When many sstables(thousand) in level 0 ，compaction very slow

After I use sstableloader from a old cluster, the new cluster node's data size double. The new cluster has the same node with old cluster( 3 nodes). The old cluster has 500G each node,after sstableloader completely, the new cluster data size nearly 1T each node. The problem is that compaction

Does the rebuild tools rebuild all each time it start Or rebuild the rest?

we want deploy one more data-center for data safe. As we rebuild one node's data from the old DC, after some hours rebuild failure due to network fault. I can restart rebuild surely,but I'm afraid restart rebuild, is it rebuild all rang of tokens which belong to the node or just rebuild the rest

how many rows can one partion key hold?

Hi all, My team is using Cassandra as our database. We have one question as below. As we know, the row with the some partition key will be stored in the some node. But how many rows can one partition key hold? What is it depend on? The node's volume or partition data size or partition rows size(th