fix(scale-cluster): Reduce load by 75%

Current load with the very large blob being used in this test is
too heavy for the i3.large nodes and disk can't handle the load.

Reducing the load by 75% reduced the throughput only by 10%,
Now compaction bandwidth is around 200MB/s compared to almost 8GB/s previosuly.

We can probably push more, but it's not the intention of the test.

Another small change is fixing the initial number of node, the base
scale test is called 15-to-25, so adjusted accordingly.

test-cases/scale/scale-cluster.yaml

@@ -6,15 +6,15 @@ prepare_write_cmd:  [ "cassandra-stress write cl=ALL n=500000 -schema 'replicati
                       "cassandra-stress write cl=ALL n=500000 -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=100 -pop seq=1500001..2000000  -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5"
                     ]
 
-stress_cmd: [ "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=100 -pop seq=1..500000          -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
-              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=100 -pop seq=500001..1000000   -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
-              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=100 -pop seq=1000001..1500000  -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
-              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=100 -pop seq=1500001..2000000  -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5"
+stress_cmd: [ "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=25 -pop seq=1..500000          -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
+              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=25 -pop seq=500001..1000000   -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
+              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=25 -pop seq=1000001..1500000  -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5",
+              "cassandra-stress mixed cl=QUORUM duration=180m -schema 'replication(strategy=NetworkTopologyStrategy,replication_factor=3)' -mode cql3 native -rate threads=25 -pop seq=1500001..2000000  -col 'n=FIXED(16) size=FIXED(8192)' -log interval=5"
              ]
 
 round_robin: true
 
-n_db_nodes: 20
+n_db_nodes: 15
 add_node_cnt: 1
 cluster_target_size: 25
 n_loaders: 4