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

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

[Fam Zheng]

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.

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;
-- 
2.4.0

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

[Wen Congyang]

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.

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;
> 

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

[Fam Zheng]

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;
> > 
> 

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

[Wen Congyang]

On 05/13/2015 02:40 PM, Fam Zheng wrote:
> 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.

It is the filesystem's problem. If we mirror a big empty raw image,
lseek(..., SEEK_DATA) may needs some seconds(about 5s for 500G empty
raw image). Even if the granularity is 64K, we need to call this
syscall 8192000(500G/64K) times. We may need more than half year...

So I think we should allow bdrv_is_allocated() and other APIs to return
a larger p_num than nb_sectors.

Thanks
Wen Congyang

> 
> 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;
>>>
>>
> .
> 

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

[Wen Congyang]

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.
> 
> Add sleep points like the later mirror iterations.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>

Reviewed-by: Wen Congyang <wency@cn.fujitsu.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;
> 

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

[Paolo Bonzini]

On 13/05/2015 05:11, 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.
> 
> Add sleep points like the later mirror iterations.

You were also planning to let bdrv_co_is_allocated/get_block_status
return a larger p_num than nb_sectors---which maybe could make
nb_sectors obsolete completely, I don't know.  But this is already an
improvement.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Thanks,

Paolo

> 
> 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;
> 

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

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 1966 bytes --]

On Wed, May 13, 2015 at 11:11:13AM +0800, 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.
> 
> 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(-)

CCing Jeff Cody for block jobs.

> 
> 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;
> -- 
> 2.4.0
> 
> 

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

[Stefan Hajnoczi]

[-- Attachment #1: Type: text/plain, Size: 491 bytes --]

On Wed, May 13, 2015 at 11:11:13AM +0800, 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.
> 
> 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(-)

Ping for Jeff

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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

[Jeff Cody]

On Wed, May 13, 2015 at 11:11:13AM +0800, 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.
> 
> 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;
> -- 
> 2.4.0
>

Thanks, applied to my block tree:
https://github.com/codyprime/qemu-kvm-jtc/commits/block

Jeff

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

[Alexandre DERUMIER]

Hi,

I'm currently testing this patch,
and it still hanging for me (mirroring a raw file from nfs)


I just wonder why block_job_sleep_ns is set to 0 ?

+                block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 0);


I have tried to increasing it to a bigger value

+                block_job_sleep_ns(&s->common, QEMU_CLOCK_REALTIME, 100000000);


And now it's working fine for me, no more qemu hang



----- Mail original -----
De: "Jeff Cody" <jcody@redhat.com>
À: "Fam Zheng" <famz@redhat.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>, "pbonzini" <pbonzini@redhat.com>, "qemu-devel" <qemu-devel@nongnu.org>, qemu-block@nongnu.org
Envoyé: Mardi 16 Juin 2015 06:06:25
Objet: Re: [Qemu-devel] [PATCH] block/mirror: Sleep periodically during	bitmap scanning

On Wed, May 13, 2015 at 11:11:13AM +0800, 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. 
> 
> 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; 
> -- 
> 2.4.0 
> 

Thanks, applied to my block tree: 
https://github.com/codyprime/qemu-kvm-jtc/commits/block 

Jeff