On Wed, Jul 11, 2007 at 01:21:55PM +1000, Neil Brown wrote:

And just by-the-way, the server doesn't really have the option of not
sending the attribute.  If i_version isn't defined, it has to fake
something using mtime, and hope that is good enough.

ctime, actually--the change attribute is also supposed to be updated on
attribute updates.

Alternately we could mandate that i_version is always kept up-to-date
and if a filesystem doesn't have anything to load from storage, it
just sets it to the current time in nanoseconds.

That would mean that a client would need to flush it's cache whenever
the inode fell out of cache on the server, but I don't think we can
reliably do better than that.

I think I like that approach.

So my vote is to increment i_version in common code every time any
change is made to the file, and alloc_inode should initialise it to
current time, which might be changed by the filesystem before it calls
unlock_new_inode. 

So the client would be invalidating its cache more often than necessary,
rather than failing to invalidate it when it should.  I agree that
that's probably the better tradeoff, although I wish I had a better idea
of the downside.  I don't know, for example, whether users might see
unpleasant results if every client has to reread its cached data on a
reboot.

The currently proposed change--just providing a model change attribute
implementation for ext4 and leaving other filesystems untouched--is a
more conservative step.

So I'm inclined to just do this ext4 thing first, and then look into
further change attribute experiments next time around....

--b.