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

[Avi Kivity]

On 02/24/2011 05:00 PM, Anthony Liguori wrote:
> On 02/24/2011 02:54 AM, Avi Kivity wrote:
> > On 02/23/2011 10:18 PM, Anthony Liguori wrote:
> > > > Then the management stack has to worry about yet another way of 
> > > > interacting via qemu.
> > >
> > >
> > > { 'StateItem': { 'key': 'str', 'value': 'str' } }
> > > { 'StateSection': { 'kind': 'str', 'name': 'str', 'items': [ 
> > > 'StateItem' ] } }
> > > { 'StateInfo': { 'sections': [ 'StateSection' ] } }
> > >
> > > { 'query-state', {}, {}, 'StateInfo' }
> > >
> > > A management tool never need to worry about anything other than this 
> > > command if it so chooses.  If we have the pre-machine init mode for 
> > > 0.16, then this can even be used to inspect state without running a 
> > > guest.
> >
> > So we have yet another information tree.  If we store the cd-rom 
> > eject state here, then we need to make an association between the 
> > device path of the cd-rom, and the StateItem key.
>
> And this linkage is key.
>
> Let's say I launch QEMU with:
>
> qemu -cdrom ~/foo.img
>
> And then in the monitor, I do:
>
> (qemu) eject ide1-cd0
>
> The question is, what command can I now use to launch the same qemu 
> instance?
>
> When I think of stateful config, what I really think of is a way to 
> spit out a command line that essentially becomes, "this is how you now 
> launch QEMU".
>
> In this case, it would be:
>
> qemu -cdrom ~/foo.img -device ide-disk,id=ide1-cd0,drive=
>
> Or, we could think of this in terms of:
>
> qemu -cdrom ~/foo.img -readconfig foo.cfg
>
> Where foo.cfg contained:
>
> [device "ide1-cd0"]
> driver="ide-disk"
> drive=""
>
> So what I'm really suggesting is that we generate foo.cfg whenever 
> monitor commands do things that change the command line and introduce 
> a new option to reflect this, IOW:
>
> qemu -cdrom ~/foo.img -config foo.cfg

If you move the cdrom to a different IDE channel, you have to update the 
stateful non-config file.

Whereas if you do

   $ qemu-img create -f cd-tray -b ~/foo.img ~/foo-media-tray.img
   $ qemu -cdrom ~/foo-media-tray.img

the cd-rom tray state will be tracked in the image file.

> > Far better to store it in the device itself.  For example, we could 
> > make a layered block format driver that stores the eject state and a 
> > "backing file" containing the actual media.  Eject and media change 
> > would be recorded in the block format driver's state.  You could then 
> > hot-unplug a USB cd-writer and hot-plug it back into a different 
> > guest, implementing a virtual sneakernet.
>
> I think you're far too hung up on "store it in the device itself".  
> The recipe to create the device model is not intrinsic to the device 
> model.  It's an independent thing that's a combination of the command 
> line arguments and any executed monitor commands.
>
> Maybe a better way to think about the stateful config file is a 
> mechanism to replay the monitor history.

Again the question is who is the authoritative source of the 
configuration.  Is it the management tool or is it qemu?

The management tool already has to keep track of (the optional parts of) 
the guest device tree.  It cannot start reading the stateful non-config 
file at random points in time.  So all that is left is the guest 
controlled portions of the device tree, which are pretty rare, and 
random events like live-copy migration.  I think that introducing a new 
authoritative source of information will create a lot of problems.

> > > The fact that the state is visible in the filesystem is an 
> > > implementation detail.
> >
> > A detail that has to be catered for by the management stack - it has 
> > to provide a safe place for it, back it up, etc.
>
> If it cares for QEMU to preserve state.  Today, this all gets thrown 
> away.

Right, but we should make it easy, not hard.

> > > It doesn't work for eject unless you interpose an acknowledged 
> > > event.  Ultimately, this is a simple problem.  If you want 
> > > reliability, we either need symmetric RPCs so that the device model 
> > > can call (and wait) to the management layer to acknowledge a change 
> > > or QEMU can post an event to the management layer, and maintain the 
> > > state in a reliable fashion.
> >
> > I don't see why it doesn't work.  Please explain.
>
> 1) guest eject
> 2) qemu posts eject event
> 3) qemu acknowledges eject to the guest
> 4) management tool sees eject event and updates guest config
>
> There's a race between 3 & 4.  It can only be addressed by interposing 
> 4 between 2 and 3 OR making qemu persist this state between 2 and 3 
> such that the management tool can reliably query it.

If "it" is my cd-rom tray block format driver, it works.  It's really 
the same in action as the stateful non-config, except it's part of the 
device/image, not a central location.

> > I disagree.  Storing NVRAM as a disk image is a simple extension of 
> > existing management tools.  Block live-copy and cd-rom eject state 
> > also make sense as per-image state if you take hotunplug and hotplug 
> > into account.
>
> Everything can be stored in a block driver but when the data is highly 
> structured, isn't it nice to expose it in a structured, human readable 
> way?  I know I'd personally prefer a text representation of CMOS than 
> a binary blob.

Have a tool expose it.  Part of the range is unspecified anyway.

Using a block format driver means that we don't have to care about a 
crash during a write, that we can snapshot it, etc.

> > Device settings should be stored with the devices, not with qemu.
> >
> > Suppose we take the cold-plug on startup via the monitor approach.  
> > So we start with a bare machine, cold plug stuff into it.  Now qemu 
> > has to reconcile the stateful non-config file with the hardware.  
> > What if something has changed?  A device moved into a different slot?
>
> Sorry, I'm confused.  Is there anything in the stateful config file 
> when we start up?  If so, the act of starting up will add a bunch of 
> hardware.

Suppose it has information about ide1-cd0's media tray.  Now we restart 
qemu and cold-plug the cdrom into ide0-cd0.  What happens to the 
information?

Or do we store both the cdrom and media tray information in there?  If 
so the management stack has to edit the file to make changes (perhaps 
via the monitor).

> > If a network card has eeprom, we can specify it with -device 
> > rtl8139,eeprom=id, where id specifies a disk image for the eeprom.
>
> We could, but then we'll end up with a bunch of little block devices.  
> That seems less than ideal to me.
>
> Technically, mac address is stored on eeprom and we store that as a 
> device property today.  We can't persist device properties even though 
> you can change the mac address of a network card and it does persist 
> across reboots.  Are you advocating that we introduce an eeprom for 
> every network card (all in a slightly different format) and have 
> special tools to manipulate the eeprom to store and view the mac address?

Yes -- if we really want to support it.  Obviously we have to store the 
entire eeprom, not just the portion containing the MAC address, so it's 
not just a key/value store.  A card may even have its firmware in flash.


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