hi,
reading the manual (qemu-nbd 6.2.0), I would say that qemu-nbd can take on
two roles: nbd server AND nbd client (not just server).
The `-c` option makes it take on both roles locally.
on my side, I see two inter-connected (eg. client/server) unix sockets :
```
# modprobe nbd
# truncate -s 256M /tmp/test.disk
# qemu-nbd -f raw -c /dev/nbd0 /tmp/test.disk
# ss -xp | head -1; ss -xp | grep nbd
Netid State Recv-Q Send-Q Local Address:Port Peer
Address:Port Process
u_str ESTAB 0 0 /var/lock/qemu-nbd-nbd0 68444 *
65480 users:(("qemu-nbd",pid=5430,fd=10))
u_str ESTAB 0 0 * 65480 *
68444 users:(("qemu-nbd",pid=5430,fd=9))
```
this being said, maybe the qem-nbd manual could be more precise about this
option...
regards, lacsaP.
Le dim. 1 mai 2022 à 19:46, Bob Power <b_po...@yahoo.com> a écrit :
> Hey all,
>
> Back again...
>
> While image mounted / in use .....
>
> [root@fedora vd]# cat <(ps -aux | head -1) <(ps -aux | grep nbd)
> USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
> root 126 0.0 0.0 0 0 ? S< 14:17 0:00
> [kworker/u9:0+nbd0-recv]
> root 6541 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd0-recv]
> root 6544 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd1-recv]
> root 6545 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd2-recv]
> root 6547 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd3-recv]
> root 6550 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd4-recv]
> root 6551 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd5-recv]
> root 6552 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd6-recv]
> root 6553 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd7-recv]
> root 6554 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd8-recv]
> root 6555 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd9-recv]
> root 6556 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd10-recv]
> root 6558 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd11-recv]
> root 6560 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd12-recv]
> root 6561 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd13-recv]
> root 6562 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd14-recv]
> root 6564 0.0 0.0 0 0 ? I< 17:28 0:00
> [nbd15-recv]
> root 6575 0.0 0.0 1982620 6700 ? Ssl 17:28 0:00
> qemu-nbd -c /dev/nbd0 test.qcow2
> root 6589 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-buf/nbd0p1]
> root 6591 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-conv/nbd0p1]
> root 6592 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-reclaim/nbd]
> root 6593 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-blockgc/nbd]
> root 6594 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-inodegc/nbd]
> root 6595 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-log/nbd0p1]
> root 6596 0.0 0.0 0 0 ? I< 17:28 0:00
> [xfs-cil/nbd0p1]
> root 6597 0.0 0.0 0 0 ? S 17:28 0:00
> [xfsaild/nbd0p1]
> root 6821 0.0 0.0 221800 2132 pts/0 S 17:33 0:00 grep
> --color=auto nbd
>
> [root@fedora vd]# nmap -p 10809 localhost
> Starting Nmap 7.92 ( https://nmap.org ) at 2022-05-01 18:05 BST
> Nmap scan report for localhost (127.0.0.1)
> Host is up (0.00010s latency).
> Other addresses for localhost (not scanned): ::1
>
> PORT STATE SERVICE
> 10809/tcp closed nbd
>
> [root@fedora vd]# systemctl list-units --type=service | grep nbd
> [root@fedora vd]# systemctl list-units --type=service | grep qemu
>
> qemu-guest-agent.service
> loaded active running QEMU Guest Agent
>
> [root@fedora vd]# netstat -pnltu
> Active Internet connections (only servers)
> Proto Recv-Q Send-Q Local Address Foreign Address
> State PID/Program name
> tcp 0 0 127.0.0.53:53 0.0.0.0:*
> LISTEN 693/systemd-resolve
> tcp 0 0 0.0.0.0:5355 0.0.0.0:*
> LISTEN 693/systemd-resolve
> tcp 0 0 127.0.0.1:631 0.0.0.0:*
> LISTEN 815/cupsd
> tcp6 0 0 ::1:631 :::*
> LISTEN 815/cupsd
> tcp6 0 0 :::5355 :::*
> LISTEN 693/systemd-resolve
> udp 0 0 0.0.0.0:5353 0.0.0.0:*
> 719/avahi-daemon: r
> udp 0 0 0.0.0.0:5355 0.0.0.0:*
> 693/systemd-resolve
> udp 0 0 0.0.0.0:48300 0.0.0.0:*
> 719/avahi-daemon: r
> udp 0 0 127.0.0.53:53 0.0.0.0:*
> 693/systemd-resolve
> udp 0 0 127.0.0.1:323 0.0.0.0:*
> 764/chronyd
> udp6 0 0 :::5353
> :::* 719/avahi-daemon: r
> udp6 0 0 :::5355
> :::* 693/systemd-resolve
> udp6 0 0 :::47159
> :::* 719/avahi-daemon: r
> udp6 0 0 ::1:323
> :::* 764/chronyd
>
>
> So qemu-nbd is running ( has not exited ) and nothing is running as a
> service: no server.
>
> I'm not saying this is a problem or a bug - it looks to me like most of
> the time it works and is supposed to work as a server and in this case it
> doesn't and isn't supposed to.
>
> Reading the man page I just expected to see it available (via an nbd
> client) on port 10809....as well as /dev/nbd0 via the usual filesystem
> interfaces.
>
> ( So I tried with -re 2 (read-only , 2 users ) out of curiosity and stuff
> broke ... )
>
> Unless I've misinterpreted what is going on, it should ideally say, for
> clarity, in the man page under "-c", that in this case it does not act as
> a server but on a 1-to-1 basis with the host via whatever method it uses
> (local socket ?)
>
> A server listening on an open port is a security consideration so it would
> be nice to know for certain there isn't supposed to be one if there isn't.
>
> Bob.
>
> On Friday, 29 April 2022, 09:01:34 BST, Pascal <patate...@gmail.com>
> wrote:
>
>
> `qemu-nbd -c` operate as a "local" server/client.
>
> Le ven. 29 avr. 2022 à 09:58, Pascal <patate...@gmail.com> a écrit :
>
> hi,
>
> `-t` switch for persistence (for me - `-p` is for port number).
> `qemu-nbd -c` operate as a "local" server.
> `qemu-nbd` still running : use `ps ax | grep nbd` and/or `ss -p | grep
> nbd` to see it.
>
> regards, lacsaP.
>
> Le ven. 29 avr. 2022 à 02:40, Connor Kuehl <cipku...@gmail.com> a écrit :
>
> re-adding the list
>
> On Thu Apr 28, 2022 at 6:51 PM CDT, Bob Power wrote:
> > Hi Connor,
> > While I'm using the qcow2 image as nbd0 - ie while it is mounted and I
> am accessing files - so client not hung up and something still running to
> interface between the qcow2 image and the system, there does not appear to
> be any nbd service operating.
>
> Yeah, my suspicion is that as part of creating /dev/nbd0, the kernel
> gets all of the information it needs from the qemu-nbd process and is
> then self-sufficient and therefore it hangs up on qemu-nbd and qemu-nbd
> exits. Just a guess.
>
> Have you tried seeing if qemu-nbd remains up to serve other clients if
> you use the -p or --persistent flag?
>
> qemu-nbd -c /dev/nbd0 -p test.qcow2
>
> --
> Connor
>
>
