Am Freitag, 20. März 2015, 12:59:14 schrieb Filipe David Manana: > On Fri, Mar 20, 2015 at 12:39 PM, David Sterba <dste...@suse.cz> wrote: > > On Thu, Mar 19, 2015 at 09:47:15PM +0000, Filipe David Manana wrote: > >> On Thu, Mar 19, 2015 at 1:21 PM, David Sterba <dste...@suse.cz> wrote: > >> > On Wed, Mar 18, 2015 at 03:23:50PM +0100, Martin Steigerwald wrote: > >> >> It explains that having a correct hardlink number for directory is > >> >> not > >> >> mandatory, but it doesn´t explain why BTRFS always has 1 in there > >> >> instead of the actual count of hardlinks. Is this an performance > >> >> optimization for BTRFS or are there any other reasons why BTRFS > >> >> does it this way?>> > > >> > I believe it's for performance reasons. New inodes do not update > >> > the > >> > parent directory metadata wrt link counts, compared to other > >> > filesystems that do that. > >> > >> Weird. Because creating a new inode implies adding the dentry to the > >> parent directory, which implies updating the directory's i_size. > > > > I wonder why the link count is not maintained then. The directory > > inode > > is modified anyway, add a few ifs here and there will not make things > > worse and we could get rid of this special behaviour, plus find could > > use the link count reliably. > > > > It can be turned on in a fully backward compatible way: > > > > * new directories/subvolumes will get dir nlink set to 2 > > * if mkdir/rmdir/move sees a parent link count of 1, no change > > * otherwise inc/dec the link count for new subdirectoris/subvolumes > > > > Jeff Liu reported a problem, without further details > > http://article.gmane.org/gmane.comp.file-systems.btrfs/14628 > > > > so I still might be missing something. > > Maybe this was all before delayed inodes were introduced, where > updating inode items always implied touching the btrees.
Interesting. Well this came up due to an exercise to find all hardlinks to /usr/local in a Linux training. And one participant of the training did this on a SLES 11 SP 3 VM with BTRFS. I did some search and indeed see nothing about link count for directories related to POSIX standard. Incrementing link count only seems to be mentioned for files: http://pubs.opengroup.org/onlinepubs/009695399/functions/link.html Even when people tend to rely on this behavior for forensics as in: http://digital-forensics.sans.org/blog/2009/06/19/directory-link-counts-and-hidden-directories/ Ciao, -- Martin 'Helios' Steigerwald - http://www.Lichtvoll.de GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
