Re: [Qemu-devel] Fwd: [libvirt] virDomainAttachDevice error during disk hotplug

[Daniel P. Berrange]

On Wed, Aug 21, 2013 at 11:03:49AM +0200, Stefan Hajnoczi wrote:
> On Tue, Aug 20, 2013 at 03:57:40PM +0100, Daniel P. Berrange wrote:
> > On Tue, Aug 20, 2013 at 04:43:48PM +0200, Stefan Hajnoczi wrote:
> > > On Fri, Aug 16, 2013 at 06:52:32PM +0530, Deepak C Shetty wrote:
> > > > 2) qemu-img info /var/run/vdsm/3333017d-1278-4bfb-8129-62bded257399
> > > > image: /var/run/vdsm/3333017d-1278-4bfb-8129-62bded257399
> > > > file format: qcow2
> > > > virtual size: 3.8G (4096000000 bytes)
> > > > disk size: 196K
> > > > cluster_size: 65536
> > > > backing file:
> > > > /rhev/data-center/000065de-04b8-42e2-986c-2de664708be7/11112d24-4cda-4200-8f6d-a1d8362c70fd/images/22224c45-6504-4ea1-bd24-12340017dd32/3333017d-1278-4bfb-8129-62bded257399
> > > > backing file format: qcow2
> > > 
> > > Good, QEMU can parse and read the file.
> > > 
> > > > 3) ls -l
> > > > /rhev/data-center/000065de-04b8-42e2-986c-2de664708be7/11112d24-4cda-4200-8f6d-a1d8362c70fd/images/22224c45-6504-4ea1-bd24-12340017dd32/3333017d-1278-4bfb-8129-62bded257399
> > > > -r--r-----. 1 vdsm kvm 197120 Aug 10 11:59
> > > > /rhev/data-center/000065de-04b8-42e2-986c-2de664708be7/11112d24-4cda-4200-8f6d-a1d8362c70fd/images/22224c45-6504-4ea1-bd24-12340017dd32/3333017d-1278-4bfb-8129-62bded257399
> > > 
> > > Read-only?
> > > 
> > > > 4) libvirtd.log snippets:
> > > > 
> > > > 2013-08-10 11:19:41.766+0000: 1103: debug : virDomainAttachDevice:9820 :
> > > > dom=0x7f92f4003f20, (VM: name=dpk_BR_vm,
> > > > uuid=9999017d-1278-4bfb-8129-62bded257399), xml=<disk device="disk"
> > > > snapshot="no" type="file">
> > > >         <source 
> > > > file="/var/run/vdsm/3333017d-1278-4bfb-8129-62bded257399"/>
> > > >         <target bus="virtio" dev="vdb"/>
> > > > <serial>22224c45-6504-4ea1-bd24-12340017dd32</serial>
> > > >         <driver cache="none" error_policy="stop" io="threads"
> > > > name="qemu" type="qcow2"/>
> > > > </disk>
> > > 
> > > I see no read-only option.
> > > 
> > > Does it work if you make the file read-write instead?
> > 
> > FYI this was solved in libvir-list.
> > 
> > The image file was on tmpfs, which does not support cache=none.
> > 
> > We hit this so often it would be a huge help if QEMU reported a
> > clearer error message then "Invalid Argument" when trying cache=none
> > on tmpfs.
> 
> The kernel only tells us EINVAL but if this is a common issue we could
> add a tmpfs check and print advice (note that it's only advice because
> future kernel versions might lift this limitation of tmpfs).

Rather than checking for tmpfs, you could simply re-try the open()
call without O_DIRECT. If that passes, then you can safely issue
an error message saying that the filesystem doesn't support O_DIRECT.
This will make it robust to other future filesystems which may also
not do O_DIRECT


Daniel
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