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

[Zach Brown <zab@zabbo.net>]

> *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.

As I understood it, lo those years ago, the use case was virtual guests
provisioned on loopback devices on files in the host file system.

In this use case either having loop implement a second writeback cache
or having it only capable of one sync IO in flight at a time is pretty
bad when you're trying to offer consistent and performant file systems
to the guest.

I believe one motivations for using loopback files is the ability to use
ocfs2's reflinks to get cluster-wide cow snapshoting of guest file
systems in files in ocfs2.

But I'll let Dave (or Chris?) share more about how this stuff is
actually used.

Really, if we had our way, file systems would offer the bio submission
and completion interface for file data.  Then loopback dev IO <-> file
system IO would be trivial.  The aio+dio iocbs are an existing
aproximation :/.

- z