[dm-devel] new path revalidation policy

[Joe Thornber]

On Sat, Feb 21, 2004 at 11:02:35PM +0100, christophe varoqui wrote:
> I've got a problem with the new path revalidation method :
> The storage controlers I use for testing offer zero valid paths during a 
>  controler switch. I just can't see how such a scenario will handled 
> correctly : how will the kernel mpath code wait for userspace to 
> revalidate a path before sending IO errors ?

how many paths are on each controller ?

> Anyway, given this part works, would you prefer to see the path 
> revalidation userspace spawn 1 process per multipath or 1 master daemon 
> plus threads ? I personnaly would vote for the former.

I would tend towards the latter, though at this stage do whatever is
easiest.

> Last question : what is the userspace action needed to revalidate a path 
> ? new libdevmapper primitive ?

No it's got nothing to do with libdevmapper.  I suggest your multipath
daemon take a config file (maybe borrow the config file code from
LVM2?), that specifies programs/scripts to be run to test a path,
frequency of testing if the path is believed to be ok, and frequency
of retesting for failback.  A trivial test script would just try and
read sector 0 of the device.

- Joe