Implement dynamic gang header sizes

ZFS gang block headers are currently fixed at 512 bytes. This is
increasingly wasteful in the era of larger disk sector sizes. This PR
allows any size allocation to work as a gang header. It also contains
supporting changes to ZDB to make gang headers easier to work with.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Closes #17004

include/sys/vdev.h

@@ -148,7 +148,7 @@ extern uint64_t vdev_psize_to_asize(vdev_t *vd, uint64_t psize);
 static inline uint64_t
 vdev_gang_header_asize(vdev_t *vd)
 {
-	return (vdev_psize_to_asize_txg(vd, SPA_GANGBLOCKSIZE, 0));
+	return (vdev_psize_to_asize_txg(vd, SPA_OLD_GANGBLOCKSIZE, 0));
 }
 
 extern int vdev_fault(spa_t *spa, uint64_t guid, vdev_aux_t aux);

include/sys/zio.h

@@ -59,21 +59,36 @@ typedef struct zio_eck {
 
 /*
  * Gang block headers are self-checksumming and contain an array
- * of block pointers.
+ * of block pointers. The old gang block size has enough room for 3 blkptrs,
+ * while new gang blocks can store more.
+ *
+ * Layout:
+ * +--------+--------+--------+-----+---------+-----------+
+ * |        |        |        |     |         |           |
+ * | blkptr | blkptr | blkptr | ... | padding | zio_eck_t |
+ * |   1    |   2    |   3    |     |         |           |
+ * +--------+--------+--------+-----+---------+-----------+
+ *   128B     128B     128B             88B        40B
  */
-#define	SPA_GANGBLOCKSIZE	SPA_MINBLOCKSIZE
-#define	SPA_GBH_NBLKPTRS	((SPA_GANGBLOCKSIZE - \
-	sizeof (zio_eck_t)) / sizeof (blkptr_t))
-#define	SPA_GBH_FILLER		((SPA_GANGBLOCKSIZE - \
-	sizeof (zio_eck_t) - \
-	(SPA_GBH_NBLKPTRS * sizeof (blkptr_t))) /\
-	sizeof (uint64_t))
+#define	SPA_OLD_GANGBLOCKSIZE	SPA_MINBLOCKSIZE
+typedef void zio_gbh_phys_t;
 
-typedef struct zio_gbh {
-	blkptr_t		zg_blkptr[SPA_GBH_NBLKPTRS];
-	uint64_t		zg_filler[SPA_GBH_FILLER];
-	zio_eck_t		zg_tail;
-} zio_gbh_phys_t;
+static inline uint64_t
+gbh_nblkptrs(uint64_t size) {
+	ASSERT(IS_P2ALIGNED(size, sizeof (blkptr_t)));
+	return ((size - sizeof (zio_eck_t)) / sizeof (blkptr_t));
+}
+
+static inline zio_eck_t *
+gbh_eck(zio_gbh_phys_t *gbh, uint64_t size) {
+	ASSERT(IS_P2ALIGNED(size, sizeof (blkptr_t)));
+	return ((zio_eck_t *)((uintptr_t)gbh + size - sizeof (zio_eck_t)));
+}
+
+static inline blkptr_t *
+gbh_bp(zio_gbh_phys_t *gbh, int bp) {
+	return (&((blkptr_t *)gbh)[bp]);
+}
 
 enum zio_checksum {
 	ZIO_CHECKSUM_INHERIT = 0,
@@ -398,7 +413,9 @@ typedef struct zio_vsd_ops {
 
 typedef struct zio_gang_node {
 	zio_gbh_phys_t		*gn_gbh;
-	struct zio_gang_node	*gn_child[SPA_GBH_NBLKPTRS];
+	uint64_t		gn_gangblocksize;
+	uint64_t		gn_allocsize;
+	struct zio_gang_node	*gn_child[];
 } zio_gang_node_t;
 
 typedef zio_t *zio_gang_issue_func_t(zio_t *zio, blkptr_t *bp,

include/zfeature_common.h

@@ -87,6 +87,7 @@ typedef enum spa_feature {
 	SPA_FEATURE_FAST_DEDUP,
 	SPA_FEATURE_LONGNAME,
 	SPA_FEATURE_LARGE_MICROZAP,
+	SPA_FEATURE_DYNAMIC_GANG_HEADER,
 	SPA_FEATURES
 } spa_feature_t;