SERVER-6718 new parameter to control replication index prefetching behavior

New parameter replIndexPrefetch.
Settable via the command line or use setParameter() command on a running server.
Only valid on a server started with --replSet parameter.
This allows a user to disable the prefetching of all index pages prior to the application
of a replicated op on a secondary.
For most use cases, prefetching all indexes for a given op will provide good performance.
For use cases that do many in-place updates to documents in a collection with many indexes
that are unaffected by such updates, using the '_id_only' setting may provide better performance
than the 'all' setting.  The '_id_only' setting only prefetches index pages for the _id index,
which will always be used to find the document needed to be updated.
The 'none' setting will be used for debugging only; there should be no production use cases for
this setting.
getParameter() supports getting the current value of this parameter.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/mongo/db/dbcommands_generic.cpp b/src/mongo/db/dbcommands_generic.cpp
index 84d57b8769e..ff128dbd1ff 100644
--- a/src/mongo/db/dbcommands_generic.cpp
+++ b/src/mongo/db/dbcommands_generic.cpp
@@ -124,6 +124,8 @@ namespace mongo {
         */
     unsigned replApplyBatchSize = 1;
 
+    const char* fetchReplIndexPrefetchParam();
+
     class CmdGet : public Command {
     public:
         CmdGet() : Command( "getParameter" ) { }
@@ -160,7 +162,9 @@ namespace mongo {
             if( all || cmdObj.hasElement("replApplyBatchSize") ) {
                 result.append("replApplyBatchSize", replApplyBatchSize);
             }
-
+            if (all || cmdObj.hasElement("replIndexPrefetch")) {
+                result.append("replIndexPrefetch", fetchReplIndexPrefetchParam());
+            }
             if ( before == result.len() ) {
                 errmsg = "no option found to get";
                 return false;