Re: [RFC] fs io with struct page instead of iovecs

[David Chinner <dgc@sgi.com>]

On Wed, Nov 07, 2007 at 09:02:05AM -0800, Zach Brown wrote:
> Badari Pulavarty wrote:
> > On Tue, 2007-11-06 at 17:43 -0800, Zach Brown wrote:
> >> At the FS meeting at LCE there was some talk of doing O_DIRECT writes from the
> >> kernel with pages instead of with iovecs.  T
> > 
> > Why ? Whats the use case ?
> 
> Well, I think there's a few:
> 
> There are existing callers which hold a kmap() across ->write, which
> isn't great.  ecryptfs() does this.  That's mentioned in the patch
> series.  Arguably loopback should be using this instead of copying some
> fs paths and trying to call aop methods directly.
> 
> I seem to remember Christoph and David having stories of knfsd folks in
> SGI wanting to do O_DIRECT writes from knfsd?  (If not, *I* kind of want
> to, after rolling some patches to align net rx descriptors :)).

The main reason is to remove the serialised writer problem when multiple
clients are writing to the one file. With XFS and direct I/O, we can have
multiple concurrent writers to the one file and have it scale rather than be
limited to what a single cpu holding the i_mutex can do....

> Lustre shows us that there is a point at which you can't saturate your
> network and storage if your cpu is copying all the data.

Buy more CPUs ;)

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group