cassandra nodetool repair what does it really do?

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Hi all (cassandra noob here),

I'm trying understand exactly what going on..with repair..trying to get to the point where we can run our repairs on a schedule.

I have setup a 4 DC (3 nodes per dc) Cassandra 2.1 cluster, 4gb Ram (1gb heap), HDD.

Due to various issues..(repair taking too long/crashing nodes with OOM), I decided to start fresh, nuke everything, I deleted /var/lib/cassandra/data, and /opt/cassandra/data

Recreated my keyspaces (no data), and ran nodetool repair -par -inc -local

I was supprised to see it took ~5min to run, watching the logs I see merkle trees being generated...

I guess..my question is, if my keyspaces have no data..whats it generating Merkle trees against?

After running a local-dc repair against each node in that dc, I decided to run a cross-dc repair, again with no data..

this time it took 4+ hours..with no data? This really feels wrong to me, what am I missing?

Thanks

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