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Re: [dm-devel] [Lsf] Preliminary Agenda and Activities for LSF



On 03/30/2011 07:13 AM, Theodore Tso wrote:
On Mar 29, 2011, at 10:17 PM, Dave Chinner wrote:

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 :)

Ric


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