I've yet to use it for any load, and am struggling to triangulate from all the articles I read on whether it does/doesn't scale efficiently.
All the issues I have hit so far have been self-inflicted, it is still one of the best new technologies I've used in years - but is has taken a while to stop thinking in SQL equivalents and start thinking natively.
It doesn't. Three words: "global write lock". Writes block reads, reads block writes. Implications: if you run a query in production that doesn't hit an index, all traffic stops. The notablescan setting is a very, very good idea. This also means all queries must have an index, so Mongo ends up with more indexes than say, postgres would.
It's impossible to configure a clustered mongo environment to not lose data: http://aphyr.com/posts/284-call-me-maybe-mongodb
Sharding configuration is baroque, and limited.
https://blog.serverdensity.com/goodbye-global-lock-mongodb-2...
Now locking is on the database level.
Couldn't you just work around that before by running a separate Mongo process per database?
Saying it's now on the database still means any single-database app is globally locked. Or does using Mongo imply you're going to be making lots of databases so this actually means anything?
It's perverse.
Even if Mongo did have a global write lock, which it doesn't as has already been covered, it yields on page faults which means that other queries are minimally impacted. See: http://docs.mongodb.org/manual/faq/concurrency/#does-a-read-...
As to your linked doc, emphasis added:
> In some situations, read and write operations can yield their locks.
> Long running read and write operations, such as queries, updates, and deletes, yield under many conditions.
In practice, I've been bitten hard by this. A new feature rolls out, and users can't log in anymore, because a query is taking 2 minutes to run.
http://www.xaprb.com/blog/2013/04/29/what-tokudb-might-mean-...
A daunting task with hundreds of shards!
But replication from MongoDB to TokuDB does not work.
The second I can migrate to another data store, I will. Unfortunately that kind of refactor isn't possible right now, but all new projects are using different tools.