Use a helper function to clarify gang block size

For gang blocks, `DVA_GET_ASIZE()` is the total space allocated for the
gang DVA including its children BP's.  The space allocated at each DVA's
vdev/offset is `vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE)`.

This commit makes this relationship more clear by using a helper
function, `vdev_gang_header_asize()`, for the space allocated at the
gang block's vdev/offset.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11744

include/sys/spa.h

@@ -405,6 +405,12 @@ typedef struct blkptr {
 /*
  * Macros to get and set fields in a bp or DVA.
  */
+
+/*
+ * Note, for gang blocks, DVA_GET_ASIZE() is the total space allocated for
+ * this gang DVA including its children BP's.  The space allocated at this
+ * DVA's vdev/offset is vdev_gang_header_asize(vdev).
+ */
 #define	DVA_GET_ASIZE(dva)	\
 	BF64_GET_SB((dva)->dva_word[0], 0, SPA_ASIZEBITS, SPA_MINBLOCKSHIFT, 0)
 #define	DVA_SET_ASIZE(dva, x)	\

include/sys/vdev.h

@@ -133,6 +133,15 @@ extern int64_t vdev_deflated_space(vdev_t *vd, int64_t space);
 
 extern uint64_t vdev_psize_to_asize(vdev_t *vd, uint64_t psize);
 
+/*
+ * Return the amount of space allocated for a gang block header.
+ */
+static inline uint64_t
+vdev_gang_header_asize(vdev_t *vd)
+{
+	return (vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE));
+}
+
 extern int vdev_fault(spa_t *spa, uint64_t guid, vdev_aux_t aux);
 extern int vdev_degrade(spa_t *spa, uint64_t guid, vdev_aux_t aux);
 extern int vdev_online(spa_t *spa, uint64_t guid, uint64_t flags,

module/zfs/metaslab.c

@@ -5544,7 +5544,7 @@ metaslab_unalloc_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
 	ASSERT3P(vd->vdev_indirect_mapping, ==, NULL);
 
 	if (DVA_GET_GANG(dva))
-		size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+		size = vdev_gang_header_asize(vd);
 
 	msp = vd->vdev_ms[offset >> vd->vdev_ms_shift];
 
@@ -5579,7 +5579,7 @@ metaslab_free_dva(spa_t *spa, const dva_t *dva, boolean_t checkpoint)
 	ASSERT3U(spa_config_held(spa, SCL_ALL, RW_READER), !=, 0);
 
 	if (DVA_GET_GANG(dva)) {
-		size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+		size = vdev_gang_header_asize(vd);
 	}
 
 	metaslab_free_impl(vd, offset, size, checkpoint);
@@ -5768,7 +5768,7 @@ metaslab_claim_dva(spa_t *spa, const dva_t *dva, uint64_t txg)
 	ASSERT(DVA_IS_VALID(dva));
 
 	if (DVA_GET_GANG(dva))
-		size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+		size = vdev_gang_header_asize(vd);
 
 	return (metaslab_claim_impl(vd, offset, size, txg));
 }
@@ -6034,7 +6034,7 @@ metaslab_check_free(spa_t *spa, const blkptr_t *bp)
 		uint64_t size = DVA_GET_ASIZE(&bp->blk_dva[i]);
 
 		if (DVA_GET_GANG(&bp->blk_dva[i]))
-			size = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+			size = vdev_gang_header_asize(vd);
 
 		ASSERT3P(vd, !=, NULL);
 

module/zfs/zio.c

@@ -995,7 +995,8 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp, boolean_t config_held,
 	 * that are in the log) to be arbitrarily large.
 	 */
 	for (int i = 0; i < BP_GET_NDVAS(bp); i++) {
-		uint64_t vdevid = DVA_GET_VDEV(&bp->blk_dva[i]);
+		const dva_t *dva = &bp->blk_dva[i];
+		uint64_t vdevid = DVA_GET_VDEV(dva);
 
 		if (vdevid >= spa->spa_root_vdev->vdev_children) {
 			errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
@@ -1024,10 +1025,10 @@ zfs_blkptr_verify(spa_t *spa, const blkptr_t *bp, boolean_t config_held,
 			 */
 			continue;
 		}
-		uint64_t offset = DVA_GET_OFFSET(&bp->blk_dva[i]);
+		uint64_t offset = DVA_GET_OFFSET(dva);
-		uint64_t asize = DVA_GET_ASIZE(&bp->blk_dva[i]);
+		uint64_t asize = DVA_GET_ASIZE(dva);
-		if (BP_IS_GANG(bp))
+		if (DVA_GET_GANG(dva))
-			asize = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+			asize = vdev_gang_header_asize(vd);
 		if (offset + asize > vd->vdev_asize) {
 			errors += zfs_blkptr_verify_log(spa, bp, blk_verify,
 			    "blkptr at %p DVA %u has invalid OFFSET %llu",
@@ -1064,8 +1065,8 @@ zfs_dva_valid(spa_t *spa, const dva_t *dva, const blkptr_t *bp)
 	uint64_t offset = DVA_GET_OFFSET(dva);
 	uint64_t asize = DVA_GET_ASIZE(dva);
 
-	if (BP_IS_GANG(bp))
+	if (DVA_GET_GANG(dva))
-		asize = vdev_psize_to_asize(vd, SPA_GANGBLOCKSIZE);
+		asize = vdev_gang_header_asize(vd);
 	if (offset + asize > vd->vdev_asize)
 		return (B_FALSE);
 
@@ -3984,6 +3985,9 @@ zio_vdev_io_assess(zio_t *zio)
 	 */
 	if (zio->io_error == ENXIO && zio->io_type == ZIO_TYPE_WRITE &&
 	    vd != NULL && !vd->vdev_ops->vdev_op_leaf) {
+		vdev_dbgmsg(vd, "zio_vdev_io_assess(zio=%px) setting "
+		    "cant_write=TRUE due to write failure with ENXIO",
+		    zio);
 		vd->vdev_cant_write = B_TRUE;
 	}