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

[Stefan Hajnoczi <stefanha@gmail.com>]

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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

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