Re: [Qemu-devel] Pending Disk I/O requests during live VM migration

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

In ioemu/vl.c: do_savevm(), before saving the current VM state for live 
VM migration (qemu_savevm_state()), it calls qemu_aio_flush() in 
ioemu/block-raw.c. Now qemu_aio_flush() internally calls qemu_aio_poll() 
where we loop through the linked list of RawAIOCB structures and remove 
the request (qemu_aio_release()) only if the request is CANCELED or NOT 
IN PROGRESS.

I am not really sure then why ioemu/hw/ide.c:pci_ide_save() has a 
comment saying "If a transfer is pending, we do not save it yet". 
Looking at qemu_aio_poll() doesn't seem to indicate so...

Also, looking at /usr/include/aio.h sees to suggest that the pending 
requests get stored in the aio_buf of "struct aiocb" structure. Do the 
responses to these requests go in the same buffer (in which case the 
aio_buf and aio_nbytes would get overwritten by the response) ??

any help in this regard would be helful.

-Kaushik

Blue Swirl wrote:
> On 10/4/07, Kaushik Bhandankar <kaushikb@cc.gatech.edu> wrote:
>   
>> 1) I guess the IDE disk code is in ioemu/hw/ide.c. But where is the IDE
>> disk controller code located ??
>>     
>
> Same file, see ide_ioport_write().
>
>   
>> 2) I do not really understand BMDMA in ioemu/hw/ide.c. google doesnt
>> seem to be helping much here. Any documentation about the QEMU IDE disks
>> would be greatly appreciated.
>>     
>
> No idea.
>
>   
>> 3) vl.c:5522 seems to be not the correct code (I guess our vl.c codes
>> are different).. Could you point me to the routine in vl.c where the
>> pending disk IO requests are flushed before VM save (I guess the VM save
>> happens in qemu_savevm_state() in vl.c wherein it invokes all the
>> registered save handlers)?
>>     
>
> It's in:
> void do_savevm(const char *name)
>
>   
>> 4) Somebody told me that the pending disk IO requests are stored
>> somewhere in quemu's I/O disk model but I am not sure exactly
>> where....Can somebody help me with this ?
>>     
>
> Qemu uses asynchronous IO, grepping aio or AIO should find something.
>
>
>   


-- 
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                -- Wm. Shakespeare