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

From: Paolo Bonzini <pbonzini@redhat.com>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: kwolf@redhat.com, Federico Simoncelli <fsimonce@redhat.com>,
	mtosatti@redhat.com, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH 1/3] Add blkmirror block driver
Date: Mon, 27 Feb 2012 12:48:41 +0100	[thread overview]
Message-ID: <4F4B6D99.3090305@redhat.com> (raw)
In-Reply-To: <CAJSP0QVWgtaBH-vWeHX59OqCvTOMYkMWorgzsmT=3GSRBFEegg@mail.gmail.com>

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.

Paolo