[Ocfs2-devel] [PATCH 1/2] Ocfs2: Add a new code 'OCFS2_INFO_FREEINODE' for o2info ioctl.

[Joel Becker]

On Thu, Nov 04, 2010 at 10:27:01AM +0800, tristan wrote:
> Joel Becker wrote:
> > On Wed, Nov 03, 2010 at 07:02:05PM +0800, Tristan Ye wrote:
> >> The new code is dedicated to calculate free inodes number of all inode_allocs,
> >> then return the info to userpace in terms of an array.
> >>
> >> Specially, flag 'OCFS2_INFO_FL_NON_COHERENT', manipulated by '--cluster-coherent'
> >> from userspace, is now going to be involved. setting the flag on means no cluster
> >> coherency considered, usually, userspace tools choose none-coherency strategy by
> >> default for the sake of performace.
> >
> > 	This looks pretty straightforward.  Note that any non-cached
> > allocator is going to lock, regardless of the coherency flag.  Do we
> > want to use ocfs2_ilookup() instead?
> 
> A bit confused here, did you mean we use 'ocfs2_ilookup' instead of 
> 'ocfs2_get_system_file_inode' here?

	I do.  ocfs2_get_system_file_inode() calls ocfs2_iget(), which
will read and lock the inode if it is not in the inode cache.  Now, the
cache-coherent case obviously wants this.
	But in the non-coherent case we have the following conditions:

1) We have an inode in the inode cache, we can use it to look up the
   blkno.
2) We have no inode in the cache, and we have to go get it.

	Case (1) is fast.  Case (2) is not, especially because it locks
the inode.  Why not merely look up blkno via
ocfs2_lookup_ino_from_name() and go from there?  Note that
ocfs2_ilookup() isn't even needed, unless you have a faster way to get
to the blkno.

> coherency flag refers to a cluster-aware lock, while 
> ocfs2_get_system_file_inode will use iget_locked() to get
> the inode if it didn't exist in cache, does iget_locked() also refer to 
> a cluster-aware lock somehow?

	Yes.  If an inode is not in cache, it will eventually call
ocfs2_read_inode_locked().

Joel

-- 

"The trouble with being punctual is that nobody's there to
 appreciate it."
	- Franklin P. Jones

Joel Becker
Senior Development Manager
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127