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[dm-devel] [RFC][mpath-tools] path info cache

I just merged a quite invasive patch that needs commenting.

The patch explore a new solution to /sbin/multipath inefficiency wrt
systems with lots of paths.

What did we have :
The previous attempt was the cache file plus hot/cold policy, which was
suboptimal in many ways :

- hard to get the security right
- early userspace annoyances wrt umounted FS
- needs to allocate a few bytes on FS
- the 5 seconds hot to cold policy is not adequate for everyone (see
recent Adic post)
- hard to settle on an a consensual FS location for the file

The proposed solution is : "/sbin/multipath asks the path cache to
multipathd through its unix socket" (the same used for the CLI).
When the ademon doesn't run, multipath has to do a full rescan.

What do we get :
- security is easy
- no more FS allocation to satisfy
- The daemon has its pathvec always up-to-date, thanks to its
event-driven model. No more hot/cold policy.

What should be debated :
- settle on a FS socket location (currently /var/cache/multipath/uxsock)
- people who care about early userspace have now 2 options :

	- No daemon in early userspace, and disable hotplug-triggered
	  multipath. /sbin/multipath is ran once.

	- Start the daemon in early userspace, let multipath be

Comments and critics are welcome.
christophe varoqui <christophe varoqui free fr>

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