It still fails with ext4:

#0  0x00007fbaa12b33f8 in raise () at /lib64/libc.so.6
#1  0x00007fbaa12b4ffa in abort () at /lib64/libc.so.6
#2  0x00007fbaa12abc17 in __assert_fail_base () at /lib64/libc.so.6
#3  0x00007fbaa12abcc2 in  () at /lib64/libc.so.6
#4  0x00005646b990f926 in mirror_run (s=0x5646bc50f480) at block/mirror.c:335
        next_sector = 36659200
        next_chunk = 286400
        nb_chunks = 80
        end = 209715200
        sectors_per_chunk = 128
        source = 0x5646bcb70000
        sector_num = 36648960
        delay_ns = 0
        delay_ns = 0
        cnt = 15360
        should_complete = <optimized out>
        s = 0x5646bc50f480
        data = <optimized out>
        bs = 0x5646bcb70000
        sector_num = <optimized out>
        end = <optimized out>
        length = <optimized out>
        last_pause_ns = <optimized out>
        bdi = {cluster_size = 65536, vm_state_offset = 107374182400, is_dirty = false, unallocated_blocks_are_zero = true, can_write_zeroes_with_unmap = true, needs_compressed_writes = false}
        backing_filename = "\000"
        ret = <optimized out>
        n = 1048576
        target_cluster_size = <optimized out>
        __PRETTY_FUNCTION__ = "mirror_run"
#5  0x00005646b990f926 in mirror_run (opaque=0x5646bc50f480) at block/mirror.c:613
        delay_ns = 0
        cnt = 15360
        should_complete = <optimized out>
        s = 0x5646bc50f480
        data = <optimized out>
        bs = 0x5646bcb70000
        sector_num = <optimized out>
        end = <optimized out>
        length = <optimized out>
        last_pause_ns = <optimized out>
        bdi = {cluster_size = 65536, vm_state_offset = 107374182400, is_dirty = false, unallocated_blocks_are_zero = true, can_write_zeroes_with_unmap = true, needs_compressed_writes = false}
        backing_filename = "\000"
        ret = <optimized out>
        n = 1048576
        target_cluster_size = <optimized out>
        __PRETTY_FUNCTION__ = "mirror_run"
#6  0x00005646b997568a in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>) at util/coroutine-ucontext.c:78
        self = 0x5646bc5115b0
        co = 0x5646bc5115b0
#7  0x00007fbaa12c8560 in __start_context () at /lib64/libc.so.6
#8  0x00005646bd2b98b0 in  ()
#9  0x0000000000000000 in  ()

qemu-system-x86_64: block/mirror.c:335: mirror_iteration: Assertion `hbitmap_next == next_sector' failed.


I can't seem to get stable snapshotting and blockpull with a loaded VM.

Interestingly enough, the last command libvirt passes to qemu is:

2016-04-14 20:47:58.196+0000: 18932: debug : qemuMonitorJSONCommandWithFd:294 : Send command '{"execute":"query-block-jobs","id":"libvirt-69"}' for write with FD -1
2016-04-14 20:47:58.196+0000: 18932: info : qemuMonitorSend:1005 : QEMU_MONITOR_SEND_MSG: mon=0x7f1874001a30 msg={"execute":"query-block-jobs","id":"libvirt-69"}
2016-04-14 20:47:58.197+0000: 18929: info : qemuMonitorIOWrite:529 : QEMU_MONITOR_IO_WRITE: mon=0x7f1874001a30 buf={"execute":"query-block-jobs","id":"libvirt-69"}


Odd that it would SIGABRT on a smile query-block-jobs.

Even more interesting is that it crashes on the first or second or third snapshot/block-commit cycle when using EXT4, but would sometimes go for 30-40 cycles on ZFS. 

Any ideas?  I'm certainly willing to test and help in any way I can.

Thanks!