Re: [PATCH/RFC 0/7] Volatile Filehandle Client-side Support

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Nov 14, 2011 at 08:07:45AM +1100, NeilBrown wrote:
> On Sun, 13 Nov 2011 11:36:32 -0500 "J. Bruce Fields" <bfields@fieldses.org>
> wrote:
> 
> > On Sun, Nov 13, 2011 at 02:45:48PM +0100, Tigran Mkrtchyan wrote:
> > > I have a server which runs on top of hadoop. The problem with hadoop
> > > is that there is no way to have persistent file handles. I am
> > > currently working on a way to do that - either simulate them or add a
> > > support for unique file id to hadoop. If linux client will support
> > > volatile file handles then I can stop inventing some workarounds.
> > 
> > I might call that "fixing" rather than inventing workarounds.
> > 
> > Our of curiosity: if we really wanted to support such filesystems, what
> > would we need in the protocol?  Just saying "filehandles aren't stable,
> > deal with it" seems insufficient. 
> 
> 1/ no guarantees if the file is not 'open'
> 2/ two possible responses to FHEXPIRED:
>     a/ perform a GETATTR and request the 'filehandle' attribute.  Client then
>        uses that filehandle instead.
>     b/ perform LOOKUP on parent filehandle with same name as before, and use
>        the resulting filehandle.
>    Server specifies which somehow (different error code?  magic attribute
>    flag somewhere?  doesn't really matter)
> 
> If a server has objects that are never renamed, it can easily use volatile
> file handles.
> If a server has objects which can be renamed and wants to use volatile file
> handles, then if such an object is open and is about to be renamed, it must
> first log to stable storage some mapping to allow it to access the file from
> the old volatile file handle.

I think then there's no limit to the lifetime of those log entries, or
to the size of the log?

--b.

> And of course it cannot allow renames during
> the grace period, but I think we already have that.
> Also, if the VFH is such that it will be  lost on a reboot, the server must
> log it to stable storage before allowing an open.