Re: [Qemu-devel] [PATCH V4] qemu-img: align result of is_allocated_sectors

[Kevin Wolf <kwolf@redhat.com>]

Am 10.07.2018 um 14:36 hat Peter Lieven geschrieben:
> Am 10.07.2018 um 14:28 schrieb Kevin Wolf:
> > Am 07.07.2018 um 13:42 hat Peter Lieven geschrieben:
> > > We currently don't enforce that the sparse segments we detect during convert are
> > > aligned. This leads to unnecessary and costly read-modify-write cycles either
> > > internally in Qemu or in the background on the storage device as nearly all
> > > modern filesystems or hardware have a 4k alignment internally.
> > > 
> > > This patch modifies is_allocated_sectors so that its *pnum result will always
> > > end at an alignment boundary. This way all requests will end at an alignment
> > > boundary. The start of all requests will also be aligned as long as the results
> > > of get_block_status do not lead to an unaligned offset.
> > > 
> > > The number of RMW cycles when converting an example image [1] to a raw device that
> > > has 4k sector size is about 4600 4k read requests to perform a total of about 15000
> > > write requests. With this path the additional 4600 read requests are eliminated while
> > > the number of total write requests stays constant.
> > > 
> > > [1] https://cloud-images.ubuntu.com/releases/16.04/release/ubuntu-16.04-server-cloudimg-amd64-disk1.vmdk
> > > 
> > > Signed-off-by: Peter Lieven <pl@kamp.de>
> > > ---
> > > V3->V4: - only focus on the end offset in is_allocated_sectors [Kevin]
> > > V2->V3: - ensure that s.alignment is a power of 2
> > >          - correctly handle n < alignment in is_allocated_sectors if
> > >            sector_num % alignment > 0.
> > > V1->V2: - take the current sector offset into account [Max]
> > >          - try to figure out the target alignment [Max]
> > > 
> > >   qemu-img.c | 44 ++++++++++++++++++++++++++++++++++++++------
> > >   1 file changed, 38 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/qemu-img.c b/qemu-img.c
> > > index e1a506f..20e3236 100644
> > > --- a/qemu-img.c
> > > +++ b/qemu-img.c
> > > @@ -1105,11 +1105,15 @@ static int64_t find_nonzero(const uint8_t *buf, int64_t n)
> > >    *
> > >    * 'pnum' is set to the number of sectors (including and immediately following
> > >    * the first one) that are known to be in the same allocated/unallocated state.
> > > + * The function will try to align the end offset to alignment boundaries so
> > > + * that the request will at least end aligned and consequtive requests will
> > > + * also start at an aligned offset.
> > >    */
> > > -static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum)
> > > +static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum,
> > > +                                int64_t sector_num, int alignment)
> > >   {
> > >       bool is_zero;
> > > -    int i;
> > > +    int i, tail;
> > >       if (n <= 0) {
> > >           *pnum = 0;
> > > @@ -1122,6 +1126,23 @@ static int is_allocated_sectors(const uint8_t *buf, int n, int *pnum)
> > >               break;
> > >           }
> > >       }
> > > +
> > > +    tail = (sector_num + i) & (alignment - 1);
> > > +    if (tail) {
> > > +        if (is_zero && i == tail) {
> > Should this be i <= tail for the case where sector_num is unaligned?
> > 
> > For example:
> > 
> >      Bytes 0     - 1024:     zero
> >      Bytes 1024  - 4096:     non-zero
> > 
> >      /* Check from 512 to 4096, alignment 2048 */
> >      is_allocated_sectors(buf, 7, &pnum, 1, 4)
> > 
> >      -> is_zero = true
> >      -> i = 1
> >      -> tail = (sector_num + i) & (alignment - 1)
> >              = (1 + 1) & (4 - 1)
> >              = 2
> >              != i
> 
> You are right. I missed that.
> 
> > 
> > > +            /* treat unallocated areas which only consist
> > > +             * of a small tail as allocated. */
> > > +            is_zero = 0;
> > (This should be false rather than 0, is_zero is a bool)
> 
> will fix.
> 
> > 
> > > +        }
> > > +        if (!is_zero) {
> > > +            /* align up end offset of allocated areas. */
> > > +            i += alignment - tail;
> > > +            i = MIN(i, n);
> > > +        } else {
> > > +            /* align down end offset of zero areas. */
> > > +            i -= tail;
> > So our example above will end up in this branch and we get:
> > 
> >      i = i - tail
> >        = 1 - 2
> >        = -1
> > 
> > I'm not sure what callers will do with a negative *pnum, but I expect it
> > won't be anything good.
> 
> But with i <= tail, we avoid ending up here.
> So with the 2 fixes you are okay with this Patch?

I think so, yes.

Kevin