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TokuMX. They ripped out the native MongoDB storage engine and replaced it with the TokuDB engine.

http://www.xaprb.com/blog/2013/04/29/what-tokudb-might-mean-...

TokuDB doesn't currently have a drop-in replacement strategy, so you'd need to migrate your whole cluster.

A daunting task with hundreds of shards!

Such a thing could never exist, because the point of tokumx is to change the storage system, so at some point you have to change the storage over and that's just going to be a rewrite of all your data. It sucks but that's the way it is.

Yes, but if I could replace the replicas and let them refill one at a time, then replace the masters, I could do a rolling conversion.

But replication from MongoDB to TokuDB does not work.

Actually, you can do that. I wrote a tool to allow replication from MongoDB to TokuMX: http://www.tokutek.com/2013/07/tokumx-1-0-3-seamless-migrati.... It doesn't allow TokuMX instances to satisfy write concern or to participate in elections, but you can slave a TokuMX replica set off a MongoDB replica set just fine, and this allows you to do a rolling migration where you replace one secondary at a time.

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