Re: [Qemu-devel] [PATCH 1/3] Add blkmirror block driver

[Stefan Hajnoczi]

On Mon, Feb 27, 2012 at 11:48 AM, Paolo Bonzini <address@hidden> wrote:
> On 02/27/2012 12:42 PM, Stefan Hajnoczi wrote:
>>> >
>>> > Once non-incremental mode is added, I suspect blkmirror will diverge
>>> > from blkverify significantly.  In particular, we would need to track
>>> > where have writes been done in the destination.  We also would need to
>>> > hooks for block/stream.c, or perhaps a completely separate
>>> > implementation of streaming.
>> I'm not familiar with non-incremental mode, could you please explain
>> it or link to it?
>
> Non-incremental mode is "pre-copy" migration.  It would stream in the
> background from the source to the destination.  In this case:
>
> - you need to differentiate streaming writes from other writes.  When
> streaming, you do not want to issue spurious writes to the source;
>
> - you can skip streaming anything that has been written to the
> destination already.  This means that you need: 1) a bitmap similar to
> is_allocated; 2) to widen writes to a cluster; 3) serialization similar
> to copy-on-read.
>
> I'm not yet sure how much of this will be generalized in the block layer
> and block/stream.c, and how much will be specific to blkmirror, but in
> general none of this applies to blkverify.

Agreed but I'm not sure it has to do with blkmirror either.  blkmirror
and blkverify perform the same function except that blkverify mirrors
reads too and checks that they match.

Who knows when and if pre-copy will be (re)implemented, it's not a
good argument to say let's duplicate block mirroring because we're not
sure about the design of a feature feature yet which might want to
hook in here.

Stefan