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Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs

From: Max Reitz
Subject: Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs
Date: Fri, 31 Mar 2017 19:57:11 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 31.03.2017 19:49, Ciprian Barbu wrote:
> Hi,
> Thank you for getting back!
> I'm trying to follow you, but I don't understand all the details. I would 
> like to ask this question though:
> What is the difference between v2.8.0 and this commit? With v2.8.0 the same 
> qemu command worked, but I admit it doesn't request sharing.

The difference is that we now make sure there are for example not two
writers for the same block device (unless both agree that it's OK to
have shared writers). We haven't done that before.

The built-in NBD server is completely OK with writes by other parties
(such as guest devices), but guest devices generally are not (because
the guest OS should know about this).

Therefore, you'd now have to set the share-rw flag for guest devices
which should allow shared writers. Otherwise you cannot set up an NBD
server on a block device that is in use by the guest device.

> We also use libvirt v1.3.4, which might be a problem, but at least we want to 
> understand if the commit in question introduced an obvious problem or if it's 
> all in the details.

Well, I don't think there is any libvirt version that knows about this
flag yet.

...maybe another way to resolve the issue would be to set share-rw to
true by default for now, noting that this will be changed in the future.
In any case, I'm afraid we'll have to wait until Kevin is around on Monday.

> Btw, the qemu command generated by libvirt is this one, sorry about that:

And sorry about not yet having thanked you for reporting this issue. :-)


> 2017-03-31 17:40:10.956+0000: starting up libvirt version: 1.3.4, package: 
> 0+amos3~u16.04 (Enea Armband Devops Team <address@hidden> Fri, 13 Jan 2017 
> 02:06:05 +0100), qemu version: 2.8.50(Debian 1:2.9+amos2~u16.04), hostname: 
> node-2.domain.tld
> LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin 
> QEMU_AUDIO_DRV=none /usr/bin/kvm -name instance-00000076,debug-threads=on -S 
> -object 
> secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-14-instance-00000076/master-key.aes
>  -machine virt-2.8,accel=kvm,usb=off,gic-version=3 -cpu host -m 256 -realtime 
> mlock=off -smp 1,sockets=1,cores=1,threads=1 -uuid 
> 2812f3c9-f564-499b-a8c7-e9e7ccf24143 -no-user-config -nodefaults -chardev 
> socket,id=charmonitor,path=/var/lib/libvirt/qemu/domain-14-instance-00000076/monitor.sock,server,nowait
>  -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew 
> -no-shutdown -boot strict=on -kernel 
> /var/lib/nova/instances/2812f3c9-f564-499b-a8c7-e9e7ccf24143/kernel -initrd 
> /var/lib/nova/instances/2812f3c9-f564-499b-a8c7-e9e7ccf24143/ramdisk -append 
> 'root=/dev/vda1 rw rootwait console=tty0 console=ttyS0 console=ttyAMA0' 
> -device i82801b11-bridge,id=pci.1,bus=pcie.0,addr=0x1 -device 
> pci-bridge,chassis_nr=2,id=pci.2,bus=pci.1,addr=0x0 -usb -drive 
> file=/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-e9e7ccf24143/disk,format=qcow2,if=none,id=drive-virtio-disk0,cache=none,aio=native
>  -device 
> virtio-blk-device,scsi=off,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
>  -drive 
> file=/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-e9e7ccf24143/disk.config,format=raw,if=none,id=drive-virtio-disk1,cache=none,aio=native
>  -device virtio-blk-device,scsi=off,drive=drive-virtio-disk1,id=virtio-disk1 
> -netdev tap,fd=27,id=hostnet0 -device 
> virtio-net-device,netdev=hostnet0,id=net0,mac=fa:16:3e:82:0a:2b -serial 
> file:/var/lib/nova/instances/2812f3c9-f564-499b-a8c7-e9e7ccf24143/console.log 
> -serial pty -vnc -k en-us -device 
> VGA,id=video0,vgamem_mb=16,bus=pci.2,addr=0x1 -device 
> virtio-balloon-device,id=balloon0 -msg timestamp=on
> Domain id=14 is tainted: high-privileges
> Regards,
> /Ciprian

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