[dm-devel] Re: dm snapshot: rework writing to snapshot origin

[Mikulas Patocka]

I think this should be applied, otherwise snapshots turn into 
unmaintainable mess.

Mikulas

> Alasdair,
> 
> I am not aware of any "cons" associated with this patch. 
> 
> Mikulas are you?  Alasdair would like both "pros" and "cons" documented
> in the patch header.
> 
> Here is a revised patch header that now documents why this patch is a
> prerequisite for snapshot-merge (specifically this patch:
> http://patchwork.kernel.org/patch/60708/):
> 
> 
> From: Mikulas Patocka <mpatocka at redhat.com>
> 
> The previous code selected one exception as "primary_pe", linked all
> other exceptions on it and used reference counting to wait until all
> exceptions are reallocated.
> 
> All the complexity with exceptions linking and reference counting has
> been removed.  Now, a bio is linked to one exception and when that
> exception is reallocated, the bio is retried to possibly wait for other
> exceptions.
> 
> The new __origin_write() interface affords the snapshot-merge support,
> snapshot_merge_process() specifically, the ability to trigger exceptions
> in other snapshots without having a need for an associated bio (which
> snapshot-merge does not generate).  As such this patch is a prerequisite
> for snapshot-merge.
> 
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
>