Re: [Qemu-devel] [RFC] Replace posix-aio with custom thread pool

[Anthony Liguori <anthony@codemonkey.ws>]

Andrea Arcangeli wrote:
> On Wed, Dec 10, 2008 at 08:08:10PM +0100, Andrea Arcangeli wrote:
>   
>> On Wed, Dec 10, 2008 at 12:50:17PM -0600, Anthony Liguori wrote:
>>     
>>> But opening twice means that you lose coherency with NFS.
>>>       
>> Not sure why. They're not running from different nfs clients. If this
>>     
>
> I just got confirmation from Trond that from nfs point of view,
> opening the file multiple times or duping it, is the same as I
> expected (all it matters is that the inode is the same so the
> pagecache radix tree is the same etc..). So opening the file each time
> a thread starts,

I've been thinking about this, the problems I see are:

1) It's impossible to accept a file descriptor for a block device 
(possibly not a problem)

2) You'd have to open all the file descriptors at once.  Otherwise, you 
get really strange behavior if the file gets deleted while the guest is 
running (for instance, with -snapshot).

Regards,

Anthony Liguori

>  would provide a local f_pos and it would work fine on
> older/current kernels too. In theory other nfs clients should also
> behave. Not sure what's best, if to hack around the bdrv api and open
> the file multiple times or wait for preadv/pwritev.
>