Re: Use case for "loop: Issue O_DIRECT aio with pages"?

[Chris Mason <chris.mason@oracle.com>]

On Thu, Mar 01, 2012 at 06:15:24PM -0500, Ted Ts'o wrote:
> > >My latest focus is on completing the work started on by Zach Brown to
> > >add a kernel interface to allow kernel code to submit aio, and having
> > >the loop device submit direct IO requests to the underlying file system.
> 
> So I've been taking a lot at your patch series, and one of the things
> which would help me understand the requirements behind what you are
> doing is *why* is it interesting to allow kernel code to submit aio
> requests, and loop devices to be able to direct I/O requests all the
> way to the underlying file systems.  In other words, what's the use
> case that you're most interested in that these patches would enable?

I'll let James talk about the parallels use case, which includes some
nice building blocks on top of just exporting a file as a block device.

Oracle's main use for this is virtualized guests.  We have files
in dom0 that we want to turn into block devices for guests, and doing it
from loop allows us to avoid passing the IO off to a userland process.
Yes, kvm does this from qemu today, but the xen guys tell me the perf
hit is pretty big on 64 bit xen handoffs for the ios.

Being able to use loop in a read/write workload where it is actually
crash safe is an important use case.  I always cringe when I hear people
using loop for writable filesystem images.

-chris