Re: [Qemu-devel] the whole virtual machine hangs when IO does notcome back!

[Fam Zheng]

On Tue, 08/12 10:09, Zhang Haoyu 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.
> >> 
> >
> >If some of the requests are dropped by host and never return to QEMU, I think
> >bdrv_drain_all() will still cause the hang. Even with virtio-blk, reset has
> >such a call. Maybe we could add some -ETIMEDOUT machanism in QEMU's block
> >layer.
> >
> >A workaround might be to configure the host storage to fail the IO after a
> >timeout.
> >
> If -ETIMEOUT returned after a short time network disconnection, may 
> unpredicted fault happened in VM ?
> e.g., the VM was reading important data(like, system data).
> Does aio replay work for this case?

Guest should do error handling with it, in a way similar to -EIO. The
connection is still down even if guest is free to retry, isn't it?

Fam