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

[Dave Chinner]

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.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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