MongoDB's runCommand is much slower than direct call

Question

I have this two mongo commands, that in my opinion should lead to the same results:

for(var i=0;i<1000;i++){db.test.insert({a:1}); db.getLastError({j:1, w:1});};

for(var i=0;i<1000;i++){db.test.insert({a:1}); db.runCommand({getLastError:1, j:1, w:1});};

both commands perform inserts correctly, however, the second one is cca 100x times slower (500ms vs 45s). Does anybody know, why this is so? The difference is present only when {j:1} is set, so it's probably somehow related to some journaling issues?

Answer 1

The second command is actually waiting for the journal commit whereas the first is not and hence the difference. When using the getLastError shell helper you cannot pass in the j option. It should be a number or string corresponding to the w parameter to the getlasterror database command as documented here.