Re: [Qemu-devel] [PATCH] Align file accesses with cache=off (O_DIRECT)

[Kevin Wolf <kwolf@suse.de>]

Laurent Vivier schrieb:
>> Disabling O_DIRECT for a single aio request is impossible (after all, 
>> aio is asynchronous), and disabling it for at least one aio request is 
> 
> Perhaps I'm wrong, but I think it is possible: the only consequence is
> the asynchronous I/O becomes synchronous...

Hm, yes. We could call raw_pread in raw_aio_read when O_DIRECT is used 
and the request is not properly aligned. Is this what you meant?

>> going to be ugly. So maybe we better turn O_DIRECT off for snapsnot 
>> saving/loading, even if it's not the generic fix I wanted to have when I 
>> started.
> 
> I don't think it is a good idea:
> 
> In linux world, there are three reasons to use O_DIRECT:
> 
> 1- to use linux AIO (not POSIX AIO).
> 
> 2- to avoid a buffer copy between user- and kernel- space
> (performance ?)
> 
> 3- to increase reliability: by using O_DIRECT you are sure your data are
> on the disk when the write is over and your system can now crash (if it
> wants).
> 
> And I think reliability is better when the snapshot is being saved...

I think we agree that it's mostly item 3 why one would use O_DIRECT with 
qemu. In terms of reliability, it is important that the data really is 
written to the disk when the guest OS thinks so. But when for example 
qemu crashes, I don't think it's too important if 40% or 50% of a 
snapshot have already been written - it's unusable anyway. A sync 
afterwards could be enough there.

>> I'm still undecided, though. What do you think?
> 
> Is it possible to align the last AIO ?

I have to admit that I neither know how to recognize the "last AIO" in 
the generic code nor do I understand what you want to achieve with it. 
The main problem are unaligned buffers and these occur on any request.

> And see comments below

Good suggestion, will change the patch accordingly.

Kevin