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

From: Bin Wu
Subject: Re: [Qemu-devel] the whole virtual machine hangs when IO does not come back!
Date: Tue, 12 Aug 2014 09:10:26 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

On 2014/8/11 22:21, Stefan Hajnoczi wrote:
On Mon, Aug 11, 2014 at 04:33:21PM +0800, Bin Wu wrote:

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.

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

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