Re: [Qemu-devel] migration: adding migration to/from a file (v2)

[Anthony Liguori <anthony@codemonkey.ws>]

Uri Lublin wrote:
> Anthony Liguori wrote:
>> Uri Lublin wrote:
>>> Anthony Liguori wrote:
>>>> Uri Lublin wrote:
>>>>>
>>>>> Migration to file, uses migration-to-fd (supports live migration).
>>>>> Migration from file, uses qemu-fopen directly.
>>>>
>>>> Eh?  Haven't we already talked about why this doesn't work?  Maybe 
>>>> there's a v3 that you meant to send?
>>>>
>>>
>>> Actually I do have a v3 which uses posix-aio-compat.c
>>> It's a much more complicated solution then just writing to a file 
>>> though.
>>> Also I am not sure if I need to use a signal or not as the migration 
>>> (to-fd) code is polling. And if I use signal should I use SIGUSR2 or 
>>> a different one and use a pipe similar to block-raw-posix.c ?
>>
>> How is the migration code polling?  It will attempt to do writes 
>> until a write returns EAGAIN.  At this point, it will wait for 
>> notification that the more writes are available.  Remember, migration 
>> is a streaming protocol, not a random access, so it only makes sense 
>> to have one outstanding request at a time.
>>
>> Your code would look something like:
>>
>> write() -> submit aio request
>> until aio completes, write returns EAGAIN
>> when aio completes, notify migration code that we are writable again
>>
>
> Basically that's what I've done in my v3.
> But since I fake EAGAIN, and the fd is writeable by select, the 
> scenario is as follows:

Oh, yes, this is a problem.  You cannot use the existing migrate_fd code 
unfortunately :-(

Or maybe you can.  If you change all instances of  
qemu_set_fd_handler2() in migrate.c (in the migrate_fd_ routines) to 
basically, s->set_fd_handler(), you can introduce a simple wrapper that 
calls qemu_set_fd_handler2() for everything else, but for yourself, use 
it as a mechanism to keep track of what the "writable" callback should 
be.  This is the callback you would invoke when the aio request completes.

> Sure looks like a bug.
I wish!  It's Unix suckiness.

Regards,

Anthony Liguori


> I'll send my v3 using posix-aio-compat for review soon.
>
> Thanks,
>     Uri.