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

[Marcelo Tosatti]

On Thu, Feb 24, 2011 at 05:28:20PM +0200, Avi Kivity wrote:
> On 02/24/2011 05:14 PM, Marcelo Tosatti wrote:
> >>  >>   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
> >
> >There is a similar situation with atomicity here. Mgmt app requests a
> >switch and dies immediately, before receiving the command reply. Qemu
> >crashes. Which image is the uptodate one, source or live-copy?
> 
> live-copy (or it's new name, RAID-1).  Once you've created it it is
> equivalent to source.  Once it switches to state=synced you can
> switch back to either source or destination (I guess by telling qemu
> to detach the one you don't want first, so it falls back to
> state=degraded).
> 
> >
> >>  You could hot-unplug the image and hot-plug it later (continuing the
> >>  copy with qemu-img),
> >
> >Then there's no need for live copy. qemu-img does that already.
> 
> It will start from the beginning.
> 
> >>  or live migrate it.
> >
> >You can live migrate (but not live migrate with live block migration)
> >with live copy in progress, its just that its not supported yet.
> 
> A RAID-1 driver will work with block live migration too.

Nobody cares about that one (block copy and block live migration in
progress). In fact, i doubt anybody cares about parallel block migration
and block copy either (mgmt can easily cope with that limitation, stop
live copy if migration is needed).

> >>  In fact I think a qemu RAID-1 driver
> >>  removes the restriction that you can't live-migrate and live-copy
> >>  simultaneously.
> >
> >As mentioned its just an implementation detail.

I meant the restriction of live-migrate and live-copy in parallel.

> I think it's an important one.  It moves the code from the generic
> layer to a driver. 

Well it is a nice idea, but devil is in the details:

- Guest writes must invalidate in progress live copy reads
and live copy writes, so you have to maintain a queue for live
copy AIO.
- Live copy writes must be aware of in progress guest AIO writes, so
you have to maintain a queue for guest AIO.
- Guest writes must be mirrored to source and destination.
- qemu-img must handle this new format. 

So my view ATM is that this is overengineering.

> It allows generic RAID-1 functionality (for high
> availablity).

Isnt HA responsability of the host filesystem?