On Wed, 2011-03-30 at 18:00 -0700, Joel Becker wrote: > On Wed, Mar 30, 2011 at 02:49:58PM -0700, Mingming Cao wrote: > > On Wed, 2011-03-30 at 13:17 +1100, Dave Chinner wrote: > > > For direct IO, the IO lock is always taken in shared mode, so we can > > > have concurrent read and write operations taking place at once > > > regardless of the offset into the file. > > > > > > > thanks for reminding me,in xfs concurrent direct IO write to the same > > offset is allowed. > > ocfs2 as well, with the same sort of strategem (including across > the cluster). > Thanks for providing view from OCFS2 side. This is good to know. > > > Direct IO semantics have always been that the application is allowed > > > to overlap IO to the same range if it wants to. The result is > > > undefined (just like issuing overlapping reads and writes to a disk > > > at the same time) so it's the application's responsibility to avoid > > > overlapping IO if it is a problem. > > > > > > > I was thinking along the line to provide finer granularity lock to allow > > concurrent direct IO to different offset/range, but to same offset, they > > have to be serialized. If it's undefined behavior, i.e. overlapping is > > allowed, then concurrent dio implementation is much easier. But not sure > > if any apps currently using DIO aware of the ordering has to be done at > > the application level. > > Oh dear God no. One of the major DIO use cases is to tell the > kernel, "I know I won't do that, so don't spend any effort protecting > me." > > Joel > Looks like so - So I think we could have a mode to turn on/off concurrent dio if the non heavy duty applications relies on filesystem to take care of the serialization. Mingming -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
