Question regarding live-migration with drive-mirror

[Fiona Ebner]

Hi,
recently one of our users provided a backtrace[0] for the following
assertion failure during a live migration that uses drive-mirror to sync
a local disk:
> bdrv_co_write_req_prepare: Assertion `!(bs->open_flags & BDRV_O_INACTIVE)' 
> failed

The way we do migration with a local disk is essentially:
1. start target instance with a suitable NBD export
2. start drive-mirror on the source side and wait for it to become ready
once
3. issue 'migrate' QMP command
4. cancel drive-mirror blockjob after the migration has finished

I reproduced the issue with the following fio script running in the
guest (to dirty lots of clusters):
> fio --name=make-mirror-work --size=100M --direct=1 --rw=randwrite \
>     --bs=4k --ioengine=psync --numjobs=5 --runtime=60 --time_based

AFAIU, the issue is that nothing guarantees that the drive mirror is
ready when the migration inactivates the block drives.

Is using copy-mode=write-blocking for drive-mirror to only way to avoid
this issue? There, the downside is that the network (used by the mirror)
would become a bottleneck for IO in the guest, while the behavior would
really only be needed during the final phase.

I guess the assert should be avoided in any case. Here's a few ideas
that came to mind:
1. migration should fail gracefully
2. migration should wait for the mirror-jobs to become ready before
inactivating the block drives - that would increase the downtime in
these situations of course
2A. additionally, drive-mirror could be taken into account when
converging the migration somehow?

I noticed the following comment in the mirror implementation
>         /* Note that even when no rate limit is applied we need to yield
>          * periodically with no pending I/O so that bdrv_drain_all() returns.
>          * We do so every BLKOCK_JOB_SLICE_TIME nanoseconds, or when there is
>          * an error, or when the source is clean, whichever comes first. */

3. change draining behavior after the job was ready once, so that
bdrv_drain_all() will only return when the job is ready again? Hope I'm
not completely misunderstanding this.

Best Regards,
Fiona

[0]:
> Thread 1 (Thread 0x7f3389d4a000 (LWP 2297576) "kvm"):
> #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
> #1  0x00007f339488d537 in __GI_abort () at abort.c:79
> #2  0x00007f339488d40f in __assert_fail_base (fmt=0x7f3394a056a8 "%s%s%s:%u: 
> %s%sAssertion `%s' failed.\n%n", assertion=0x5595f85bfd70 "!(bs->open_flags & 
> BDRV_O_INACTIVE)", file=0x5595f85cb576 "../block/io.c", line=2026, 
> function=<optimized out>) at assert.c:92
> #3  0x00007f339489c662 in __GI___assert_fail 
> (assertion=assertion@entry=0x5595f85bfd70 "!(bs->open_flags & 
> BDRV_O_INACTIVE)", file=file@entry=0x5595f85cb576 "../block/io.c", 
> line=line@entry=2026, function=function@entry=0x5595f85cc510 
> <__PRETTY_FUNCTION__.8> "bdrv_co_write_req_prepare") at assert.c:101
> #4  0x00005595f83218f2 in bdrv_co_write_req_prepare (child=0x5595f91cab90, 
> offset=60867018752, bytes=196608, req=0x7f324a2e9d70, flags=0) at 
> ../block/io.c:2026
> #5  0x00005595f8323384 in bdrv_aligned_pwritev 
> (child=child@entry=0x5595f91cab90, req=req@entry=0x7f324a2e9d70, 
> offset=60867018752, bytes=196608, align=align@entry=1, qiov=0x5595f9030d58, 
> qiov_offset=0, flags=0) at ../block/io.c:2140
> #6  0x00005595f832485a in bdrv_co_pwritev_part (child=0x5595f91cab90, 
> offset=<optimized out>, offset@entry=60867018752, bytes=<optimized out>, 
> bytes@entry=196608, qiov=<optimized out>, qiov@entry=0x5595f9030d58, 
> qiov_offset=<optimized out>, qiov_offset@entry=0, flags=flags@entry=0) at 
> ../block/io.c:2353
> #7  0x00005595f8315a09 in blk_co_do_pwritev_part 
> (blk=blk@entry=0x5595f91db8c0, offset=60867018752, bytes=196608, 
> qiov=qiov@entry=0x5595f9030d58, qiov_offset=qiov_offset@entry=0, 
> flags=flags@entry=0) at ../block/block-backend.c:1365
> #8  0x00005595f8315bdd in blk_co_pwritev_part (flags=0, qiov_offset=0, 
> qiov=qiov@entry=0x5595f9030d58, bytes=<optimized out>, offset=<optimized 
> out>, blk=0x5595f91db8c0) at ../block/block-backend.c:1380
> #9  blk_co_pwritev (blk=0x5595f91db8c0, offset=<optimized out>, 
> bytes=<optimized out>, qiov=qiov@entry=0x5595f9030d58, flags=flags@entry=0) 
> at ../block/block-backend.c:1391
> #10 0x00005595f8328a59 in mirror_read_complete (ret=0, op=0x5595f9030d50) at 
> ../block/mirror.c:260
> #11 mirror_co_read (opaque=0x5595f9030d50) at ../block/mirror.c:400
> #12 0x00005595f843a39b in coroutine_trampoline (i0=<optimized out>, 
> i1=<optimized out>) at ../util/coroutine-ucontext.c:177
> #13 0x00007f33948b8d40 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> #14 0x00007f324a3ea6e0 in ?? ()
> #15 0x0000000000000000 in ?? ()