Re: Setting up NBD server / client - block nbd4: Other side returned error (22) issue

[Wouter Verhelst <w@uter.be>]

Hi Simon,

On Thu, Jul 22, 2021 at 01:41:00PM +0100, Simon Fernandez wrote:
>    Hi folks,
>    I’m trying setup a client that connect to a 500Gb ext4 disk on another
>    machine running nbd server.
>    Server: raspberry pi 3: uname -r 4.19.66+
>    Client: raspberry pi 3: uname -r 5.10.17-v7+
>    I’ve followed the
>    information here [https://github.com/NetworkBlockDevice/nbd] and a lot of
>    other sites but I think I’m doing something wrong on the server setup
>    because I get this on the client:
>    $ sudo mount /dev/nbd2 /home/pi/Vols/nbd1-recipes/
>    mount: /home/pi/Vols/nbd1-recipes: can't read superblock on /dev/nbd2
>    Also it appears to be listening on default port - NOT ones specified
>    in config file.
>    This is how I’ve set up the server & client, it’s probably something
>    obvious to more experienced folk.
>    == rpi-C3 (192.168.1.14) server setup
>    > sudo apt-get update
>    > sudo apt-get install nbd-server
>    > sudo nano /etc/nbd-server/config 
>    [generic]
>    # If you want to run everything as root rather than the nbd user, you
>    # may either say "root" in the two following lines, or remove them
>    # altogether. Do not remove the [generic] section, however.
>            user = nbd
>            group = nbd
>            includedir = /etc/nbd-server/conf.d
>            allowlist = true
>    # What follows are export definitions. You may create as much of them as
>    # you want, but the section header has to be unique.
>    [images]
>            exportname = /home/pi/media/500G_FAITH/images
>            port = 510029

The per-export port numbers are no longer supported. You should see a
warning about that in syslog when you start nbd-server.

>    [recipes]
>            exportname = /home/pi/media/500G_FAITH/recipes
>            port = 510030
>    [movie]
>            exportname = /home/pi/media/500G_FAITH/movies
>            port = 510031
>    > sudo modprobe nbd  # NOT nbd-server!
>    > lsmod | grep nbd
>    nbd                    40960  0
>    > sudo service nbd-server force-reload  # if already running
>    == client side
>    > sudo apt-get install nbd-client  # done
>    > sudo modprobe nbd
>    > nbd-client 192.168.1.14 -l
>    Negotiation: ..
>    images
>    recipes
>    movie
>    > sudo nbd-client 192.168.1.14 -N recipes /dev/nbd2
>    Negotiation: ..size = 0MB
>    Connected /dev/nbd2
>    > sudo nbd-client 192.168.1.14 -N movie /dev/nbd3
>    Negotiation: ..size = 0MB
>    Connected /dev/nbd3
>    > sudo nbd-client 192.168.1.14 -N images /dev/nbd1
>    Negotiation: ..size = 0MB
>    Connected /dev/nbd1

These all show that the export is 0 bytes large.

Do the paths that you point to in the exportname parameters exist? Are
they files? Are they readable and writable by the "nbd" user? Do they
have a non-zero size?

If the answer to any of these is "no", then that's your problem.

Note that you can also have nbd-server create the exported files on the
fly if you specify a wanted size; see the "filesize" parameter for that.

>    > dmesg | tail -n 10
>    [ 1417.883364] Buffer I/O error on dev nbd1, logical block 0, async page
>    read
>    [ 1417.884614] block nbd1: Other side returned error (22)

Error 22 is NBD_EINVAL. It is expected that you receive that, if you
connect a zero-sized NBD device, because all reads with a non-zero
offset will go beyond the end of the device, and that is an invalid
request.

Everything else you try returns that error, for the same reason.

[...]
>    I’d be very grateful for any pointers, or other resources that my be of
>    help.

Hope that helped,

-- 
     w@uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}