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

[Pavel Machek <pavel@ucw.cz>]

On Thu 2015-05-07 12:53:46, Andy Lutomirski wrote:
> On Thu, May 7, 2015 at 12:09 PM, Richard Weinberger
> <richard.weinberger@gmail.com> wrote:
> > On Thu, May 7, 2015 at 7:20 PM, Zach Brown <zab@redhat.com> 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.
> >
> > But how can Joey random user know that some of his
> > applications are using O_NOMTIME and his KISS backup
> > program does no longer function as expected?
> >
> 
> Joey random user can't have a working KISS backup anyway, though,
> because we screw up mtime updates on mmap writes.  I have patches
> gathering dust that fix that, though.

I'm using unison, and yes, I believe I already seen failures from
mmap(). I'd like to see that fixed, and I can test the patches.

Thanks,
									Pavel
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