SERVER-16908 disable MMAPv1 rollback during fg index build, for perf

1 files changed, 5 insertions, 0 deletions

diff --git a/src/mongo/db/index/btree_based_bulk_access_method.cpp b/src/mongo/db/index/btree_based_bulk_access_method.cpp
index 12fcd1bfbd2..fa1121ac5e9 100644
--- a/src/mongo/db/index/btree_based_bulk_access_method.cpp
+++ b/src/mongo/db/index/btree_based_bulk_access_method.cpp
@@ -150,6 +150,11 @@ namespace mongo {
             }
 
             WriteUnitOfWork wunit(_txn);
+            // Improve performance in the btree-building phase by disabling rollback tracking.
+            // This avoids copying all the written bytes to a buffer that is only used to roll back.
+            // Note that this is safe to do, as this entire index-build-in-progress will be cleaned
+            // up by the index system.
+            _txn->recoveryUnit()->setRollbackWritesDisabled();
 
             // Get the next datum and add it to the builder.
             BSONObjExternalSorter::Data d = i->next();