Re: [Qemu-devel] [sheepdog] [PATCH v2 0/9] sheepdog: reconnect server after connection failure

[Liu Yuan]

On Wed, Jul 24, 2013 at 11:42:49PM +0800, Liu Yuan wrote:
> On Wed, Jul 24, 2013 at 06:07:21PM +0900, MORITA Kazutaka wrote:
> > At Wed, 24 Jul 2013 16:28:30 +0800,
> > Liu Yuan wrote:
> > > 
> > > On Wed, Jul 24, 2013 at 04:56:24PM +0900, MORITA Kazutaka wrote:
> > > > Currently, if a sheepdog server exits, all the connecting VMs need to
> > > > be restarted.  This series implements a feature to reconnect the
> > > > server, and enables us to do online sheepdog upgrade and avoid
> > > > restarting VMs when sheepdog servers crash unexpectedly.
> > > > 
> > > 
> > > It doesn't work on my test. I tried start linux-0.2.img stored in sheepdog
> > > cluster and then
> > > 
> > > 1. did some buffered writes
> > > 2. restart sheep that this QEMU VM connected to.
> > > 3. $ sync
> > > 
> > > I got following error:
> > > 
> > > $ ../qemu/x86_64-softmmu/qemu-system-x86_64 --enable-kvm -m 1024 -hda 
> > > sheepdog:test
> > > qemu-system-x86_64: failed to get the header, Resource temporarily 
> > > unavailable
> > > qemu-system-x86_64: Failed to connect to socket: Connection refused
> > > qemu-system-x86_64: Failed to connect to socket: Connection refused
> > > qemu-system-x86_64: Failed to connect to socket: Connection refused
> > > qemu-system-x86_64: Failed to connect to socket: Connection refused
> > > qemu-system-x86_64: Failed to connect to socket: Connection refused
> > > ...repeat...
> > > 
> > > QEMU version is master tip
> > 
> > Your sheep daemon looks like unreachable from qemu.  I tried the same
> > procedure, but couldn't reproduce it.
> > 
> > Is the problem reproducible?  Can you make sure that you can connect
> > to the sheep daemon from collie while the error message shows up?
> > 
> 
> Yesh. Well I try to repeat it with following process:
> 
> 1. did some buffered write
> 2. kill the sheep
> 3. $ sync # at guest, now 'sync' hang for response
> 4. restart sheep
> 
> After 4 'sync' still hangs until timeout with a message
> "hda:dma_timer_expiry: dma status == 0x21"
> 
> Guest end up freeze.
> 
> QEMU output is the same:
> qemu-system-x86_64: failed to get the header, Resource temporarily unavailable
> qemu-system-x86_64: Failed to connect to socket: Connection refused
> qemu-system-x86_64: Failed to connect to socket: Connection refused
> qemu-system-x86_64: Failed to connect to socket: Connection refused
> qemu-system-x86_64: Failed to connect to socket: Connection refused
> 
> But notice, if I did restart sheep with guest doing nothing, your patch set 
> work
> like a charm.

I have debug it a bit. The problem is that at stage 3, 'sync' invoke
add_aio_request() in the sheepdog driver and add_aio_request *succeed* with aio
put on the inflight_aio_head list, *not* on the failed_aio_head list. So in the
reconnect_to_sdog(), we have no way to resend the targeted aio and 'sync' wait
for ever.

Thanks
Yuan