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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection
Date: Wed, 15 May 2013 10:53:21 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 15.05.2013 um 10:25 hat Paolo Bonzini geschrieben:
> Il 15/05/2013 09:59, Kevin Wolf ha scritto:
> >>> 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...
> We don't need a way to pass data from before to after hooks, a simple
> scan of a linked list will do.

So in this case the linked list is the way.

> >> 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?
> Yes, otherwise the guest can allocate arbitrary amounts of memory in the
> host just by starting a few very large I/O operations.

I think I would rather throttle I/O in this case, i.e. requests wait
until they can get the space. At least for a synchronous mirror we
have to do something like this.

> > 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.
> That's not enough for the case when the host crashes and you have to
> restart the mirroring or complete it offline.

You're thinking of a persistent bitmap here? Makes sense then, I didn't
think about that.


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