[Bug 1875139] Re: Domain fails to start when 'readonly' device not writable

[Peter Krempa]

This was indeed caused by libvirt starting to use -blockdev. The issue
is that qemu's 'auto-read-only' property which is used by libvirt for
the backing files doesn't properly work if the 'host_device' backend
encounters a read-only LV.

The above situation also happens if you have an read-only LV in the
backing chain.

In the current upstream situation of qemu's APIs there currently isn't
anything that libvirt can do in this case, because any solution would
either not fix the problem completely or would require sacrificing other
features, the auto-read-only property needs to be fixed in qemu.

I've also filed https://bugzilla.redhat.com/show_bug.cgi?id=1828252 to
track this issue.

** Bug watch added: Red Hat Bugzilla #1828252
   https://bugzilla.redhat.com/show_bug.cgi?id=1828252

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https://bugs.launchpad.net/bugs/1875139

Title:
  Domain fails to start when 'readonly' device not writable

Status in QEMU:
  New

Bug description:
  This issue is introduced in QEMU 4.2.0 (4.1.0 is working fine)

  My root disk is a LVM2 volume thin snapshot that is marked as read-only
  But when I try to start the domain (using virt-manager) I get the following 
error:

  Error starting domain: internal error: process exited while connecting
  to monitor: 2020-04-26T06:55:06.342700Z qemu-system-x86_64: -blockdev
  {"driver":"host_device","filename":"/dev/vg/vmroot-20200425","aio":"native
  ","node-name":"libvirt-3-storage","cache":{"direct":true,"no-
  flush":false},"auto-read-only":true,"discard":"unmap"} The device is
  not writable: Permission denied

  Changing the lvm snapshot to writeable allows me to start the domain.
  (Making it changes possible during domain is running)

  I don't think QEMU should fail when it can't open a (block) device when the 
read-only option is set.
  (why is write access needed?)

  Reproduce steps:
  * Create LVM read-only volume (I don't think any data is needed)
  * Create domain with read-only volume as block device
  * Try to start the domain

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