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

[Anthony Liguori]

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

> Your concern in the previous patch was that write() to a regular file 
> might block. That's why I'm calling select before calling write. Do 
> you think select will mark the fd as writeable but write would still 
> block ?

select() doesn't help.  It will return that the file descriptor is 
writable and then the subsequent write will block.

Regards,

Anthony Liguori

> Thanks,
>     Uri.