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

[bfields@fieldses.org (J. Bruce Fields)]

On Fri, May 08, 2015 at 09:44:25AM -0500, Eric Sandeen wrote:
> On 5/7/15 10:24 PM, Andy Lutomirski wrote:
> > 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.
> 
> I was going to suggest the same.  Make infrastructure available for an app
> to request O_NOMTIME, but a mount option must be set to allow it, so the
> administrator doesn't get an unhappy surprise at backup-restore time.
> 
> (Not a big fan of more twiddly knobs, but that seems to put the control
> in all the right places).

It seems more like a permanent feature of the filesystem than a
per-mount option: once you've turned off mtime updates you lose
information that can't be regained after remounting.  A mkfs option
might make more sense?  But I guess those aren't very generic.

(I do hope we can get an O_NOMTIME flag, it will make me smile every
time I see it....)

--b.