HPCC-31062 Update comment

Signed-off-by: Shamser Ahmed <[email protected]>

common/workunit/workunit.cpp

@@ -9023,7 +9023,13 @@ void CLocalWorkUnit::setStatistic(StatisticCreatorType creatorType, const char *
 
     // Special case - update aggregates for dfu FileAccessCost.  This is needed because although
     // fileservices can update dfu cost in the workunit, it does not have a mechanism to update
-    // the aggregates for this stat.
+    // the aggregates for this stat (i.e. fileservices doesn't have access to a StatsAggregator.
+    // And say the StatsAggregator were created here, it would need to load all the global stats
+    // from dali,  mark the stat as 'dirty', and then have the aggregator refresh the aggregates.
+    // This would be slow and unnessarily cumbersome.) Note also that if the stat is written
+    // with the expectation that the hThor/Thor stats aggregator would generate the aggregate for
+    // this stat, it would not be the case as the aggregates are only created for stats marked
+    // 'dirty'.
     if (scopeType == SSTdfuworkunit && kind == StCostFileAccess && deltaValue)
     {
         StringBuffer currentScope(scope), parent;