Re: [PATCH RFC] vfs: add a O_NOMTIME flag

[Sage Weil]

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?

One might argue that we gave the user a similar power with O_NOATIME (the 
power to break applications that assume atime is accurate).  Here we give 
developers/users the power to not update mtime and suffer the consequences 
(like, obviously, breaking mtime-based backups).  It should be pretty 
obvious to anyone using the flag what the consequences are.

Note that we can suffer similar lapses in mtime with fdatasync followed by 
a system crash.  And as Andy points out it's semi-broken for writable 
mmap.  The crash case is obviously a slightly different thing, but the 
idea that mtime can't always be trusted certainly isn't crazy talk.

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.

Thanks!
sage
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