Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps

[Jeff Layton]

On Wed, 2023-09-20 at 13:48 +0200, Christian Brauner wrote:
> > > > While we initially thought we can do this unconditionally it turns out
> > > > that this might break existing workloads that rely on timestamps in very
> > > > specific ways and we always knew this was a possibility. Move
> > > > multi-grain timestamps behind a vfs mount option.
> > > 
> > > Surely this is a safe choice as it moves the responsibility to the sysadmin
> > > and the cases where finegrained timestamps are required. But I kind of
> > > wonder how is the sysadmin going to decide whether mgtime is safe for his
> > > system or not? Because the possible breakage needn't be obvious at the
> > > first sight...
> > > 
> > 
> > That's the main reason I really didn't want to go with a mount option.
> > Documenting that may be difficult. While there is some pessimism around
> > it, I may still take a stab at just advancing the coarse clock whenever
> > we fetch a fine-grained timestamp. It'd be nice to remove this option in
> > the future if that turns out to be feasible.
> > 
> > > If I were a sysadmin, I'd rather opt for something like
> > > finegrained timestamps + lazytime (if I needed the finegrained timestamps
> > > functionality). That should avoid the IO overhead of finegrained timestamps
> > > as well and I'd know I can have problems with timestamps only after a
> > > system crash.
> > 
> > > I've just got another idea how we could solve the problem: Couldn't we
> > > always just report coarsegrained timestamp to userspace and provide access
> > > to finegrained value only to NFS which should know what it's doing?
> > > 
> > 
> > I think that'd be hard. First of all, where would we store the second
> > timestamp? We can't just truncate the fine-grained ones to come up with
> > a coarse-grained one. It might also be confusing having nfsd and local
> > filesystems present different attributes.
> 
> As far as I can tell we have two options. The first one is to make this
> into a mount option which I really think isn't a big deal and lets us
> avoid this whole problem while allowing filesytems exposed via NFS to
> make use of this feature for change tracking.
> 
> The second option is that we turn off fine-grained finestamps for v6.6
> and you get to explore other options.
> 
> It isn't a big deal regressions like this were always to be expected but
> v6.6 needs to stabilize so anything that requires more significant work
> is not an option.

Oh, absolutely.

I wasn't proposing to do that work for v6.6. For that, we absolutely
either need the mount option or to just revert the mgtime conversions.

My plan was to take a stab at doing this for a later kernel release.
This is very much a "back to the drawing board" idea. It may not pan out
after all, but if it does then we could consider removing the mount
option at that point.
-- 
Jeff Layton <jlayton@xxxxxxxxxx>