Re: [Lsf] Preliminary Agenda and Activities for LSF

Excerpts from Ric Wheeler's message of 2011-03-30 07:28:34 -0400:

On 03/30/2011 07:13 AM, Theodore Tso wrote:

quoted

On Mar 29, 2011, at 10:17 PM, Dave Chinner wrote:

quoted

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.

Even if the overlapping read/writes are taking place in different processes?

DIO has never been standardized, and was originally implemented as gentleman's agreements between various database manufacturers and proprietary unix vendors.  The lack of formal specifications of what applications are guaranteed to receive is unfortunate....

-- Ted

What possible semantics could you have?

If you ever write concurrently from multiple processes without locking, you 
clearly are at the mercy of the scheduler and the underlying storage which could 
fragment a single write into multiple IO's sent to the backend device.

I would agree with Dave, let's not make it overly complicated or try to give 
people "atomic" unbounded size writes just because they set the O_DIRECT flag :)

We've talked about this with the oracle database people at least, any
concurrent O_DIRECT ios to the same area would be considered a db bug.
As long as it doesn't make the kernel crash or hang, we can return
one of these: http://www.youtube.com/watch?v=rX7wtNOkuHo

IBM might have a different answer, but I don't see how you can have good
results from mixing concurrent IOs.

-chris