Re: dm-statistics: report precise timestamps and histogram

["Bryn M. Reeves" <bmr@redhat.com>]

On Tue, Aug 18, 2015 at 05:41:12PM -0400, Mike Snitzer wrote:
> On Tue, Aug 18 2015 at  5:07pm -0400,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
> > On Tue, Aug 18 2015 at  4:26pm -0400,
> > Mikulas Patocka <mpatocka@redhat.com> wrote:
> > 
> > > Mike, please submit this patch to the upstream kernel before 4.2 is 
> > > released, so that the all the kernels that support the options 
> > > "precise_timestamps" and "histogram" also report these values back.
> > 
> > It is really late to be sending pull requests for 4.2 (next release is
> > likely 4.2 final).  I've already sent 6 'fixes' pull requests to Linus
> > for 4.2.  This patch is just too late, sorry.
> > 
> > But what I will do is tag this commit for 4.2-stable inclusion.
> > 
> > Userspace will have to check the DM ioctl version to know if DM provides
> > these extra keywords.
> 
> I've staged the following for 4.3:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=bd49784fd1e8f42c7600fbfa206361324857f373
> 
> Bryn, please verify this change is sufficient for your needs.

Yes; this will be usable by dmstats.

Unfortuntely the fact that there are kernels out there without this
means that we will need to either carry a workaround in dmstats
userspace forever or just disallow use of these features on those
kernels.

The workaround is fairly unobtrusive and is already written so it's not
the end of the world (it also has future uses for which kernel support
will not help so it won't be solely used for precise/histogram
tracking).

Regards,
Bryn.