[robotadam]

MongoDB has a single, global read/write lock for the entire database. So when a write is actually processing, nothing else can happen. In a moderately write-heavy environment, a single slow write can result in pretty bad performance in my experience. Reads can happen at the same time, although there have been problems with not yielding the lock on scans, although there are changes in 1.6 to address that.

[ethangunderson]

This used to be the case in earlier versions, but now it only issues a server level write lock for atomic updates, reads can still happen.

It's not ideal, but it is an improvement.