[dm-devel] Desynchronizing dm-raid1

[malahal at us.ibm.com]

Heinz Mauelshagen [mauelshagen at redhat.com] wrote:
> > > RH_MAYBE_DIRTY sounds superfluous at first glance, because when all writes
> > > to a region drained, we can set RH_CLEAN_CANDIDATE, run the sync() and check
> > > if that state persists in order to trigger the dirty log update.

If I understand your above description: a region's state is set to
RH_DIRTY when an I/O is scheduled in the region and is set to
RH_CLEAN_CANDIDATE when all I/O is completed. In other words, a region's
state is RH_CLEAN_CANDIDATE when there is no pending I/O to that region.
Did I get it right so far?

Then we invoke sync(). Now, if the region's state is RH_CLEAN_CANDIDATE,
you set the region's state to RH_CLEAN. If the region's state is
anything other than RH_CLEAN_CANDIDATE, you don't do anything. Am I
correct?


> > I don't think the state RH_MAYBE_DIRTY is superfluous.  If the region
> > state is RH_CLEAN_CANDIDATE after the sync(), that means no 'write'
> > happened since we set RH_CLEAN_CANDIDATE. If there was any write, the
> > region state would be 'RH_DIRTY' or 'RH_MAYBE_DIRTY'.
> 
> Hrm, sound like a contradiction in your statement.
> Either it stays RH_CLEAN_CANDIDATE because of no writes *or*
> it's state-changing to RH_DIRTY, no ?

The state would be RH_CLEAN_CANDIDATE if there were ***NO*** writes as
part of sync(). The next statement only describes what would happen if
there were any writes as part of sync().

--Malahal.
PS: Any comments from the original submitter if he thinks the state
is really superfluous?