Re: Pending Disk io requests during live migration

[Kaushik Bhandankar <kaushikb@cc.gatech.edu>]

Thanks Tim.... I have also mailed qemu-devel.....

I actually want to enhance the VMM in such a way that once the VM has 
migrated, the old VMM can transfer the responses of the pending disk I/O 
requests to the new VMM (using some communication channel between the VMMs).

You said "They're somewhere in qemu's disk i/o model. You could modify 
the IDE controller to remember outstanding requests so they could be 
reissued,or possibly you could add save/restore handlers in the disks 
themselves". I have several doubts here...

* I am newbie so dont know where is the source for qemu's disk I/O model.
* Where is the IDE controller source code located (I guess the emulated 
IDE disk code is in ioemu/hw/ide.c)?
* The save/restore handler for IDE disks are the 
pci_ide_save()/pci_ide_load() in ioemu/hw/ide.c. Knowing where the VMM 
stores the pending disk IO requests would allow me to modify these 
handlers to remember the outstanding disk requests.


Further, I am unclear what the BMDMA structure in ide.c is meant for. I 
could not find relevant documentation about it.


Thanks,
Kaushik
--------------------------------------------------------------------------------------------------------------------------------------------

Tim Deegan wrote:
> Hi,
>
> At 01:22 -0400 on 04 Oct (1191460920), Kaushik Bhandankar wrote:
>   
>> register_savevm() in tools/ioemu/vl.c is simply used to register the 
>> save & load routines......... register_savevm() is called in 
>> tools/ioemu/hw/ide.c:pci_piix_ide_init to register pci_ide_save() and 
>> pci_ide_load() as the save & load routines for IDE disks.......
>>
>> But I am still unsure as to where these save/load routines for IDE disks 
>> get invoked....
>>     
>
> qemu_savevm() in vl.c walks the list of registered save handlers.
> qemu_loadvm() in vl.c expects a load handler to have been registered 
> for each chunk of the save file. 
>
>   
>>> Basically, ide.c:pci_ide_save() saves the state of the IDE disk in a 
>>> QEMUFile and this file is sent over the network (can somebody point me 
>>> to the code where this happens ??))
>>>       
>
> Seacrh for 'qemu' in tools/python/xen/xend/XendCheckpoint.py
>
>   
>> so that the new VMM (where the VM 
>>     
>>> has migrated) invokes ide.c:pci_ide_load() to retrieve the IDE Disk 
>>> contents form the file. As of now, the pending disk I/O requests do 
>>> not get saved in this file so these pending disk I/O requests are not 
>>> available when executing pci_ide_load().
>>>       
>
> Yes.  I looked at this before but it seemed like a PITA to track down
> the request in whtever DMA callback it was living in, so I just made
> pci_ide_load signal an abort and let the OS pick up the pieces.  Have
> you got a system where this doesn't work, or are you just trying to do
> something a bit less nasty?
>
>   
>>> I am still trying to figure out where the pending Disk I/O requests 
>>> get stored in the VMM so that during live VM migration, these requests 
>>> can be put in the QEMUFile (As mentioned above)
>>>       
>
> They're somewhere in qemu's disk i/o model.  You could modify the IDE
> controller to remember outstanding requests so they could be reissued,
> or possibly you could add save/restore handlers in the disks themselves.
> Also it might be worth asking on qemu-devel since the IDE save/restore
> code is independent of Xen.
>
> Cheers,
>
> Tim.
>
>