sync - MongoDB upsert operation blocks inconsistently (with syncdelay set to 0) -

there database 9 million rows, 3 million distinct entities. such database loaded everyday mongodb using perl driver. runs smoothly on first load. second, third, etc. process slowed down. blocks long times every , then.

i realised because of automatic flushing disk every 60 seconds, tried setting syncdelay 0 , tried nojournalling option. have indexed fields used upsert. have observed blocking inconsistent , not @ same time same line.

i have 17 g ram , enough hard disk space. replicating on 2 servers 1 arbiter. not have significant processes running in background. there solution/explanation such blocking?

update: mongostat tool says in 'res' column around 3.6 g used.