Re: [Qemu-devel] Use getaddrinfo for migration

[Daniel P. Berrange]

On Fri, Mar 02, 2012 at 02:25:36PM +0400, Michael Tokarev wrote:
> Not a reply to the patch but a general observation.
> 
> I noticed that the tcp migration uses gethostname
> (or getaddrinfo after this patch) from the main
> thread - is it really the way to go?  Note that
> DNS query which is done may block for a large amount
> of time.  Is it really safe in this context?  Should
> it resolve the name in a separate thread, allowing
> guest to run while it is doing that?
> 
> This question is important for me because right now
> I'm evaluating a network-connected block device driver
> which should do failover, so it will have to resolve
> alternative name(s) at runtime (especially since list
> of available targets is dynamic).
> 
> From one point, _usually_, the delay there is very
> small since it is unlikely you'll do migration or
> failover overseas, so most likely you'll have the
> answer from DNS handy.  But from another point, if
> the DNS is malfunctioning right at that time (eg,
> one of the two DNS resolvers is being rebooted),
> the delay even from local DNS may be noticeable.

Yes, I think you are correct - QEMU should take care to ensure that
DNS resolution can not block the QEMU event loop thread.

There is the GLib extension (getaddrinfo_a) which does async DNS
resolution, but for sake of portability it is probably better
to use a thread to do it.

Regards,
Daniel
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