linux-kernel - Re: [PATCH RFC] vfs: add a O

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Message-ID: <20150508171115.GA13050@lenny.home.zabbo.net>
Date:	Fri, 8 May 2015 10:11:15 -0700
From:	Zach Brown <zab@...hat.com>
To:	Sage Weil <sage@...dream.net>
Cc:	Dave Chinner <david@...morbit.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [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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