ddt: add FDT feature and support for legacy and new on-disk formats

This is the supporting infrastructure for the upcoming dedup features.

Traditionally, dedup objects live directly in the MOS root. While their
details vary (checksum, type and class), they are all the same "kind" of
thing - a store of dedup entries.

The new features are more varied than that, and are better thought of as
a set of related stores for the overall state of a dedup table.

This adds a new feature flag, SPA_FEATURE_FAST_DEDUP. Enabling this will
cause new DDTs to be created as a ZAP in the MOS root, named
DDT-<checksum>. The is used as the root object for the normal type/class
store objects, but will also be a place for any storage required by new
features.

This commit adds two new fields to ddt_t, for version and flags. These
are intended to describe the structure and features of the overall dedup
table, and are stored as-is in the DDT root. In this commit, flags are
always zero, but the intent is that they can be used to hang optional
logic or state onto for new dedup features. Version is always 1.

For a "legacy" dedup table, where no DDT root directory exists, the
version will be 0.

ddt_configure() is expected to determine the version and flags features
currently in operation based on whether or not the fast_dedup feature is
enabled, and from what's available on disk. In this way, its possible to
support both old and new tables.

This also provides a migration path. A legacy setup can be upgraded to
FDT by creating the DDT root ZAP, moving the existing objects into it,
and setting version and flags appropriately. There's no support for that
here, but it would be straightforward to add later and allows the
possibility that newer features could be applied to existing dedup
tables.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15892

include/sys/ddt.h

@@ -39,6 +39,12 @@ extern "C" {
 
 struct abd;
 
+/*
+ * DDT-wide feature flags. These are set in ddt_flags by ddt_configure().
+ */
+/* No flags yet. */
+#define	DDT_FLAG_MASK	(0)
+
 /*
  * DDT on-disk storage object types. Each one corresponds to specific
  * implementation, see ddt_ops_t. The value itself is not stored on disk.
@@ -185,11 +191,15 @@ typedef struct {
 
 	avl_tree_t	ddt_tree;	/* "live" (changed) entries this txg */
 
-	avl_tree_t	ddt_repair_tree;	/* entries being repaired */
+	avl_tree_t	ddt_repair_tree; /* entries being repaired */
 
-	enum zio_checksum ddt_checksum;		/* checksum algorithm in use */
-	spa_t		*ddt_spa;		/* pool this ddt is on */
-	objset_t	*ddt_os;		/* ddt objset (always MOS) */
+	enum zio_checksum ddt_checksum;	/* checksum algorithm in use */
+	spa_t		*ddt_spa;	/* pool this ddt is on */
+	objset_t	*ddt_os;	/* ddt objset (always MOS) */
+
+	uint64_t	ddt_dir_object;	/* MOS dir holding ddt objects */
+	uint64_t	ddt_version;	/* DDT version */
+	uint64_t	ddt_flags;	/* FDT option flags */
 
 	/* per-type/per-class entry store objects */
 	uint64_t	ddt_object[DDT_TYPES][DDT_CLASSES];

include/sys/ddt_impl.h

@@ -33,6 +33,14 @@
 extern "C" {
 #endif
 
+/* DDT version numbers */
+#define	DDT_VERSION_LEGACY	(0)
+#define	DDT_VERSION_FDT		(1)
+
+/* Names of interesting objects in the DDT root dir */
+#define	DDT_DIR_VERSION		"version"
+#define	DDT_DIR_FLAGS		"flags"
+
 /*
  * Ops vector to access a specific DDT object type.
  */

include/sys/dmu.h

@@ -376,6 +376,7 @@ typedef struct dmu_buf {
 #define	DMU_POOL_TMP_USERREFS		"tmp_userrefs"
 #define	DMU_POOL_DDT			"DDT-%s-%s-%s"
 #define	DMU_POOL_DDT_STATS		"DDT-statistics"
+#define	DMU_POOL_DDT_DIR		"DDT-%s"
 #define	DMU_POOL_CREATION_VERSION	"creation_version"
 #define	DMU_POOL_SCAN			"scan"
 #define	DMU_POOL_ERRORSCRUB		"error_scrub"

include/zfeature_common.h

@@ -82,6 +82,7 @@ typedef enum spa_feature {
 	SPA_FEATURE_AVZ_V2,
 	SPA_FEATURE_REDACTION_LIST_SPILL,
 	SPA_FEATURE_RAIDZ_EXPANSION,
+	SPA_FEATURE_FAST_DEDUP,
 	SPA_FEATURES
 } spa_feature_t;