Re: [Qemu-devel] [PATCH] RFC: Add new block driver for the VDI format

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> On 07/06/2009 04:37 PM, Anthony Liguori wrote:
>>
>> I'd really like to get rid of synchronous IO functions in the block 
>> layer.  One way to do this is to insist that all new block drivers 
>> only implement the AIO functions.
>>
>> I think we should make this decree but I'd like to know if other 
>> people think this is unreasonable first.  One potential model of 
>> block drivers would involve synchronous IO and threads.  I'm not a 
>> big fan of that model and I don't think it's an easy conversion from 
>> today's synchronous IO drivers to that model because the locking and 
>> re-entrance needs careful consideration.
>>
>
> I agree that sync+threads is not easy, but well performing async is 
> much, much harder.  Consider that qcow2 still has synchronous 
> operations, and that eliminating the RMW when writing a partial 
> cluster concurrently (a very common operation with 64K clusters) is 
> very hard to do ayncly and much easier syncly.

Supporting parallel RMW operations is certainly difficult, but you're 
confusing parallel RMW ops with asynchronous RMW ops.  You just have to 
queue requests and handle them in order.  It's only mildly more 
difficult to deal with asynchronous I/O and it avoids all the nastiness 
associated with threads and locking.

Fundamentally, threads don't help the RMW problem because you probably 
would just hold a look for the entire RMW operation so you're 
effectively queuing any RMW op.

> Given in addition the large numbers of format drivers, I think we 
> should prefer sync+threads over trying to convert all format drivers 
> to full async.

It's just shifting the problem from one place to another.  Instead of 
figuring out the state machine, you have to figure out how to do the 
locking.  The danger of the later is that it gives you the illusion that 
it's an easy problem and is therefore prone to error.

Regards,

Anthony Liguori