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

[Mike Snitzer <snitzer redhat com>]

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 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 redhat com>
Signed-off-by: Mike Snitzer <snitzer redhat com>