diff --git a/include/sigar_private.h b/include/sigar_private.h
index d41d9eda..f02e4555 100644
--- a/include/sigar_private.h
+++ b/include/sigar_private.h
@@ -321,7 +321,9 @@ int sigar_group_name_get(sigar_t *sigar, int gid, char *buf, int buflen);
     SIGAR_DISK_STATS_INIT(&fsusage->disk)
 
 /* key used for filesystem (/) -> device (/dev/hda1) mapping */
-#define SIGAR_FSDEV_ID(sb) ((sb).st_ino + (sb).st_dev)
+/* and disk_usage cache for service_time */
+#define SIGAR_FSDEV_ID(sb) \
+    (S_ISBLK((sb).st_mode) ? (sb).st_rdev : ((sb).st_ino + (sb).st_dev))
 
 #if defined(WIN32) || defined(NETWARE)
 int sigar_get_iftype(const char *name, int *type, int *inst);