Re: [Qemu-devel] [PATCH] block: extend BLOCK_IO_ERROR event with nospace indicator

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 8 Sep 2014 17:33:18 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> Am 08.09.2014 um 16:42 hat Luiz Capitulino geschrieben:
> > On Fri, 29 Aug 2014 16:07:27 -0400
> > Luiz Capitulino <lcapitulino@redhat.com> wrote:
> > 
> > > Management software, such as RHEV's vdsm, want to be able to allocate
> > > disk space on demand. The basic use case is to start a VM with a small
> > > disk and then the disk is enlarged when QEMU hits a ENOSPC condition.
> > > 
> > > To this end, the management software has to be notified when QEMU
> > > encounters ENOSPC. The solution implemented by this commit is simple:
> > > it extends the BLOCK_IO_ERROR with a 'nospace' key, which is true
> > > when QEMU is stopped due to ENOSPC.
> > > 
> > > Note that support for querying this event is already present in
> > > query-block by means of the 'io-status' key. Also, the new 'nospace'
> > > BLOCK_IO_ERROR field shares the same semantics with 'io-status',
> > > which basically means that werror= has to be set to either
> > > 'stop' or 'enospc' to enable 'nospace'.
> > > 
> > > Finally, this commit also updates the 'io-status' key doc in the
> > > schema with a list of supported device models.
> > > 
> > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > 
> > Kevin, are you going to take this via block layer tree?
> 
> Yes, thanks, I've applied it now.
> 
> What was our conclusion wrt the human-readable strerror() string for
> debugging? Didn't we want to add that as well?

I can do it on top of this patch. So, just adding a new field for this
is fine?

> 
> > > ---
> > > 
> > > Three important observations:
> > > 
> > >  1. We've talked with oVirt and OpenStack folks. oVirt folks say that
> > >     this implementation is enough for their use-case. OpenStack don't
> > >     need this feature
> > > 
> > >  2. While testing this with a raw image on a (smaller) ext2 file mounted
> > >     via the loopback device, I get half "Invalid argument" I/O errors and
> > >     half  "No space" errors". This means that half of the BLOCK_IO_ERROR
> > >     events that are emitted for this test-case will have nospace=false
> > >     and the other half nospace=true. I don't know why I'm getting those
> > >     "Invalid argument" errors, can anyone of the block layer comment
> > >     on this? I don't get that with a qcow2 image (I get nospace=true for
> > >     all events)
> 
> Sounds familiar, but I never got around to debugging. Would probably be
> worth some digging where the EINVAL comes from.
> 
> > >  3. I think this should go via block tree
> > > 
> > >  block.c              | 22 ++++++++++++++--------
> > >  qapi/block-core.json |  8 +++++++-
> > >  2 files changed, 21 insertions(+), 9 deletions(-)
> 
> Kevin
>