Re: i_version changes

[Andreas Dilger <adilger@sun.com>]

On Feb 13, 2008  09:07 -0500, Trond Myklebust wrote:
> On Wed, 2008-02-13 at 13:52 +0100, Christoph Hellwig wrote:
> > Btw, stupid question:  the commit message for the i_version changes
> > mentions this is to work around lack of granularity for ctime updates.
> > But all modern filesystems (and I includ ext4 in that here) have 64bit
> > timestamps already, so that should be enough.  It would certainly
> > avoid all this additional code, and especially the additional space
> > used in struct inode which can hurt quite a lot.
> 
> Support for 64-bit on-disk time stamps alone does not suffice. In order
> to label all file/directory changes unambiguously, you would also need
> nanosecond timekeeping support.
> 
> An example is XFS, which has had on-disk support for 64-bit time stamps
> since forever, but is in practice limited by the Linux default 250Hz
> internal clock. We've seen plenty of examples of NFS clients missing
> updates on the resulting filesystem due to the fact that they occurred
> within 1/250 sec of each other.

The other issue which unfortunately makes ctime a non-starter is the
ability of ctime to go backward due to clock changes.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.