What does "fastmod" means in mongodb logs - mongodb

Question

I have thought fastmod specifies some operations like update-in-place.
In my app I'm doing update by _id using '$' modifiers, for example:
$colleciton->update(
array('_id' => $id),
array(
'$inc' => array('hits' => new MongoInt32(1)),
'$set' => array(
'times.gen' => gettimeofday(true),
'http.code' => new MongoInt32(200)
)
),
array('safe'=>false,'multiple'=>false,'upsert'=>false)
);
I've got such logs:
Wed Jul 25 11:08:36 [conn7002912] update mob.stat_pages query: { _id: BinData } update: { $inc: { hits: 1 }, $set: { times.gen: 1343203715.684896, http.code: 200 } } nscanned:1 nupdated:1 keyUpdates:0 locks(micros) w:342973 342ms
In logs as you can see I don't have any "fastmod" flags. There is no "moved" flag, because I set fields 'times.gen' and 'http.code' on insert, so padding factor is 1.0.
Am I doing something wrong, or I misunderstood meaning of fastmod?

Answer 1

You are correct that "fastmod" in the logs means an in-place update. Some possible reasons for the omission of logged fastmod/in-place operations:
You are actually setting or incrementing a field that doesn't exist, so it must be added, not an in place operation
The logs only show slow queries (default >100ms), so the in-place ones are probably happening too fast to be logged
You seem to be using 2.1 or 2.2 judging by the log - did the messages disappear if/when you switched to the new version?
In terms of looking into this further:
Have a look at the profiler, try with different settings, note: profiling adds load - so use carefully.
You can also try setting the slowms value lower, either on start up or:
> db.setProfilingLevel(0,20) // slow threshold=20ms