[Sheepdog] [RFC PATCH v4 3/3] block: add sheepdog driver for distributed storage support

[MORITA Kazutaka]

At Fri, 04 Jun 2010 13:04:00 +0200,
Kevin Wolf wrote:
> 
> Am 03.06.2010 18:23, schrieb MORITA Kazutaka:
> >>> +static void sd_aio_cancel(BlockDriverAIOCB *blockacb)
> >>> +{
> >>> +	SheepdogAIOCB *acb = (SheepdogAIOCB *)blockacb;
> >>> +
> >>> +	acb->canceled = 1;
> >>> +}
> >>
> >> Does this provide the right semantics? You haven't really cancelled the
> >> request, but you pretend to. So you actually complete the request in the
> >> background and then throw the return code away.
> >>
> >> I seem to remember that posix-aio-compat.c waits at this point for
> >> completion of the requests, calls the callbacks and only afterwards
> >> returns from aio_cancel when no more requests are in flight.
> >>
> >> Or if you can really cancel requests, it would be the best option, of
> >> course.
> >>
> > 
> > Sheepdog cannot cancel the requests which are already sent to the
> > servers.  So, as you say, we pretend to cancel the requests without
> > waiting for completion of them.  However, are there any situation
> > where pretending to cancel causes problems in practice?
> 
> I'm not sure how often it would happen in practice, but if the guest OS
> thinks the old value is on disk when in fact the new one is, this could
> lead to corruption. I think if it can happen, even without evidence that
> it actually does, it's already relevant enough.
> 

I agree.

> > To wait for completion of the requests here, we may need to create
> > another thread for processing I/O like posix-aio-compat.c.
> 
> I don't think you need a thread to get the same behaviour, you just need
> to call the fd handlers like in the main loop. It would probably be the
> first driver doing this, though, and it's not an often used code path,
> so it might be a bad idea.
> 
> Maybe it's reasonable to just complete the request with -EIO? This way
> the guest couldn't make any assumption about the data written. On the
> other hand, it could be unhappy about failed requests, but that's
> probably better than corruption.
> 

Completing with -EIO looks good to me.  Thanks for the advice.
I'll send an updated patch tomorrow.

Regards,

Kazutaka