Avoid unneccessary zio allocation and wait

In function dmu_buf_hold_array_by_dnode, the usage of zio is only for 
the reading operation. Only create the zio and wait it in the reading 
scenario as a performance optimization.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Finix Yan <[email protected]>
Closes #11251 
Closes #11256

Author: Finix1979

module/zfs/dmu.c

@@ -499,7 +499,7 @@ dmu_buf_hold_array_by_dnode(dnode_t *dn, uint64_t offset, uint64_t length,
 	uint64_t blkid, nblks, i;
 	uint32_t dbuf_flags;
 	int err;
-	zio_t *zio;
+	zio_t *zio = NULL;
 
 	ASSERT(length <= DMU_MAX_ACCESS);
 
@@ -531,14 +531,17 @@ dmu_buf_hold_array_by_dnode(dnode_t *dn, uint64_t offset, uint64_t length,
 	}
 	dbp = kmem_zalloc(sizeof (dmu_buf_t *) * nblks, KM_SLEEP);
 
-	zio = zio_root(dn->dn_objset->os_spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+	if (read)
+		zio = zio_root(dn->dn_objset->os_spa, NULL, NULL,
+		    ZIO_FLAG_CANFAIL);
 	blkid = dbuf_whichblock(dn, 0, offset);
 	for (i = 0; i < nblks; i++) {
 		dmu_buf_impl_t *db = dbuf_hold(dn, blkid + i, tag);
 		if (db == NULL) {
 			rw_exit(&dn->dn_struct_rwlock);
 			dmu_buf_rele_array(dbp, nblks, tag);
-			zio_nowait(zio);
+			if (read)
+				zio_nowait(zio);
 			return (SET_ERROR(EIO));
 		}
 
@@ -555,15 +558,15 @@ dmu_buf_hold_array_by_dnode(dnode_t *dn, uint64_t offset, uint64_t length,
 	}
 	rw_exit(&dn->dn_struct_rwlock);
 
-	/* wait for async i/o */
-	err = zio_wait(zio);
-	if (err) {
-		dmu_buf_rele_array(dbp, nblks, tag);
-		return (err);
-	}
-
-	/* wait for other io to complete */
 	if (read) {
+		/* wait for async read i/o */
+		err = zio_wait(zio);
+		if (err) {
+			dmu_buf_rele_array(dbp, nblks, tag);
+			return (err);
+		}
+
+		/* wait for other io to complete */
 		for (i = 0; i < nblks; i++) {
 			dmu_buf_impl_t *db = (dmu_buf_impl_t *)dbp[i];
 			mutex_enter(&db->db_mtx);