Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

[Avi Kivity <avi@redhat.com>]

On 02/23/2011 07:49 PM, Marcelo Tosatti wrote:
> On Wed, Feb 23, 2011 at 03:01:14PM +0200, Avi Kivity wrote:
> >  On 02/23/2011 01:14 AM, Anthony Liguori wrote:
> >  >
> >  >-drive already ties into the qemuopts infrastructure and we have
> >  >readconfig and writeconfig.  I don't think we're missing any major
> >  >pieces to do this in a more proper fashion.
> >
> >  The problem with qemu config files is that it splits the
> >  authoritative source of where images are stored into two.  Is it in
> >  the management tool's database or is it in qemu's config file?
> >
> >  For the problem at hand, one solution is to make qemu stop after the
> >  copy, and then management can issue an additional command to
> >  rearrange the disk and resume the guest.  A drawback here is that if
> >  management dies, the guest is stopped until it restarts.  We also
> >  make management latency guest visible, even if it doesn't die at an
> >  inconvenient place.
> >
> >  An alternative approach is to have the copy be performed by a new
> >  layered block format driver:
> >
> >  - create a new image, type = live-copy, containing three pieces of
> >  information
> >     - source image
> >     - destination image
> >     - copy state (initially nothing is copied)
> >  - tell qemu switch to the new image
> >  - qemu starts copying, updates copy state as needed
> >  - copy finishes, event is emitted; reads and writes still serviced
> >  - management receives event, switches qemu to destination image
> >  - management removes live-copy image
> >
> >  If management dies while this is happening, it can simply query the
> >  state of the copy.
> >  Similarly, if qemu dies, the copy state is persistent (could be 0/1 or
> >  real range of blocks).
>
> You don't know if a given block is uptodate or not without the dirty
> bitmap. So unless you also keep track of dirty log somehow, this is
> meaningless.

First, you no longer need the dirty bitmap.  Since all writes go through 
the layered block format driver, you know first-hand what is dirty and 
what isn't.

> So a commit file as proposed indicates copy state (in 0/1 fashion). The
> difference in your proposal is that such information is stored inside a
> special purpose image format?
>

It could also store the already synced range.

The difference is that the file is self contained.  You could hot-unplug 
the image and hot-plug it later (continuing the copy with qemu-img), or 
live migrate it.  In fact I think a qemu RAID-1 driver removes the 
restriction that you can't live-migrate and live-copy simultaneously.

-- 
error compiling committee.c: too many arguments to function