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Date: Wed, 24 Apr 2013 11:26:59 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Adding Disk-Level Introspection to QEMU
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To: Wolfgang Richter <wolf@cs.cmu.edu>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel <qemu-devel@nongnu.org>

Il 23/04/2013 20:31, Wolfgang Richter ha scritto:
> On Tue, Apr 23, 2013 at 2:21 PM, Stefan Hajnoczi <stefanha@gmail.com
> <mailto:stefanha@gmail.com>> wrote:
> 
>     The tracing subsystem is geared towards tracepoint instrumentation
>     rather than binary dumps.
> 
>     Can you share some specific applications?
> 
> 
> Well, my main application is in exposing a "cloud-inotify" service by
> interpreting
> sector writes in real-time and publishing the updates as file system
> manipulations.
> By using introspection we don't need agents running inside the guest.
> 
> Example: guest writes to sector 5786907; I reverse-map that sector and
> notice
> it belongs to '/etc/passwd' within that guest; I immediately emit a message
> (currently using Redis pub-sub functionality) to any interested
> subscribers that
> '/etc/passwd' changed within this guest running on a certain host within the
> datacenter.

If you are okay with writes being "bundled" and you are able to handle
reordered writes within a small timeframe (usually 0.1-1s), then you can
use drive-mirror with an NBD destination.

Paolo