Re: [PATCH 1/6] xstat: Add a pair of system calls to make extended file stats available

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

On Thu, Apr 26, 2012 at 02:45:54PM +0100, David Howells wrote:
> Steve French <smfrench gmail com> wrote:
> 
> > I also would prefer that we simply treat the time granularity as part
> > of the superblock (mounted volume) ie returned on fstat rather than on
> > every stat of the filesystem.   For cifs mounts we could conceivably
> > have different time granularity (1 or 2 second) on mounts to old
> > servers rather than 100 nanoseconds.
> 
> The question is whether you want to have to do a statfs in addition to a stat?
> I suppose you can potentially cache the statfs based on device number.
> 
> That said, there are cases where caching filesystem-level info based on i_dev
> doesn't work.  OpenAFS springs to mind as that only has one superblock and
> thus one set of device numbers, but keeps all the inodes for all the different
> volumes it may have mounted there.
> 
> I don't know whether this would be a problem for CIFS too - say on a windows
> server you fabricate P:, for example, by joining together several filesystems
> (with junctions?).  How does this appear on a Linux client when it steps from
> one filesystem to another within a mounted share?

In the NFS case we do try to preserve filesystem boundaries as well as
we can--the protocol has an fsid field and the client creates a new
mount each time it sees it change.  And the protocol defines time_delta
as a per-filesystem attribute (though, somewhat hilariously, there's
also a per-filesystem "homogeneous" attribute that a server can clear to
indicate the per-filesystem attributes might actually vary within the
filesystem.)

--b.