Re: barriers vs. reads - O_DIRECT

[Werner Almesberger <wa@almesberger.net>]

Jamie Lokier wrote:
> Note that what filesystems and databases want is write-write *partial
> dependencies*.  The per-device I/O barrier is just a crude
> approximation.

True ;-) So what would an ideally flexible model look like ?
Partial order ? Triggers plus virtual requests ? There's also
the little issue that this should still yield an interface
that people can understand without taking a semester of
graph theory ;-)

> 3. What if a journal is on a different device to its filesystem?

"Don't do this" comes to mind :-)

> Isn't the barrier itself an I/O operation which can be waited on?
> I agree something could depend on the reads at the moment.

Making barriers waitable might be very useful, yes. That could
also be a step towards implementing those cross-device barriers.

- Werner

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 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
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