Re: [RFC PATCH] Generic name to handle and open by handle syscalls

On Tue, Feb 23, 2010 at 02:28:36PM +0530, Aneesh Kumar K. V wrote:
> On Mon, 22 Feb 2010 16:06:59 -0700, Jonathan Corbet <corbet@lwn.net> wrote:
> > On Fri, 19 Feb 2010 11:12:26 +0530
> > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> > 
> > > The below set of patches implement open by handle support using exportfs
> > > operations.
> > 
> > I have a couple of questions...starting with: what is the use case for
> > this functionality?  There must, clearly, be some kind of application
> > which needs to be able to open by file handle, but I'm not sure what
> > that would be.
> >
> 
> User space NFS server would be one example. Also if we want to NFS
> export another network file system which have a user space server, that
> would be another reason.

Some history - the XFS handle interface was once used for a
userspace NFS server on Irix. IIRC it was replaced by a kernel based
server in about 1995 because the syscall overhead was a
performance limiting factor and since then only XFS specific
applications have used the interface. OOC, does an up-to-date
userspace NFS server that could make use of this even exist today?

As it is, these days the XFS handle interface is not intended for
such a use. From the XFS libhandle manpage:

DESCRIPTION
       These functions provide a way to perform certain filesystem
       operations without using  a  file  descriptor  to  access
       filesystem objects.  They  are intended for use by a limited
       set of system utilities such as backup programs. They are
       supported only by the XFS filesystem.  Link with the
       libhandle library to access these functions.

i.e. it is intended to be used for tight integration of userspace
filesystem utilities into the filesystem.

As for example uses, the first is xfsdump and xfsrestore - XFS's
optimised backup and restore programs.  The second is DMF - SGI's
HSM that is built on top of XFS. Both of these could not do what
they do without the handle interface...

FWIW, These applications both require XFS's handles to be stable for the
life of an inode as well as unique across the system. Hence the
XFS handle contains a fsid and is stable across reboots.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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