Re: [Qemu-devel] [PATCH v4 26/26] iotests: Add test for different refcount widths

[Eric Blake <eblake@redhat.com>]

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On 12/03/2014 05:37 AM, Max Reitz wrote:
> Add a test for conversion between different refcount widths and errors
> specific to certain widths (i.e. snapshots with refcount_bits=1).
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  tests/qemu-iotests/112     | 296 +++++++++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/112.out | 155 ++++++++++++++++++++++++
>  tests/qemu-iotests/group   |   1 +
>  3 files changed, 452 insertions(+)
>  create mode 100755 tests/qemu-iotests/112
>  create mode 100644 tests/qemu-iotests/112.out


> +# This test will set refcount_bits on its own which would conflict with the
> +# manual setting; compat will be overridden as well
> +_unsupported_imgopts refcount_bits 'compat=0.10'

Should this be 'compat' rather than 'compat=0.10' as the filter?  The
reason I ask is that if I can pass an explicit 'compat=1.1'...

> +echo
> +echo '=== refcount_bits and compat=0.10 ==='
> +echo
> +
> +# Should work
> +IMGOPTS="$IMGOPTS,compat=0.10,refcount_bits=16" _make_test_img 64M

...is it going to conflict with this explicit compat=0.10?

I didn't actually try this setup, though, so if the test passes with an
explicit imgopt request for compat=1.1, then I'm fine with it as-is.

Reviewed-by: Eric Blake <eblake@redhat.com>

Side note:

Now that we can produce MUCH smaller images where the reftable can
easily require enough contiguous clusters to require the creation of at
least one refblock that cannot be self-referential, it would probably be
good to modify an existing test and/or add a new test to prove that we
don't trip up when trying to create and read such an image.  But I'm
fine with doing that as a later patch, so don't hold up this series for
it (unless you want to add a 27/26).  See my mail here where I calculate
the minimum size required to guarantee that situation at just under 256
megabytes with 512 byte clusters:
https://lists.gnu.org/archive/html/qemu-devel/2014-11/msg03455.html

But now that I looked that up, I just realized that that email did not
calculate what it would take to get an L1 table to likewise occupy
enough contiguous clusters to guarantee a refblock where every entry is
pointing to clusters of the L1 table and therefore cannot be
self-referential.  But as both L1 and L2 table entries are 64 bits, it
looks like the math is the same as for 64-bit width refcounts, so it
happens to be the same magic size of just under 256 megabytes - and in
this case, the magic value is hit even without relying on this series'
addition of 64-bit refcount widths.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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