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

From: J. Bruce Fields
Date: Tue May 12 2015 - 10:36:45 EST

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.

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