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Re: [dm-devel] [RFC] pathchecker

On Mon, Mar 01, 2004 at 12:03:34PM -0800, Joel Becker wrote:
> 	The "wait for DM event" part.  Do we have an event yet?
> 2.6.4-udm1 doesn't seem to send any events to userspace on fail_path().
> Are we thinking an upcall, or perhaps polling the status?

The event handling should be working; fail_path() uses a work queue to
schedule trigger_event() being called (we can't call it directly from
interrupt context).

dm has a very simple model for events:

- Userland issues the wait for event ioctl, which blocks until an event occurs
- A target (eg. mpath) triggers an event
- Userland returns from the wait for event ioctl.  At this point it
  should query the status of the device to work out what happened.

An event number is passed into 'wait for event' to indicate the last
known event.  This way we can avoid missing events while previous
events are being processed.  Only recent versions of dmsetup support
this event number handling.

- Joe

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