8025 dbuf_read() creates unnecessary zio_root() for bonus buf

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>

dbuf_read() creates a zio_root() to track and wait for all the zio's
that may happen as part of this call. However, if the blkptr_t for
this buffer is NULL or a hole, we will not create any more zio's, so
this zio_root() is unnecessary. This is always the case when calling
dbuf_read() on a bonus buffer, because it has no blkptr (it's part of
the containing dnode). For workloads that read a lot of bonus buffers
(e.g. file creation and removal), creating and destroying these
unnecessary zio's can decrease performance by around 3%.

The fix is to only create/destroy the zio_root() in dbuf_read() if the
blkptr is not NULL and not a hole.

Closes #329

usr/src/uts/common/fs/zfs/dbuf.c

@@ -1088,7 +1088,6 @@ int
 dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 {
 	int err = 0;
-	boolean_t havepzio = (zio != NULL);
 	boolean_t prefetch;
 	dnode_t *dn;
 
@@ -1132,9 +1131,13 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 		DB_DNODE_EXIT(db);
 	} else if (db->db_state == DB_UNCACHED) {
 		spa_t *spa = dn->dn_objset->os_spa;
+		boolean_t need_wait = B_FALSE;
 
-		if (zio == NULL)
+		if (zio == NULL &&
+		    db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)) {
 			zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+			need_wait = B_TRUE;
+		}
 		dbuf_read_impl(db, zio, flags);
 
 		/* dbuf_read_impl has dropped db_mtx for us */
@@ -1146,7 +1149,7 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 			rw_exit(&dn->dn_struct_rwlock);
 		DB_DNODE_EXIT(db);
 
-		if (!havepzio)
+		if (need_wait)
 			err = zio_wait(zio);
 	} else {
 		/*
@@ -1181,7 +1184,6 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 		mutex_exit(&db->db_mtx);
 	}
 
-	ASSERT(err || havepzio || db->db_state == DB_CACHED);
 	return (err);
 }