$options['safe'] = true;
to make Mongo slow down and do inserts sequentially instead of in parallel.
I had 274 PDF documents stored on ext3 disk (avg. size was 5MB, none over 20MB)
A PHP loop loaded each PDF, then did a $GridFS->storeFile( $data, $meta );
This caused server to spike from load average of 0.2 to over 20.0
Apparently what was happening is the loop slammed Mongo (then memory, disk) with 274 convert/stores to do all at once.
$GridFS->storeFile( $data, $meta, array( 'safe' => true ) );
made the loop slow down and wait for each insert to finish and be confirmed before going on to next PDF.
Moral: Safe option can be used to maintain sanity and is not always just for data validity.