, Aneesh Kumar K. V wrote:
> On Fri, 22 Jan 2010 20:28:12 -0600, Neil Schemenauer <nas@arctrix.com> wrote:
> > Hi,
> > 
> > It looks like Btrfs does not follow Unix traditions for st_nlink
> > attribute of directories. It seems to be always one, no matter the
> > number of sub-directories.
> > 
> > Is this intentional? I couldn't find it discussed anywhere. I
> > gather the Mac OS HFS+ doesn't follow traditional st_nlink behavior
> > as well. The 'find' man page has this note:
> 
> I have sent patches with message-id
> 1264279089-14913-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
> to the list. Let me know if they works for your

Thanks for taking a look at this Aneesh, but in btrfs we always have a
link count of one on directories.

It's a design decision so that we don't end up limited in the total
number of subdirs we can create.  reiser3 did something similar,
switching to 1 when the link count got high.  I think the other
filesystems may have added something along these lines as well by now.

Btrfs just leaves it at one all the time.

-chris