Re: [Qemu-devel] O_DIRECT on glusterfs (was Re: [PATCH v9 4/4] block: Support GlusterFS as a QEMU block backend)

[Anand Avati]

On 10/03/2012 11:17 AM, Paolo Bonzini wrote:
> Il 03/10/2012 19:58, Anand Avati ha scritto:
> > > I think these 3 lines should be removed.  We're bypassing the host
> > > buffer cache just by virtue of using a userspace driver, and that's what
> > > cache=none cares about.
> >
> > O_DIRECT also has an effect on the behavior of the "client side" (the
> > part within the qemu) of Gluster stack as well. I presume the intention
> > of O_DIRECT is to minimize use of memory (whether as host' page cache or
> > buffered data in user space). To that end it is a good idea to leave
> > O_DIRECT flag set.
> >
> > The behavior of whether gluster bricks need to get the O_DIRECT
> > propagated or not is a different issue. We are exploring the possibility
> > of not sending O_DIRECT flag over the wire to mimic NFS behavior. That
> > would be independent of the qemu block driver setting the open flag.
>
> What is the effect of O_DIRECT on the client exactly?

To avoid caching in the io-cache module, disable read-ahead etc (if 
those translators are loaded). The behavior in write-behind is tunable. 
You could either disable write-behind entirely (which will happen once 
libgfapi supports 0-copy/RDMA) or perform a sliding-window like 
size-limited write-behind (defaults to 1MB).

Avati