Fast Clone Deletion

Deleting a clone requires finding blocks are clone-only, not shared
with the snapshot. This was done by traversing the entire block tree
which results in a large performance penalty for sparsely
written clones.

This is new method keeps track of clone blocks when they are
modified in a "Livelist" so that, when it’s time to delete,
the clone-specific blocks are already at hand.

We see performance improvements because now deletion work is
proportional to the number of clone-modified blocks, not the size
of the original dataset.

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Sara Hartse <sara.hartse@delphix.com>
Closes #8416

include/sys/spa_impl.h

@@ -49,6 +49,7 @@
 #include <sys/dsl_crypt.h>
 #include <sys/zfeature.h>
 #include <sys/zthr.h>
+#include <sys/dsl_deadlist.h>
 #include <zfeature_common.h>
 
 #ifdef	__cplusplus
@@ -317,6 +318,11 @@ struct spa {
 	list_t		spa_log_summary;
 	uint64_t	spa_log_flushall_txg;
 
+	zthr_t		*spa_livelist_delete_zthr; /* deleting livelists */
+	zthr_t		*spa_livelist_condense_zthr; /* condensing livelists */
+	uint64_t	spa_livelists_to_delete; /* set of livelists to free */
+	livelist_condense_entry_t	spa_to_condense; /* next to condense */
+
 	char		*spa_root;		/* alternate root directory */
 	uint64_t	spa_ena;		/* spa-wide ereport ENA */
 	int		spa_last_open_failed;	/* error if last open failed */