[dm-devel] Re: [PATCH 2/4] dm-snapshot-refactor-associations

[Mike Snitzer]

On Wed, Oct 07 2009 at  2:24pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Wed, Oct 07 2009 at  1:38pm -0400,
> Jonathan Brassow <jbrassow at redhat.com> wrote:
> 
> > If this is really something that we have to do, then fine.  However, it 
> > appears that you are doing this to pull out 'ti' and 'cow'... things that 
> > are separate by structure from the underlying exception store... which is 
> > ultimately what you want to hand over.  It seems to me that there is 
> > probably a better/cleaner way to do this, while simultaneously leaving 
> > 'ti' and 'cow' in their current place.
> 
> Yes, both 'ti' and 'cow' are seperate structures within the exception store.
> Yes, we want to handover the exception store itself (but keeping 'ti' and
> 'cow' fixed).
> 
> So you're saying have the handover be:
> 1) swap the 'complete' exception_table
> 2) swap the 'cow' and 'ti' in the 2 exception stores
> 3) swap the exception stores
> 
> Something like the following?:
> 
> static void handover_exceptions(struct dm_snapshot *old,
>                                 struct dm_snapshot *new)
> {
>         union {
>                 struct exception_table table_swap;
>                 struct dm_target *ti_swap;
>                 struct dm_dev *cow_swap;
>                 struct dm_exception_store *store_swap;
>         } u;
> 
>         u.table_swap = new->complete;
>         new->complete = old->complete;
>         old->complete = u.table_swap;
> 
>         u.ti_swap = new->store->ti;
>         new->store->ti = old->store->ti;
>         old->store->ti = u.ti_swap;
> 
>         u.cow_swap = new->store->cow;
>         new->store->cow = old->store->cow;
>         old->store->cow = u.cow_swap;
> 
>         u.store_swap = new->store;
>         new->store = old->store;
>         old->store = u.store_swap;
> 
>         old->active = 0;
>         new->handover = 0;
> }

Jon,

I discussed this with Mikulas and he agreed that the above would work.
But he raised the fact that reverting dm-snapshot-refactor-associations
leaves the dm_snapshot and  dm_exception_store structures muddled with
regard to where per-target members are located.  I'm inclined to agree
with him.

The dm-snapshot-refactor-associations patch offers cleanup so that:
1) dm_exception_store only contains exception store-specific members
2) dm_snapshot is the structure that contains per-target members

So while this dm-snapshot-refactor-associations patch could be viewed as
"avoidable churn" it actually takes the data structures in a more
controlled direction.

I would rather dm-snapshot-refactor-associations be applied.

Mike