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Re: [Qemu-devel] [Qemu-block] RFC cdrom in own thread?


From: Markus Armbruster
Subject: Re: [Qemu-devel] [Qemu-block] RFC cdrom in own thread?
Date: Thu, 18 Jun 2015 08:57:49 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Peter Lieven <address@hidden> writes:

> Am 17.06.2015 um 10:35 schrieb Kevin Wolf:
>> Am 16.06.2015 um 17:34 hat Stefan Hajnoczi geschrieben:
>>> On Tue, Jun 16, 2015 at 3:44 PM, Peter Lieven <address@hidden> wrote:
>>>> I wonder how difficult it would be to have the IDE CDROM run in its own
>>>> thread?
>>>> We usually have ISOs mounted on an NFS share as CDROM. Problem: If the NFS
>>>> Share
>>>> goes down, it takes down monitor, qmp, vnc etc. with it.
>>>>
>>>> Maybe its already possible to do this via cmdline args?
>>>>
>>>> Any ideas, comments?
>>> If QEMU hangs in the read/write/flush/discard code path due to NFS
>>> downtime it is a bug.
>>>
>>> QEMU is expected to hang in open/reopen because those are performed in
>>> a blocking fashion.
>>>
>>> Which of these cases applies to what you are seeing?  Maybe it can be fixed.
>> Don't forget bdrv_drain_all(), which is called a lot by the monitor. So
>> no matter what you do (and this includes moving to a thread as in a
>> hypothetical "ATAPI dataplane"), you end up with a hang sooner or later.
>
> I will have a look where qemu hangs. The problem exists with an NFS share
> mounted by the kernel and also with libnfs. So it might be a bdrv_drain_all.
> I regularly query info block and info blockstats. Do these commands always
> call bdrv_drain_all()?.

As far as I can tell, they don't.

In general, it's hard to see.  Wish it wasn't.



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