svn commit: r316938 - head/sbin/savecore

[Mark Johnston]

On Fri, Apr 14, 2017 at 01:49:51PM -0600, Alan Somers wrote:
> On Fri, Apr 14, 2017 at 1:41 PM, Ngie Cooper <ngie at freebsd.org> wrote:
> > Author: ngie
> > Date: Fri Apr 14 19:41:48 2017
> > New Revision: 316938
> > URL: https://svnweb.freebsd.org/changeset/base/316938
> >
> > Log:
> >   savecore: fix space calculation with respect to `minfree` in check_space(..)
> >
> >   - Use strtoll(3) instead of atoi(3), because atoi(3) limits the
> >     representable data to INT_MAX. Check the values received from
> >     strtoll(3), trimming trailing whitespace off the end to maintain
> >     POLA.
> >   - Use `KiB` instead of `kB` when describing free space, total space,
> >     etc. I am now fully aware of `KiB` being the IEC standard for 1024
> >     bytes and `kB` being the IEC standard for 1000 bytes.
> >   - Store available number of KiB in `available` so it can be more
> >     easily queried and compared to ensure that there are enough KiB to
> >     store the dump image on disk.
> >   - Print out the reserved space on disk, per `minfree`, so end-users
> >     can troubleshoot why check_space(..) is reporting that there isn't
> >     enough free space.
> >
> >   MFC after:    7 weeks
> >   Reviewed by:  Anton Rang <rang at acm.com> (earlier diff), cem (earlier diff)
> >   Tested with:  positive/negative cases (see review); make tinderbox
> >   Sponsored by: Dell EMC Isilon
> >   Differential Revision:        D10379
> 
> The free space calculation is still uselessly conservative, because it
> doesn't account for the fact that core dumps will always be either
> spare or compressed.  The result is that savecore will frequently
> refuse to save corefiles even when there's plenty of space.  I
> proposed removing the space check altogether in
> https://reviews.freebsd.org/D2587.  However, I agreed to wait until
> after the compressed core dump feature was merged, because then mostly
> accurate space checks will be possible.  AFAIK the compressed core
> dump feature still hasn't been finished.

I had held off on it for a while because it was going to conflict with
the work to add encrypted dump support, which of course has finished.

The patch to add compression support is here and should largely still
work:
https://people.freebsd.org/~markj/patches/core-compression/20141110-kern_dump.diff

I've been hesitant about pushing it forward:
- The dump_write* APIs need some simplification after the addition of
  encrypted dump support and support for dumping to 4Kn drives.
- I'm not sure how encryption should compose with compression. It seems
  intuitively obvious that we should compress before encrypting if the
  compression is to be of any use, but I don't know enough to know
  whether the compression might somehow compromise the effectiveness of
  the encryption.

If anyone has some insight on the second of these two points, I'd
appreciate hearing it.