diff --git a/src/server/pdc_server_region/pdc_server_region_cache.c b/src/server/pdc_server_region/pdc_server_region_cache.c
index 92c408fa6..d2f2abe56 100644
--- a/src/server/pdc_server_region/pdc_server_region_cache.c
+++ b/src/server/pdc_server_region/pdc_server_region_cache.c
@@ -393,8 +393,11 @@ PDC_region_cache_register(uint64_t obj_id, int obj_ndim, const uint64_t *obj_dim
     struct pdc_region_info *region_cache_info;
     if (obj_ndim != ndim && obj_ndim > 0) {
         printf("PDC_region_cache_register reports obj_ndim != ndim, %d != %d\n", obj_ndim, ndim);
+        return FAIL;
     }
 
+    pthread_mutex_lock(&pdc_obj_cache_list_mutex);
+
     obj_cache_iter = obj_cache_list;
     while (obj_cache_iter != NULL) {
         if (obj_cache_iter->obj_id == obj_id) {
@@ -460,7 +463,15 @@ PDC_region_cache_register(uint64_t obj_id, int obj_ndim, const uint64_t *obj_dim
     memcpy(region_cache_info->buf, buf, sizeof(char) * buf_size);
     total_cache_size += buf_size;
 
+    pthread_mutex_unlock(&pdc_obj_cache_list_mutex);
+
     if (total_cache_size > maximum_cache_size) {
+        int server_rank = 0;
+#ifdef ENABLE_MPI
+        MPI_Comm_rank(MPI_COMM_WORLD, &server_rank);
+#endif
+        printf("==PDC_SERVER[%d]: server cache full %.1f / %.1f MB, will flush to storage\n", server_rank,
+               total_cache_size / 1048576.0, maximum_cache_size / 1048576.0);
         PDC_region_cache_flush_all();
     }
 
@@ -571,11 +582,11 @@ PDC_transfer_request_data_write_out(uint64_t obj_id, int obj_ndim, const uint64_
             region_cache_iter = region_cache_iter->next;
         }
     }
+    pthread_mutex_unlock(&pdc_obj_cache_list_mutex);
     if (!flag) {
         PDC_region_cache_register(obj_id, obj_ndim, obj_dims, buf, write_size, region_info->offset,
                                   region_info->size, region_info->ndim, unit);
     }
-    pthread_mutex_unlock(&pdc_obj_cache_list_mutex);
 
     // PDC_Server_data_write_out2(obj_id, region_info, buf, unit);
 #ifdef PDC_TIMING
@@ -862,10 +873,9 @@ PDC_region_cache_clock_cycle(void *ptr)
                 gettimeofday(&finish_time, NULL);
                 elapsed_time = finish_time.tv_sec - current_time.tv_sec +
                                (finish_time.tv_usec - current_time.tv_usec) / 1000000.0;
-                fprintf(
-                    stderr,
-                    "==PDC_SERVER[%d]: flushed %d regions from cache to storage (every %.1fs), took %.4fs\n",
-                    server_rank, nflush, flush_frequency_s, elapsed_time);
+                fprintf(stderr,
+                        "==PDC_SERVER[%d]: flushed %d regions to storage (full/every %.0fs), took %.4fs\n",
+                        server_rank, nflush, flush_frequency_s, elapsed_time);
             }
             pthread_mutex_unlock(&pdc_obj_cache_list_mutex);
         }