DataFlash: added a perf counter for microSD overruns

this allows us to tell if a microSD can handle a high logging rate

libraries/DataFlash/DataFlash_File.cpp

@@ -68,7 +68,8 @@ DataFlash_File::DataFlash_File(const char *log_directory) :
 #if CONFIG_HAL_BOARD == HAL_BOARD_PX4 || CONFIG_HAL_BOARD == HAL_BOARD_VRBRAIN
     ,_perf_write(perf_alloc(PC_ELAPSED, "DF_write")),
     _perf_fsync(perf_alloc(PC_ELAPSED, "DF_fsync")),
-    _perf_errors(perf_alloc(PC_COUNT, "DF_errors"))
+    _perf_errors(perf_alloc(PC_COUNT, "DF_errors")),
+    _perf_overruns(perf_alloc(PC_COUNT, "DF_overruns"))
 #endif
 {}
 
@@ -194,6 +195,7 @@ void DataFlash_File::WriteBlock(const void *pBuffer, uint16_t size)
     uint16_t space = BUF_SPACE(_writebuf);
     if (space < size) {
         // discard the whole write, to keep the log consistent
+        perf_count(_perf_overruns);
         return;
     }
 
@@ -648,10 +650,10 @@ void DataFlash_File::_io_timer(void)
           chunk, ensuring the directory entry is updated after each
           write.
          */
+        BUF_ADVANCEHEAD(_writebuf, nwritten);
 #if CONFIG_HAL_BOARD != HAL_BOARD_AVR_SITL && CONFIG_HAL_BOARD_SUBTYPE != HAL_BOARD_SUBTYPE_LINUX_NONE
         ::fsync(_write_fd);
 #endif
-        BUF_ADVANCEHEAD(_writebuf, nwritten);
     }
     perf_end(_perf_write);
 }

libraries/DataFlash/DataFlash_File.h

@@ -89,6 +89,7 @@ private:
     perf_counter_t  _perf_write;
     perf_counter_t  _perf_fsync;
     perf_counter_t  _perf_errors;
+    perf_counter_t  _perf_overruns;
 #endif
 };