Re: [Qemu-devel] the whole virtual machine hangs when IO does not come back!

[Bin Wu <wu.wubin@huawei.com>]

On 2014/8/11 22:21, Stefan Hajnoczi wrote:
> On Mon, Aug 11, 2014 at 04:33:21PM +0800, Bin Wu wrote:
>> Hi,
>>
>> I tested the reliability of qemu in the IPSAN environment as follows:
>> (1) create one VM on a X86 server which is connected to an IPSAN, and the VM
>> has only one system volume which is on the IPSAN;
>> (2) disconnect the network between the server and the IPSAN. On the server,
>> I have a "multipath" software which can hold the IO for a long time
>> (configurable) when the network is disconnected;
>> (3) about 30 seconds later, the whole VM hangs there, nothing can be done to
>> the VM!
>>
>> Then, I used "gstack" tool to collect the stacks of all qemu threads, it
>> looked like:
>>
>> Thread 8 (Thread 0x7fd840bb5700 (LWP 6671)):
>> #0  0x00007fd84253a4f6 in poll () from /lib64/libc.so.6
>> #1  0x00007fd84410ceff in aio_poll ()
>> #2  0x00007fd84429bb05 in qemu_aio_wait ()
>> #3  0x00007fd844120f51 in bdrv_drain_all ()
>> #4  0x00007fd8441f1a4a in bmdma_cmd_writeb ()
>> #5  0x00007fd8441f216e in bmdma_write ()
>> #6  0x00007fd8443a93cf in memory_region_write_accessor ()
>> #7  0x00007fd8443a94a6 in access_with_adjusted_size ()
>> #8  0x00007fd8443a9901 in memory_region_iorange_write ()
>> #9  0x00007fd8443a19bd in ioport_writeb_thunk ()
>> #10 0x00007fd8443a13a8 in ioport_write ()
>> #11 0x00007fd8443a1f55 in cpu_outb ()
>> #12 0x00007fd8443a5b12 in kvm_handle_io ()
>> #13 0x00007fd8443a64a9 in kvm_cpu_exec ()
>> #14 0x00007fd844330962 in qemu_kvm_cpu_thread_fn ()
>> #15 0x00007fd8427e77b6 in start_thread () from /lib64/libpthread.so.0
>> #16 0x00007fd8425439cd in clone () from /lib64/libc.so.6
>> #17 0x0000000000000000 in ?? ()
> Use virtio-blk.  Read, write, and flush are asynchronous in virtio-blk.
>
> Note that the QEMU monitor commands are typically synchronous so they
> will still block the VM.
>
> Stefan
Thank you for your attention. I tested virtio-blk and it's true that the 
VM doesn't hange.
Why  does the virtio-blk implement this in asynchronous way, but 
virtio-scsi in synchronous
way?