Re: [RFC PATCH 2/2] qemu-img convert: Fix sparseness detection

[Kevin Wolf <kwolf@redhat.com>]

Am 19.05.2021 um 15:24 hat Peter Lieven geschrieben:
> Am 20.04.21 um 18:52 schrieb Vladimir Sementsov-Ogievskiy:
> > 20.04.2021 18:04, Kevin Wolf wrote:
> >> Am 20.04.2021 um 16:31 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >>> 15.04.2021 18:22, Kevin Wolf wrote:
> >>>> In order to avoid RMW cycles, is_allocated_sectors() treats zeroed areas
> >>>> like non-zero data if the end of the checked area isn't aligned. This
> >>>> can improve the efficiency of the conversion and was introduced in
> >>>> commit 8dcd3c9b91a.
> >>>>
> >>>> However, it comes with a correctness problem: qemu-img convert is
> >>>> supposed to sparsify areas that contain only zeros, which it doesn't do
> >>>> any more. It turns out that this even happens when not only the
> >>>> unaligned area is zeroed, but also the blocks before and after it. In
> >>>> the bug report, conversion of a fragmented 10G image containing only
> >>>> zeros resulted in an image consuming 2.82 GiB even though the expected
> >>>> size is only 4 KiB.
> >>>>
> >>>> As a tradeoff between both, let's ignore zeroed sectors only after
> >>>> non-zero data to fix the alignment, but if we're only looking at zeros,
> >>>> keep them as such, even if it may mean additional RMW cycles.
> >>>>
> >>>
> >>> Hmm.. If I understand correctly, we are going to do unaligned
> >>> write-zero. And that helps.
> >>
> >> This can happen (mostly raw images on block devices, I think?), but
> >> usually it just means skipping the write because we know that the target
> >> image is already zeroed.
> >>
> >> What it does mean is that if the next part is data, we'll have an
> >> unaligned data write.
> >>
> >>> Doesn't that mean that alignment is wrongly detected?
> >>
> >> The problem is that you can have bdrv_block_status_above() return the
> >> same allocation status multiple times in a row, but *pnum can be
> >> unaligned for the conversion.
> >>
> >> We only look at a single range returned by it when detecting the
> >> alignment, so it could be that we have zero buffers for both 0-11 and
> >> 12-16 and detect two misaligned ranges, when both together are a
> >> perfectly aligned zeroed range.
> >>
> >> In theory we could try to do some lookahead and merge ranges where
> >> possible, which should give us the perfect result, but it would make the
> >> code considerably more complicated. (Whether we want to merge them
> >> doesn't only depend on the block status, but possibly also on the
> >> content of a DATA range.)
> >>
> >> Kevin
> >>
> >
> > Oh, I understand now the problem, thanks for explanation.
> >
> > Hmm, yes that means, that if the whole buf is zero, is_allocated_sectors must not align it down, to be possibly "merged" with next chunk if it is zero too.
> >
> > But it's still good to align zeroes down, if data starts somewhere inside the buf, isn't it?
> >
> > what about something like this:
> >
> > diff --git a/qemu-img.c b/qemu-img.c
> > index babb5573ab..d1704584a0 100644
> > --- a/qemu-img.c
> > +++ b/qemu-img.c
> > @@ -1167,19 +1167,39 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum,
> >          }
> >      }
> >  
> > +    if (i == n) {
> > +        /*
> > +         * The whole buf is the same.
> > +         *
> > +         * if it's data, just return it. It's the old behavior.
> > +         *
> > +         * if it's zero, just return too. It will work good if target is alredy
> > +         * zeroed. And if next chunk is zero too we'll have no RMW and no reason
> > +         * to write data.
> > +         */
> > +        *pnum = i;
> > +        return !is_zero;
> > +    }
> > +
> >      tail = (sector_num + i) & (alignment - 1);
> >      if (tail) {
> >          if (is_zero && i <= tail) {
> > -            /* treat unallocated areas which only consist
> > -             * of a small tail as allocated. */
> > +            /*
> > +             * For sure next sector after i is data, and it will rewrite this
> > +             * tail anyway due to RMW. So, let's just write data now.
> > +             */
> >              is_zero = false;
> >          }
> >          if (!is_zero) {
> > -            /* align up end offset of allocated areas. */
> > +            /* If possible, align up end offset of allocated areas. */
> >              i += alignment - tail;
> >              i = MIN(i, n);
> >          } else {
> > -            /* align down end offset of zero areas. */
> > +            /*
> > +             * For sure next sector after i is data, and it will rewrite this
> > +             * tail anyway due to RMW. Better is avoid RMW and write zeroes up
> > +             * to aligned bound.
> > +             */
> >              i -= tail;
> >          }
> >      }
> 
> I think we forgot to follow up on this. Has anyone tested this
> suggestion?
> 
> Otherwise, I would try to rerun the tests I did with the my old and
> Kevins suggestion.

I noticed earlier this week that these patches are still in my
development branch, but didn't actually pick it up again yet. So feel
free to try it out.

Kevin