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

On Thu, 7 May 2015, Trond Myklebust wrote:

On Thu, May 7, 2015 at 9:01 PM, Sage Weil [off-list ref] wrote:

quoted

On Thu, 7 May 2015, Zach Brown wrote:

quoted

On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:

quoted

On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:

quoted

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.

So, what is the expectation here for filesystems that cannot support
this flag? NFSv3 in particular would break pretty catastrophically if
someone decided on a whim to turn off mtime: they will have turned off
the client's ability to detect cache incoherencies.

Is this based on mtime or ctime?  If the former, would things could also 
break if a user does, say, some stat(2), write(2), utimes(2) shenanigans?

So, my assumption is that if the mount option isn't there allowing this 
then O_NOMTIME would be a no-op (as opposed to EPERM or something)... but 
maybe that's not the right thing to do.  Whatever we do there, though, I 
suppose NFS would do the same thing?

sage