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

[Christoph Hellwig]

On Thu, Dec 11, 2008 at 04:53:35PM +0100, Andrea Arcangeli wrote:
> >   * It can't handle block allocation.  Kernel handles that by doing
> >     such writes synchronously via VFS layer (instead of the separate
> >     aio code paths).  Leads to horrible performance and bug reports
> >     such as "installs on sparse files are very slow".
> 
> I think here you mean O_DIRECT regardless of aio/sync API, I doubt aio
> has any relevance to block allocation in any way, so whatever problem
> we have with kernel API and O_DIRECT should also be there with
> sync-api + userland threads and O_DIRECT.

userland threads with O_DIRECT means only the thread doing block
allocation gets stalled, not any other.