From: Sage Weil <sage@newdream.net>
To: Zach Brown <zab@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>,
Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-api@vger.kernel.org
Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag
Date: Thu, 7 May 2015 18:01:23 -0700 (PDT) [thread overview]
Message-ID: <alpine.DEB.2.00.1505071752520.28239@cobra.newdream.net> (raw)
In-Reply-To: <20150507172053.GA659@lenny.home.zabbo.net>
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
next prev parent reply other threads:[~2015-05-08 1:01 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-06 22:00 [PATCH RFC] vfs: add a O_NOMTIME flag Zach Brown
2015-05-06 22:14 ` Trond Myklebust
2015-05-06 22:19 ` Sage Weil
[not found] ` <alpine.DEB.2.00.1505061515550.28239-vIokxiIdD2AQNTJnQDzGJqxOck334EZe@public.gmane.org>
2015-05-06 22:41 ` Zach Brown
[not found] ` <20150506224113.GA17282-fypN+1c5dIyjpB87vu3CluTW4wlIGRCZ@public.gmane.org>
2015-05-06 22:46 ` Sage Weil
2015-05-06 23:21 ` Theodore Ts'o
[not found] ` <1430949612-21356-1-git-send-email-zab-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2015-05-07 0:26 ` Dave Chinner
2015-05-07 17:20 ` Zach Brown
2015-05-07 18:43 ` Zach Brown
2015-05-08 1:01 ` Sage Weil [this message]
[not found] ` <alpine.DEB.2.00.1505071752520.28239-vIokxiIdD2AQNTJnQDzGJqxOck334EZe@public.gmane.org>
2015-05-08 1:23 ` Trond Myklebust
2015-05-08 15:19 ` Sage Weil
[not found] ` <CAHQdGtQjMHA8rVPkggB2zMz=k3O667+APH_1EY_2FtYmHL7-hw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2015-05-08 22:13 ` Dave Chinner
2015-05-08 22:24 ` Sage Weil
[not found] ` <alpine.DEB.2.00.1505081517470.28239-vIokxiIdD2AQNTJnQDzGJqxOck334EZe@public.gmane.org>
2015-05-10 23:13 ` Trond Myklebust
[not found] ` <CAHQdGtTFTN2XuvmarFZ9HPQV=cuhh7FosdHSrJME_U4htr=i8w-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2015-05-11 7:31 ` Dave Chinner
2015-05-11 16:39 ` Sage Weil
2015-05-11 17:12 ` Trond Myklebust
[not found] ` <CAHQdGtT3rCf-ycAYw-=7HGaemg1+HfY8sw3+kb54VHONxDyP3w-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2015-05-11 17:30 ` Sage Weil
2015-05-12 1:21 ` Dave Chinner
2015-05-12 23:12 ` Sage Weil
2015-05-13 0:57 ` Dave Chinner
[not found] ` <alpine.DEB.2.00.1505111020120.28239-vIokxiIdD2AQNTJnQDzGJqxOck334EZe@public.gmane.org>
2015-05-12 13:41 ` John Stoffel
2015-05-11 14:47 ` Theodore Ts'o
[not found] ` <20150511144719.GA14088-AKGzg7BKzIDYtjvyW6yDsg@public.gmane.org>
2015-05-11 16:24 ` Sage Weil
[not found] ` <alpine.DEB.2.00.1505110920520.28239-vIokxiIdD2AQNTJnQDzGJqxOck334EZe@public.gmane.org>
2015-05-11 23:10 ` Theodore Ts'o
2015-05-12 5:08 ` Kevin Easton
[not found] ` <20150512050821.GA9404-Qr0l8DEfScZEV+tojptmR0B+6BGkLq7r@public.gmane.org>
2015-05-12 11:45 ` Austin S Hemmelgarn
[not found] ` <5551E7EB.8040301-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2015-05-12 13:54 ` John Stoffel
2015-05-12 14:36 ` J. Bruce Fields
[not found] ` <20150512143637.GA6370-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>
2015-05-12 14:53 ` Austin S Hemmelgarn
2015-05-12 21:51 ` Dave Chinner
2015-05-13 15:16 ` Austin S Hemmelgarn
2015-05-12 22:39 ` NeilBrown
[not found] ` <20150513083951.5eb63bc0-wvvUuzkyo1EYVZTmpyfIwg@public.gmane.org>
2015-07-14 13:13 ` Pavel Machek
2015-07-15 4:54 ` NeilBrown
2015-07-22 13:47 ` Pavel Machek
2015-05-12 21:35 ` Sage Weil
2015-05-13 12:32 ` Jan Kara
2015-05-08 14:43 ` Austin S Hemmelgarn
2015-05-08 17:11 ` Zach Brown
2015-05-08 14:29 ` John Stoffel
2015-07-14 11:50 ` Pavel Machek
[not found] ` <20150507172053.GA659-fypN+1c5dIyjpB87vu3CluTW4wlIGRCZ@public.gmane.org>
2015-05-07 19:09 ` Richard Weinberger
2015-05-07 19:53 ` Andy Lutomirski
[not found] ` <554BC4D8.9010507@nod.at>
2015-05-07 20:06 ` Andy Lutomirski
[not found] ` <CALCETrWNDMq0nK3ac-uZweV5BKK_yWTQHH5D0YkyEu7bcONo9g-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2015-05-08 2:42 ` Dave Chinner
2015-07-14 11:44 ` Pavel Machek
2015-05-08 2:37 ` Dave Chinner
2015-05-08 3:24 ` Andy Lutomirski
[not found] ` <CALCETrUksu5ZB4QBfC8DMwYO2OFjfPW2eWsTweZGN_gybzcsmw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2015-05-08 14:44 ` Eric Sandeen
2015-05-11 20:36 ` J. Bruce Fields
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