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

[Andy Lutomirski <luto@amacapital.net>]

On May 8, 2015 8:11 AM, "Dave Chinner" <david@fromorbit.com> wrote:
>
> On Thu, May 07, 2015 at 10:20:53AM -0700, 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:
> > > > Add the O_NOMTIME flag which prevents mtime from being updated which can
> > > > greatly reduce the IO overhead of writes to allocated and initialized
> > > > regions of files.
> > >
> > > Hmmm. How do backup programs now work out if the file has changed
> > > and hence needs copying again? ie. applications using this will
> > > break other critical infrastructure in subtle ways.
> >
> > By using backup infrastructure that doesn't use cmtime.  Like btrfs
> > send/recv.  Or application level backups that know how to do
> > incrementals from metadata in giant database files, say, without
> > walking, comparing, and copying the entire thing.
>
> "Use magical thing that doesn't exist"? Really?
>
> e.g. you can't do incremental backups with tools like xfsdump if
> mtime is not being updated.  The last thing an admin wants when
> doing disaster recovery is to find out that the app started using
> O_NOMTIME as a result of the upgrade they did 6 months ago. Hence
> the last 6 months of production data isn't in the backups despite
> the backup procedure having been extensively tested and verified
> when it was first put in place.
>
> > > > 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.
>
> Understood.
>
> > So are you suggesting not implementing this
>
> No.
>
> > Or are we talking about adding some speed bumps
> > that ceph can flip on that might give Joe Speedracer pause?
>
> Yes, but not just Joe Speedracer - if it can be turned on silently
> by apps then it's a great big landmine that most users and sysadmins
> will not know about until it is too late.

What about programs like tar that explicitly override mtime?  No admin
buy-in is required for that.  Admittedly, that doesn't affect ctime,
nor is it as likely to bite unexpectedly as a nomtime flag.

I think it would be reasonably safe if a mount option had to be set to
allow O_NOCMTIME or such.

--Andy

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
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