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

[Trond Myklebust <trond.myklebust@primarydata.com>]

On Thu, May 7, 2015 at 9:01 PM, Sage Weil <sage@newdream.net> 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?
>
> 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.

Cheers
  Trond