Re: Making discard/fstrim reliable

["Richard W.M. Jones" <rjones@redhat.com>]

On Wed, Apr 02, 2014 at 04:02:57PM -0400, Jeff Moyer wrote:
> "Richard W.M. Jones" <rjones@redhat.com> writes:
> > Yes and no.  We examine the partitions, logical volumes and so on in
> > order to get a list of mountable filesystems, and then the list is
> > iterated over in this loop.  The precise code for finding the
> > filesystems is here:
> >
> > https://github.com/libguestfs/libguestfs/blob/master/src/listfs.c#L45
> >
> > ^ That code is running on the host side.  It issues various calls to
> > the appliance side which are executed by code in multiple files here:
> >
> > https://github.com/libguestfs/libguestfs/tree/master/daemon
> 
> Sorry, that's a lot to take in.  Can you distill this down to exactly
> the parts involved in the problem you're seeing?  Pretend I don't know
> anything about libguestfs (I don't).

Libguestfs runs up a small appliance, running the daemon above.  The
library side sends the daemon commands which the daemon runs.  The
commands that are run map nearly 1-1 to the libguestfs API.

The test program is this:

  # Add the disk to qemu.
  $g->add_drive ($disk, format => $format, readonly => 0, discard => "enable");
  # Run qemu.
  $g->launch ();

  # Run mkfs -t ext4 /dev/sda in the appliance.
  $g->mkfs ("ext4", "/dev/sda");
  # Run mount -o nodiscard /dev/sda /
  # [This is different from my earlier description, but it is done
  # deliberately so rm doesn't trim the space, because the test is
  # trying to test fstrim, not rm.  Discard of rm is tested in another test.]
  $g->mount_options ("nodiscard", "/dev/sda", "/");

  # Create a 10000000 byte file /data containing '!' characters.
  $g->fill (33, 10000000, "/data");
  # Sync the (guest) disk.
  $g->sync ();

  # The host file is examined here.
  my $full_size = (stat ($disk))[12];

  # Remove the large data file.  Because of -o nodiscard this won't trim.
  $g->rm ("/data");
  # Run fstrim in the appliance.
  $g->fstrim ("/");

  # This issues a sync for the guest disk, and kills qemu.
  $g->shutdown ();

  # The host file is examined again.
  my $trimmed_size = (stat ($disk))[12];
  if ($full_size - $trimmed_size < 1000) {
    # then print an error and die
  }

> I suggest you create a golden image that you copy for each test that
> already has the data committed to it.  The test would then just issue
> the rm and discard.

Sure I'll try this.  Will take a little time to implement it and run
the tests in various environments.

> It's only 64MB in size, right?  That could certainly affect things!

So I'm guessing this is too small?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Fedora Windows cross-compiler. Compile Windows programs, test, and
build Windows installers. Over 100 libraries supported.
http://fedoraproject.org/wiki/MinGW