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

[NeilBrown]

On Tue, 12 May 2015 10:36:37 -0400 bfields@xxxxxxxxxxxx (J. Bruce Fields)
wrote:

> On Tue, May 12, 2015 at 09:54:27AM -0400, John Stoffel wrote:
> > >>>>> "Austin" == Austin S Hemmelgarn <ahferroin7@xxxxxxxxx> writes:
> > 
> > Austin> On 2015-05-12 01:08, Kevin Easton wrote:
> > >> On Mon, May 11, 2015 at 07:10:21PM -0400, Theodore Ts'o wrote:
> > >>> On Mon, May 11, 2015 at 09:24:09AM -0700, Sage Weil wrote:
> > >>>>> Let me re-ask the question that I asked last week (and was apparently
> > >>>>> ignored).  Why not trying to use the lazytime feature instead of
> > >>>>> pointing a head straight at the application's --- and system
> > >>>>> administrators' --- heads?
> > >>>> 
> > >>>> Sorry Ted, I thought I responded already.
> > >>>> 
> > >>>> The goal is to avoid inode writeout entirely when we can, and
> > >>>> as I understand it lazytime will still force writeout before the inode
> > >>>> is dropped from the cache.  In systems like Ceph in particular, the
> > >>>> IOs can be spread across lots of files, so simply deferring writeout
> > >>>> doesn't always help.
> > >>> 
> > >>> Sure, but it would reduce the writeout by orders of magnitude.  I can
> > >>> understand if you want to reduce it further, but it might be good
> > >>> enough for your purposes.
> > >>> 
> > >>> I considered doing the equivalent of O_NOMTIME for our purposes at
> > >>> $WORK, and our use case is actually not that different from Ceph's
> > >>> (i.e., using a local disk file system to support a cluster file
> > >>> system), and lazytime was (a) something I figured was something I
> > >>> could upstream in good conscience, and (b) was more than good enough
> > >>> for us.
> > >> 
> > >> A safer alternative might be a chattr file attribute that if set, the
> > >> mtime is not updated on writes, and stat() on the file always shows the
> > >> mtime as "right now".  At least that way, the file won't accidentally
> > >> get left out of backups that rely on the mtime.
> > >> 
> > >> (If the file attribute is unset, you immediately update the mtime then
> > >> too, and from then on the file is back to normal).
> > >> 
> > 
> > Austin> I like this even better than the flag suggestion, it provides
> > Austin> better control, means that you don't need to update
> > Austin> applications to get the benefits, and prevents backup software
> > Austin> from breaking (although backups would be bigger).
> > 
> > Me too, it fails in a safer mode, where you do more work on backups
> > than strictly needed.  I'm still against this as a mount option
> > though, way way way too many bullets in the foot gun.  And as someone
> > else said, once you mount with O_NOMTIME, then unmount, then mount
> > again without O_NOMTIME, you've lost information.  Not good.  
> 
> That was me.  Zach also pointed out to me that'd mean figuring out where
> to store that information on-disk for every filesystem you care about.
> I like the idea of something persistent, but maybe it's more trouble
> than it's worth--I honestly don't know.
> 

When this persistent flag is in effect, the values stored in mtime and atime,
and probably ctime, become irrelevant.  Surely we can choose some magic value
to store there that would never happen in practice.

e.g. ctime is signed and so goes back to 1902 (is that right?).  As ctime
cannot be set (via POSIX) to anything but "now", and as there were no Unix
systems in 1902, such values are impossible.

So a specific large negative value in ctime could safely be take to mean
"don't update time stamps, and always report them as 'now'".

Or do we need to keep ctime 'real'?


BTW When you "swap" to a file the mtime doesn't get updated.  No one seems to
complain about that.  I guess it is a rather narrow use-case though.


NeilBrown

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