Re: [Qemu-devel] [PATCH] blockdev: reset werror/rerror on drive_del

[Eric Blake <eblake@redhat.com>]

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On 06/04/2013 01:24 PM, Markus Armbruster wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> Il 04/06/2013 18:37, Markus Armbruster ha scritto:
>>> I figure the bit that can't be resolved now is letting the user switch
>>> off "stop on error" safely before a drive_del.  Even if we had a command
>>> for that, there'd still be a window between that command's execution and
>>> drive_del's.  Your patch solves the problem by having drive_del switch
>>> it off unconditionally.  Oookay, but please document it, because it's
>>> not exactly obvious.
>>
>> It is not obvious, but it is not surprising either when you see it (i.e.
>> you won't really be surprised by the errors in the guest and won't need
>> to know that, under the hood, rerror has been changed from the value you
>> specified).
>>

> This is drive_add's documentation in hmp-commands.hx:
> 
>     Remove host block device.  The result is that guest generated IO is
>     no longer submitted against the host device underlying the disk.
>     Once a drive has been deleted, the QEMU Block layer returns -EIO
>     which results in IO errors in the guest for applications that are
>     reading/writing to the device.
> 
> Suggest to add:
> 
>     These errors are always reported to the guest, regardless of the
>     drive's error actions (drive options rerror, werror).
> 
> Independently, libvirt needs fixing.

Total agreement that libvirt needs to use a saner disk hot-unplug
sequence when it is known that qemu provides one.  I've filed
https://bugzilla.redhat.com/show_bug.cgi?id=970761
to remind us to fix libvirt.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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