Re: [PATCH] Add a set_era_counter config key to the dm-cache era policy shim

[Joe Thornber <thornber@redhat.com>]

On Thu, Nov 07, 2013 at 09:24:44PM +0000, Mears, Morgan wrote:
> On Thurs, Nov 07, 2013 at 03:51PM +0000, Joe Thornber wrote:
> > On Wed, Nov 06, 2013 at 08:59:38PM +0000, Mears, Morgan wrote:
> > > Add the ability to set the era counter maintained by the dm-cache era
> > > policy shim to an arbitrary 32-bit value, to allow era rollback after
> > > the underlying device is restored from a snapshot.
> > 
> > I wonder if we should pass in the old value, and have the call fail if the old
> > value is incorrect.  This would allow applications to spot if they were
> > competing to set the era.  Some thing like:
> > 
> >      set_era_counter <old value>:<new value>
> 
> Yes, I like this.  My inclination is to make the <old value>: portion optional so that the counter value can be forced if desired (for example, to set it to a saved value during a create); objections?

In this case userspace should make a status request to get the current
era, then send the message in the <old value>:<new value> format.  If
it fails then clearly another process is interfering and you have big
issues.

> > In general alarm bells ring if you use one of smp_*() without the other.  See
> > linux/Documentation/memory-barriers.txt for lots of discussion.
> 
> Thanks Joe, will fix.

Already done in my cache-coherency-changes branch.  I've changed the
current_era counter to be an atomic64_t.

- Joe