Ocfs2 users - Apr 2012 - enabling indexed-dirs using tunefs.ocfs2

Hello list,

I am considering enabling indexed-dirs on an OCFS2 filesystem.  It has
about 4TB and 6 million files.  In addition to turning on the feature in
the superblock, as far as I understood, it will run through all
directories and create the indexes.

Does someone on this list have any idea how long this would
approximately take? Unfortunately this is a live system and I can't
afford a very long downtime. I am assuming the FS must be completely
unmounted on all nodes during the process.

Or is there the possibility to just enable the feature and not convert
all directories immediately?  Will the kernel create indexes when it
runs across a directory that has not yet been indexed on the fly or will
this only apply to newly created directories? (if the latter is the
case, in theory, I could run through all directories and move all
contents to a new directory on the fly)

And, are the any other FS features that are good for general FS
performance worth enabling while I'm at it? (it's been originally
created with fairly old tools, it only has backup-super,
strict-journal-super, sparse, inline-data and unwritten enabled right
now.

Thanks in advance for any experience you can share with me.

Cheers,
	Christophe