Re: [Qemu-devel] [PATCH v2] mirror: double performance of the bulk stage if the disc is full

[Paolo Bonzini <pbonzini@redhat.com>]

----- Original Message -----
> From: "Denis V. Lunev" <den@openvz.org>
> To: "Vladimir Sementsov-Ogievskiy" <vsementsov@virtuozzo.com>, qemu-block@nongnu.org, qemu-devel@nongnu.org
> Cc: stefanha@redhat.com, famz@redhat.com, mreitz@redhat.com, jcody@redhat.com, eblake@redhat.com,
> pbonzini@redhat.com, "Kevin Wolf" <kwolf@redhat.com>
> Sent: Wednesday, July 20, 2016 7:36:00 PM
> Subject: Re: [PATCH v2] mirror: double performance of the bulk stage if the disc is full
> 
> On 07/14/2016 08:19 PM, Vladimir Sementsov-Ogievskiy wrote:
> > Mirror can do up to 16 in-flight requests, but actually on full copy
> > (the whole source disk is non-zero) in-flight is always 1. This happens
> > as the request is not limited in size: the data occupies maximum available
> > capacity of s->buf.
> >
> > The patch limits the size of the request to some artificial constant
> > (1 Mb here), which is not that big or small. This effectively enables
> > back parallelism in mirror code as it was designed.
> >
> > The result is important: the time to migrate 10 Gb disk is reduced from
> > ~350 sec to 170 sec.
> >
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > CC: Stefan Hajnoczi <stefanha@redhat.com>
> > CC: Fam Zheng <famz@redhat.com>
> > CC: Kevin Wolf <kwolf@redhat.com>
> > CC: Max Reitz <mreitz@redhat.com>
> > CC: Jeff Cody <jcody@redhat.com>
> > CC: Eric Blake <eblake@redhat.com>
> > ---
> >
> > v2: in case of s->buf_size larger than default use it to limit io_sectors
> >
> >   block/mirror.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/mirror.c b/block/mirror.c
> > index b1e633e..3ac3b4d 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -23,7 +23,9 @@
> >   
> >   #define SLICE_TIME    100000000ULL /* ns */
> >   #define MAX_IN_FLIGHT 16
> > -#define DEFAULT_MIRROR_BUF_SIZE   (10 << 20)
> > +#define MAX_IO_SECTORS ((1 << 20) >> BDRV_SECTOR_BITS) /* 1 Mb */
> > +#define DEFAULT_MIRROR_BUF_SIZE \
> > +    (MAX_IN_FLIGHT * MAX_IO_SECTORS * BDRV_SECTOR_SIZE)
> >   
> >   /* The mirroring buffer is a list of granularity-sized chunks.
> >    * Free chunks are organized in a list.
> > @@ -322,6 +324,8 @@ static uint64_t coroutine_fn
> > mirror_iteration(MirrorBlockJob *s)
> >       int nb_chunks = 1;
> >       int64_t end = s->bdev_length / BDRV_SECTOR_SIZE;
> >       int sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> > +    int max_io_sectors = MAX((s->buf_size >> BDRV_SECTOR_BITS) /
> > MAX_IN_FLIGHT,
> > +                             MAX_IO_SECTORS);
> >   
> >       sector_num = hbitmap_iter_next(&s->hbi);
> >       if (sector_num < 0) {
> > @@ -385,7 +389,9 @@ static uint64_t coroutine_fn
> > mirror_iteration(MirrorBlockJob *s)
> >                                             nb_chunks * sectors_per_chunk,
> >                                             &io_sectors, &file);
> >           if (ret < 0) {
> > -            io_sectors = nb_chunks * sectors_per_chunk;
> > +            io_sectors = MIN(nb_chunks * sectors_per_chunk,
> > max_io_sectors);
> > +        } else if (ret & BDRV_BLOCK_DATA) {
> > +            io_sectors = MIN(io_sectors, max_io_sectors);
> >           }
> >   
> >           io_sectors -= io_sectors % sectors_per_chunk;
> guys, what about this patch?

I think that at this point it has missed hard freeze.  It's not up to me
whether to consider it a bugfix.

Paolo