Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection

[Kevin Wolf <kwolf@redhat.com>]

Am 14.05.2013 um 18:45 hat Paolo Bonzini geschrieben:
> Il 14/05/2013 17:48, Wolfgang Richter ha scritto:
> > On Tue, May 14, 2013 at 6:04 AM, Paolo Bonzini <pbonzini@redhat.com
> > <mailto:pbonzini@redhat.com>> wrote:
> > 
> >     Il 14/05/2013 10:50, Kevin Wolf ha scritto:
> >     > Or, to translate it into our existing terminology, drive-mirror
> >     > implements a passive mirror, you're proposing an active one (which we
> >     > do want to have).
> >     >
> >     > With an active mirror, we'll want to have another choice: The
> >     mirror can
> >     > be synchronous (guest writes only complete after the mirrored
> >     write has
> >     > completed) or asynchronous (completion is based only on the original
> >     > image). It should be easy enough to support both once an active mirror
> >     > exists.
> > 
> >     Right, I'm waiting for Stefan's block-backup to give me the "right"
> >     hooks for the active mirror.
> > 
> >     The bulk phase will always be passive, but an active-asynchronous mirror
> >     has some interesting properties and it makes sense to implement it.
> > 
> > 
> > Do you mean you'd model the 'active' mode after 'block-backup,' or actually
> > call functions provided by 'block-backup'?
> 
> No, I'll just reuse the same hooks within block/mirror.c (almost... it
> looks like I need after_write too, not just before_write :( that's a
> pity).

Makes me wonder if using a real BlockDriver for the filter from the
beginning wouldn't be better than accumulating more and more hooks and
having to find ways to pass data from 'before' to 'after' hooks...

> Basically:
> 
> 1) before the write, if there is space in the job's buffers, allocate a
> MirrorOp and a data buffer for the write.  Also record whether the block
> was dirty before;
> 
> 2) after the write, do nothing if there was no room to allocate the data
> buffer.  Else clear the block from the dirty bitmap.  If the block was
> dirty, read the whole cluster from the source as in passive mirroring.
> If it wasn't, copy the data from guest memory to the preallocated buffer
> and write it to the destination;

Does the "if there was no room" part mean that the mirror is active only
sometimes?

And why even bother with a dirty bitmap for an active mirror? The
background job that sequentially processes the whole image only needs a
counter, no bitmap.

At which point it looks like implementing it separate from mirror.c
could make more sense.

Kevin