question

Repair is the process of comparing data between replicas and synchronising them so the replicas have the latest copies. In order to do the comparison, Cassandra builds a Merkle tree of each replica and compares them to determine differences between the replicas.

The root of the tree is a hash (digest) of the replica's data. This then branches into 2 halves of the data where each half gets hashed. Each branch repeatedly gets split and hashed until there are 15 "levels" (tree depth). Note that the depth of the Merkle tree differs depending on the version of Cassandra in use.

With a depth of 15, the tree ends up with 32,768 "leaves" (2¹⁵). If a replica has 30 million partitions, the root is a hash of all the 30M partitions. In the next level where the are 2 branches, each branch is a hash of 15M partitions (30M / 2) and so on until it gets to the leaves.

Each leaf at the maximum depth of 15 is a hash of around 900 partitions (30M / 32K). This is the smallest unit of the Merkle tree so if there is a digest mismatch (hash values) of 2 leaves between 2 replicas, 900 partitions will get streamed by the repair (even if only 1 partition is out of sync).

To answer your question, it isn't a hash of just one partition. Cheers!