dm: optimize flushes

Device mapper sends flush bios to all the targets and the targets send it
to the underlying device. That may be inefficient, for example if a table
contains 10 linear targets pointing to the same physical device, then
device mapper would send 10 flush bios to that device - despite the fact
that only one bio would be sufficient.

This commit optimizes the flush behavior. It introduces a per-target
variable flush_bypasses_map - it is set when the target supports flush
optimization - currently, the dm-linear and dm-stripe targets support it.
When all the targets in a table have flush_bypasses_map,
flush_bypasses_map on the table is set. __send_empty_flush tests if the
table has flush_bypasses_map - and if it has, no flush bios are sent to
the targets via the "map" method and the list dm_table->devices is
iterated and the flush bios are sent to each member of the list.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Suggested-by: Yang Yang <yang.yang@vivo.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/md/dm-core.h b/drivers/md/dm-core.h
index 14a44c0f8286..3637761f3585 100644
--- a/drivers/md/dm-core.h
+++ b/drivers/md/dm-core.h
@@ -206,6 +206,8 @@ struct dm_table {
 
 	bool integrity_supported:1;
 	bool singleton:1;
+	/* set if all the targets in the table have "flush_bypasses_map" set */
+	bool flush_bypasses_map:1;
 
 	/*
 	 * Indicates the rw permissions for the new logical device.  This