Re: [Qemu-devel] [PATCH 1.8 5/6] qemu-img: add option to align writes to cluster_sectors during convert

[Peter Lieven]

On 25.11.2013 16:11, Paolo Bonzini wrote:
> Il 25/11/2013 14:57, Peter Lieven ha scritto:
> > Signed-off-by: Peter Lieven <address@hidden>
> Ok, given this patch I think the cluster_size is the right one to use
> here---and also the way you used the optimal unmap granularity makes
> sense; you could also use MAX(optimal unmap granularity, optimal
> transfer length granularity).
>
> However, there is no need to write one cluster at a time.  What matters,
> I think, is to align the *end* of the transfer, so that the next
> transfer can start aligned.
>
> > +            if (align && cluster_sectors > 0) {
> > +                int64_t next_aligned_sector = (sector_num + cluster_sectors);
> So this should be "+ n", not "+ cluster_sectors".
>
> Perhaps it could be conditional on "n > cluster_sectors" (small requests
> happen when you have sparse region, and breaking them doesn't help).
>
> Finally, I believe there is no need for a separate "-a" knob.
>
> The patch looks fine to me with these small changes, though.
>
> Also, a couple of ideas for separate patches.  Perhaps the default value
> of "-S" could be cluster_size if specified?  This would avoid making raw
> images too fragmented, and compounding filesystem-level fragmentation
> with qcow2-level fragmentation.  And 4K is too small a default in my
> opinion; it could be easily changed to 64K, though 4K was of course an
> improvement compared to 512 before commit a22f123 (qemu-img: Require
> larger zero areas for sparse handling, 2011-08-26).
I would vote for 64K or 256K, we already use the first for some time. However, 
it turned out
that (much) bigger values decrease performance. Setting it
to cluster_size can be dangerous. As described in my case its 15MB and
I think for vhd its 1MB. This can be a lot of zeros that have to be written.

Peter
> Paolo
>
> > +                next_aligned_sector -= next_aligned_sector % cluster_sectors;
> > +                if (sector_num + n > next_aligned_sector) {
> > +                    n = next_aligned_sector - sector_num;
> > +                }
> > +            }
> > +
> >               if (n > bs_offset + bs_sectors - sector_num) {
> >                   n = bs_offset + bs_sectors - sector_num;
> >               }
> > diff --git a/qemu-img.texi b/qemu-img.texi
> > index 87f9d0f..9b1720f 100644
> > --- a/qemu-img.texi
> > +++ b/qemu-img.texi
> > @@ -179,11 +179,14 @@ Error on reading data
> >   
> >   @end table
> >   
> > address@hidden convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] [-m @var{iobuf_size}] @var{filename} address@hidden [...]] @var{output_filename}
> > address@hidden convert [-c] [-p] [-n] [-a] [-f @var{fmt}] [-t @var{cache}] [-O 
> > @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S 
> > @var{sparse_size}] [-m @var{iobuf_size}] @var{filename} address@hidden [...]] 
> > @var{output_filename}
> >   
> >   Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
> >   using format @var{output_fmt}. It can be optionally compressed (@code{-c}
> >   option) or use any format specific options like encryption (@code{-o} option).
> > +If the @code{-a} option is specified write requests will be aligned
> > +to the cluster size of the output image if possible. This is the default
> > +for compressed images.
> >   
> >   Only the formats @code{qcow} and @code{qcow2} support compression. The
> >   compression is read-only. It means that if a compressed sector is


--

Mit freundlichen Grüßen

Peter Lieven

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