[tmountain]

On the topic of the global write lock, you can run mongostat and see what the write lock percentage is at any given time. How much throughput you can squeeze out of a server is partially dependent on your sync delay setting.

If your disk is saturated during a sync due to a lot of writes, you might see the write lock at 100% for several seconds and everything will momentarily grind to a hault. If you set sync delay to zero and just rely on the OS to sync, you'll never see this happen.

We're using MongoDB in production and have a few boxes setup. One runs without sync delay and is used to process feed data. If Mongo crashed, and the data was irrevocably lost, it wouldn't really be the end of the world as the data only hangs around for about ten days anyway.

The second box runs with a sync delay and is used to store session data. In either scenario, Mongo is designed to be run with a slave at all times since it's not single server durable.

Regarding performance, the feeds box without sync delay can consistently write 25,000 records per second without ever going above 75% write lock. I find that pretty amazing. Doing preliminary benchmarks and performing batch writes that completely lock the server, I was getting about 250,000 writes / second.