nbd device is read-only

[Niosop]

Using nbd-client version 3.7 from ubuntu 14.04.1, connecting to gonbdserver, the created nbd device is read-only. blockdev --getro returns 1.

Using blockdev --setrw will cause it --getro to return 0, but writes will fail and dmesg will show "block nbd0: Write on read-only".

Using the nbd-server package to serve the device gives a writable device. Uncommenting the Dispatch debug line shows only NBD_CMD_READs being dispatched until the final NBD_CMD_DISC.

My impression is that something during the initial setup is signaling that it's a read only device somehow.

Any ideas on what I can try to debug this? gonbdserver.conf is:

servers:
- protocol: tcp
address: 127.0.0.1:6666
exports:
- name: foo
driver: file
path: /tmp/test
workers: 2
logging:

Connecting using: nbd-client -N foo 127.0.0.1 6666 /dev/nbd0

[Niosop]

On ubuntu 16.04, using nbd-client 3.13-262-g69a15c0-dirty, it is created properly as r/w. Closing this, but if you want any more information I can set up the old environment again to do testing.

[abligh]

Thanks for the report. I will take a look at this as i have a 14.04 environment. From memory there was in one nbd-client release a forward compatibility problem that produced this result; perhaps that got into 14.04. If so it would be useful if there was a workaround (other than upgrade nbd-client).

[abligh]

I think it's the following bug in nbd:
https://www.mail-archive.com/nbd-general@lists.sourceforge.net/msg03878.html

If I'm reading that right, the issue is that nbd-client is sending flags to the kernel that it shouldn't be, specifically the NO_ZEROES flag gonbdserver (correctly) advertises, rather than masking them off. I can probably work around that one with a back compatibility flag. I'll look at adding that today.

[abligh]

If you still have the test environment around, any chance you could try the latest version of gonbdserver (specifically with commit 929da65) with 14.04 and add disablenozeroes: true to the server level configuration (i.e. immediately beneath 'address'). I haven't tested this but I think it should work around the issue.

[Niosop]

I don't, blew it away to install 16.04. I will try and recreate it in the morning and let you know how it goes.

[abligh]

I have replicated this issue, which is indeed for the reason I said above. I have confirmed that commit 929da65 fixes it. You will need to make a change to your config file as follows to support it.

--- gonbdserver.conf    2016-04-06 16:40:55.000000000 +0100
+++ gonbdserver.conf.back-compatible    2016-05-04 11:18:03.000000000 +0100
@@ -1,6 +1,7 @@
 servers:
 - protocol: tcp
   address: 127.0.0.1:6666
+  disablenozeroes: true
   exports:
   - name: foo
     driver: file

[abligh]

Also see https://bugs.launchpad.net/ubuntu/+source/nbd/+bug/1578185