Re: [Qemu-devel] [PATCH 4/4] block/qcow2: add zlib-fast compression algorithm

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Jun 27, 2017 at 03:23:19PM +0200, Peter Lieven wrote:
> Am 27.06.2017 um 15:16 schrieb Daniel P. Berrange:
> > On Tue, Jun 27, 2017 at 02:34:10PM +0200, Peter Lieven wrote:
> > > this adds support for optimized zlib settings which almost
> > > tripples the compression speed while maintaining about
> > > the same compressed size.
> > > 
> > > Signed-off-by: Peter Lieven <pl@kamp.de>
> > > ---
> > >   block/qcow2-cluster.c |  3 ++-
> > >   block/qcow2.c         | 11 +++++++++--
> > >   block/qcow2.h         |  1 +
> > >   qemu-img.texi         |  1 +
> > >   4 files changed, 13 insertions(+), 3 deletions(-)
> > > diff --git a/qemu-img.texi b/qemu-img.texi
> > > index 043c1ba..83a5db2 100644
> > > --- a/qemu-img.texi
> > > +++ b/qemu-img.texi
> > > @@ -627,6 +627,7 @@ The following options are available if support for the respective libraries
> > >   has been enabled at compile time:
> > >      zlib            Uses standard zlib compression (default)
> > > +   zlib-fast       Uses zlib compression with optimized compression parameters
> > This looks like a poor design from a future proofing POV. I would much
> > rather see us introduce a more flexible modelling of compression at the
> > QAPI layer which lets us have tunables for each algorith, in the same
> > way that the qcow2 built-in encryption now has ability to set tunables
> > for each algorithm.
> > 
> > eg your "zlib-fast" impl which just uses zlib with a window size of -15
> > could be expressed as
> > 
> >     qemu-img create -o compress.format=zlib,compress.window=-15
> 
> It would also need a compress.level, but okay. This will make things
> more complicated as one might not know what good values for
> the algoritm are.
> 
> Wouldn't it be better to have sth like compress.level=1..9 and let
> the implementation decide which parameters it choose for the algorithm?

Yep, there's definitely a choice of approaches - exposing low level
details of each library as parameters, vs trying to come up with a
more abstracted, simplified notation and having the driver pick some
of the low level details implicitly from that. Both are valid, and I
don't have a particular opinion either way.

> Do you have a pointer where I can find out how to imlement that
> in QAPI? Maybe the patches that introduces the encryption settings?

It is a little messy since we don't fully use QAPI in the block create
code path. If you want to look at what I did for encryption though, see
the block/qcow2.c file. In the qcow2_create() method I grab the
encrypt.format option. Later in the qcow2_set_up_encryption() method
I then extract & parse the format specific options from the QemuOpts
array. My code is in Max's block branch, or on block-luks-qcow2-10 branch
at https://github.com/berrange/qemu

Regards,
Daniel
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