Re: [Qemu-devel] [PATCH] block/mirror: Sleep periodically during bitmap scanning

[Fam Zheng <famz@redhat.com>]

On Wed, 05/13 13:17, Wen Congyang wrote:
> On 05/13/2015 11:11 AM, Fam Zheng wrote:
> > Before, we only yield after initializing dirty bitmap, where the QMP
> > command would return. That may take very long, and guest IO will be
> > blocked.
> 
> Do you have such case to reproduce it? If the disk image is too larger,
> and I think qemu doesn't cache all metedata in the memory. So we will
> yield in bdrv_is_allocated_above() when we read the metedata from the
> disk.

True for qcow2, but raw-posix has no such yield points, because it uses
lseek(..., SEEK_HOLE). I do have a reproducer - just try a big raw image on
your ext4.

Fam

> 
> Thanks
> Wen Congyang
> 
> > 
> > Add sleep points like the later mirror iterations.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/mirror.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/mirror.c b/block/mirror.c
> > index 1a1d997..baed225 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -467,11 +467,23 @@ static void coroutine_fn mirror_run(void *opaque)
> >      sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
> >      mirror_free_init(s);
> >  
> > +    last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> >      if (!s->is_none_mode) {
> >          /* First part, loop on the sectors and initialize the dirty bitmap.  */
> >          BlockDriverState *base = s->base;
> >          for (sector_num = 0; sector_num < end; ) {
> >              int64_t next = (sector_num | (sectors_per_chunk - 1)) + 1;
> > +            int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> > +
> > +            if (now - last_pause_ns > SLICE_TIME) {
> > +                last_pause_ns = now;
> > +                block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 0);
> > +            }
> > +
> > +            if (block_job_is_cancelled(&s->common)) {
> > +                goto immediate_exit;
> > +            }
> > +
> >              ret = bdrv_is_allocated_above(bs, base,
> >                                            sector_num, next - sector_num, &n);
> >  
> > @@ -490,7 +502,6 @@ static void coroutine_fn mirror_run(void *opaque)
> >      }
> >  
> >      bdrv_dirty_iter_init(s->dirty_bitmap, &s->hbi);
> > -    last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> >      for (;;) {
> >          uint64_t delay_ns = 0;
> >          int64_t cnt;
> > 
>