Re: [Qemu-devel] [RFC v3 ATCH 0/5] char: expose CirMemCharDriver and provide QMP interface

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 12, 2012 at 07:57:21PM +0800, Lei Li wrote:
> This RFC series attempts to convert the MemCharDriver to use a circular
> buffer for input and output, expose it to users by introducing QMP commands
> memchar_write and memchar_read and via the command line like the other
> CharDriverStates.
> 
> Serial ports in qemu always use CharDriverStates as there backends,
> Right now, all of our backends always try to write the data from the
> guest to a socket or file. The concern from OpenStack is that this could
> lead to unbounded disk space usage since they log the serial output.
> For more detail of the background info:
> https://bugs.launchpad.net/nova/+bug/832507

Unbounded disk usage is only relevant if telling QEMU to write directly
to its file backend. If you use a socket backend the mgmt app can provide
whatever policy it desires.

> So we want to use a circular buffer in QEMU instead, and then OpenStack
> can periodically read the buffer in QEMU and log it.

With any circular buffer you obviously have a race condition where
the buffer may overflow before the application can consume the data.
By implementing the circular buffer in QEMU you are imposing a
specific policy for overflow on the applications using QEMU, namely
that data gets overwritten/lost.

If the circular buffering is done outside QEMU, then the application
can implement a variety of policies on overflow. At the very least
they can detect when overflow would occur, and insert a marker to
the effect that there is a log discontinuity. Or they can pause the
VM for a period of time, or reduce its scheduling priority, or any
number of different things.

The further advantage of doing it outside QEMU, is that OpenStack will
work with all existing QEMU/KVM/libvirt versions.

I think most of the discussion in the quoted OpenStack bug is rather
short sighted only focusing on the immediate needs for their current
'get_log_output' API. I don't think this will be satisfactory for
OpenStack in the medium-long term. IMHO OpenStack needs to provide
a more comprensive logging capability than what is does today, and
thus this proposed QEMU feature would have a short lifetime of usefulness
to OpenStack. I've already recommended that OpenStack take advantage
of conserver to setup logging of all VMs, though there were some
issues around that. It is entirely possible for OpenStack to provide
its own logging system to process VM logs into fixed size files, as
well as satisfying the needs of its get_log_output API for circular
buffers.

Regards,
Daniel
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