/build/qemu-YZq7uh/qemu-2.3+dfsg/nbd.c:nbd_init():L670: Failed to set NBD socket

Can anyone answer if already knows what could be issue

Thanks for reporting this bug. I can't reproduce this on a wily
system. Can you please

show 'dpkg -l | grep qemu' output
show 'uname -a' output
make sure nbd module was loaded ('sudo modprobe nbd')
make sure you ran qemu-nbd under sudo?
check whether this happens with a clean empty qcow2 (qemu-img create -f qcow2 x.qcow2 1G)

 status incomplete

Sure PFB outputs

$ dpkg -l | grep qemu

ii ipxe-qemu 1.0.0+git-20141004.86285d1-1ubuntu3 all PXE boot firmware - ROM images for qemu
ii qemu-block-extra:amd64 1:2.3+dfsg-5ubuntu9.2 amd64 extra block backend modules for qemu-system and qemu-utils
ii qemu-kvm 1:2.3+dfsg-5ubuntu9.2 amd64 QEMU Full virtualization
ii qemu-system-common 1:2.3+dfsg-5ubuntu9.2 amd64 QEMU full system emulation binaries (common files)
ii qemu-system-x86 1:2.3+dfsg-5ubuntu9.2 amd64 QEMU full system emulation binaries (x86)
ii qemu-utils 1:2.3+dfsg-5ubuntu9.2 amd64 QEMU utilities

$ uname -a
Linux stack-virtual-machine 4.2.0-16-generic #19-Ubuntu SMP Thu Oct 8 15:35:06 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

Yes Nbd module is loaded & running qemu-nbd with sudo
It is happening with all kinds of images

Let me know if any other things required to know

This nbd module working for 15.04 without making any extra changes

Thanks. Could you please point to a url to an online image which causes this for you?

After it happens is there anything in the kernel log about it? (sudo dmesg)

$ sudo dmesg

[116257.604122] systemd-sysv-generator[20102]: Overwriting existing symlink /run/systemd/generator.late/umountiscsi.service with real service
[116257.922912] systemd-sysv-generator[20153]: Overwriting existing symlink /run/systemd/generator.late/umountiscsi.service with real service
[116257.996768] nbd: registered device at major 43
[116258.225552] systemd-sysv-generator[20194]: Overwriting existing symlink /run/systemd/generator.late/umountiscsi.service with real service
[116258.545912] systemd-sysv-generator[20216]: Overwriting existing symlink /run/systemd/generator.late/umountiscsi.service with real service
[116263.958887] blk_update_request: I/O error, dev fd0, sector 0
[116263.958893] floppy: error -5 while reading block 0
[116324.660261] br100: port 4(vnet2) entered disabled state
[116324.662037] device vnet2 left promiscuous mode
[116324.662041] br100: port 4(vnet2) entered disabled state
[116396.555916] device vnet2 entered promiscuous mode
[116396.567809] br100: port 4(vnet2) entered forwarding state
[116396.567817] br100: port 4(vnet2) entered forwarding state
[116397.684084] kvm: zapping shadow pages for mmio generation wraparound
[116399.579847] kvm [27153]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xabcd
[172179.503248] br100: port 4(vnet2) entered disabled state
[172179.505056] device vnet2 left promiscuous mode
[172179.505060] br100: port 4(vnet2) entered disabled state
[172228.177299] device vnet2 entered promiscuous mode
[172228.189445] br100: port 4(vnet2) entered forwarding state
[172228.189474] br100: port 4(vnet2) entered forwarding state
[172229.355539] kvm: zapping shadow pages for mmio generation wraparound
[172231.225382] kvm [504]: vcpu0 disabled perfctr wrmsr: 0xc1 data 0xabcd

This is coming with all images
Lastly tried below image link
https://download.cirros-cloud.net/0.3.0/cirros-0.3.0-x86_64-disk.img

I get the error you get only when the device is busy.

Just to be sure, could you show

ps -ef | grep qemu-nbd

[Expired for qemu (Ubuntu) because there has been no activity for 60 days.]

Bug still exists.

Did you do

sudo modprobe nbd

first?

[Expired for qemu (Ubuntu) because there has been no activity for 60 days.]