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SubjectRe: [PATCH RFC] vfs: add a O_NOMTIME flag
On Thu, May 07, 2015 at 06:01:23PM -0700, Sage Weil wrote:
> On Thu, 7 May 2015, Zach Brown wrote:
> > On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:
> > > On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:
> > > > The criteria for using O_NOMTIME is the same as for using O_NOATIME:
> > > > owning the file or having the CAP_FOWNER capability. If we're not
> > > > comfortable allowing owners to prevent mtime/ctime updates then we
> > > > should add a tunable to allow O_NOMTIME. Maybe a mount option?
> > >
> > > I dislike "turn off safety for performance" options because Joe
> > > SpeedRacer will always select performance over safety.
> >
> > Well, for ceph there's no safety concern. They never use cmtime in
> > these files.
> >
> > So are you suggesting not implementing this and making them rework their
> > IO paths to avoid the fs maintaining mtime so that we don't give Joe
> > Speedracer more rope? Or are we talking about adding some speed bumps
> > that ceph can flip on that might give Joe Speedracer pause?
>
> I think this is the fundamental question: who do we give the ammunition
> to, the user or app writer, or the sysadmin?

Yeah, I think this is right. Dave doesn't want the possibility of it
bleeding in to installations through irresponsible default use in apps
without explicit buy-in from the people responsible for the backups.

> [...]
>
> Or, we can be conservative and require a mount option so that the admin
> has to explicitly allow behavior that might break some existing
> assumptions about mtime/ctime ('-o user_noatime' I guess?).
>
> I'm happy either way, so long as in the end an unprivileged ceph daemon
> avoids the useless work. In our case we always own the entire mount/disk,
> so a mount option is just fine.

It seems that the thread has headed towards responding to my suggestion
of a possible mount option with an enthusiastic "yes, please, no
surprises."

So I'll try that.

- z


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