On Monday May 28, dgc@xxxxxxx wrote:
> On Mon, May 28, 2007 at 12:57:53PM +1000, Neil Brown wrote:
> > What exactly do you want to know, and why do you care?
>
> If someone explicitly mounts "-o barrier" and the underlying device
> cannot do it, then we want to issue a warning or reject the
> mount.
I guess that makes sense.
But apparently you cannot tell what a device supports until you write
to it.
So maybe you need to write some metadata with as a barrier, then ask
the device what it's barrier status is. The options might be:
YES - barriers are fully handled
NO - best effort, but due to missing device features, it might not
work
DISABLED - admin has requested that barriers be ignored.
??
>
> > The idea is that every "struct block_device" supports barriers. If the
> > underlying hardware doesn't support them directly, then they get
> > simulated by draining the queue and issuing a flush.
>
> Ok. But you also seem to be implying that there will be devices that
> cannot support barriers.
It seems there will always be hardware that doesn't meet specs. If a
device doesn't support SYNCHRONIZE_CACHE or FUA, then implementing
barriers all the way to the media would be hard..
>
> Even if all devices do eventually support barriers, it may take some
> time before we reach that goal. Why not start by making it easy to
> determine what the capabilities of each device are. This can then be
> removed once we reach the holy grail....
I'd rather not add something that we plan to remove. We currently
have -EOPNOTSUP. I don't think there is much point having more than
that.
I would really like to get to the stage where -EOPNOTSUP is never
returned. If a filesystem cares, it could 'ask' as suggested above.
What would be a good interface for asking?
What if the truth changes (as can happen with md or dm)?
NeilBrown
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