[Ocfs2-devel] ocfs2: serialize unaligned aio

[Mark Fasheh]

On Sun, Jun 26, 2011 at 12:22:48AM -0700, Joel Becker wrote:
> On Wed, Jun 22, 2011 at 02:23:38PM -0700, Mark Fasheh wrote:
> > Fix a corruption that can happen when we have (two or more) outstanding
> > aio's to an overlapping unaligned region.  Ext4
> > (e9e3bcecf44c04b9e6b505fd8e2eb9cea58fb94d) and xfs recently had to fix
> > similar issues.
> > 
> > In our case what happens is that we can have an outstanding aio on a region
> > and if a write comes in with some bytes overlapping the original aio we may
> > decide to read that region into a page before continuing (typically because
> > of buffered-io fallback).  Since we have no ordering guarantees with the
> > aio, we can read stale or bad data into the page and then write it back out.
> > 
> > If the i/o is page and block aligned, then we avoid this issue as there
> > won't be any need to read data from disk.
> > 
> > I took the same approach as Eric in the ext4 patch and introduced some
> > serialization of unaligned async direct i/o.  I don't expect this to have an
> > effect on the most common cases of AIO.  Unaligned aio will be slower
> > though, but that's far more acceptable than data corruption.
> 
> 	The patch looks good, but I'm a little confused.  Why doesn't
> this matter for buffered I/O?  Just because that data is going through
> the pagecache?  For a second, I couldn't see how unaligned dio was
> possible, until I remembered this was block aligned, not sector aligned.

Buffered I/O is synchronous so we don't really have any situations in which
there can be two buffered I/O's at the same time.


> 	Don't most of the major DIO users (read: databases) do
> sector-aligned I/O?  Won't this affect them?

In 2.6? Anyway, Sunil will have to answer that question... I would guess
though that since xfs and ext4 have the same patch and there don't seem to
be major reports from Oracle of DB performance tanking. That's hardly solid
evidence of course.
	--Mark

--
Mark Fasheh