I think the row whose row key falls into the token range of the high latency node is likely to have more columns than the other nodes. I have three nodes with RF = 3, so all the nodes have all the data. And CL = Quorum, meaning each request is sent to all three nodes and response is sent back to client when two of them respond. What exactly does "Read Count" from "nodetool cfstats" mean then, should it be the same across all the nodes? I checked with Hector, it uses Round Robin LB strategy. And I also tested writes, and the writes are distributed across the cluster evenly. Below is the output from nodetool. Any one has a clue what might happened?
Node1: Read Count: 318679 Read Latency: 72.47641436367003 ms. Write Count: 158680 Write Latency: 0.07918750315099571 ms. Node 2: Read Count: 251079 Read Latency: 86.91948475579399 ms. Write Count: 158450 Write Latency: 0.1744694540864626 ms. Node 3: Read Count: 149876 Read Latency: 168.14125553123915 ms. Write Count: 157896 Write Latency: 0.06468631250949992 ms. nodetool ring Address DC Rack Status State Load Effective-Ownership Token 113427455640312821154458202477256070485 10.1.3.152 datacenter1 rack1 Up Normal 35.85 GB 100.00% 0 10.1.3.153 datacenter1 rack1 Up Normal 35.86 GB 100.00% 56713727820156410577229101238628035242 10.1.3.155 datacenter1 rack1 Up Normal 35.85 GB 100.00% 113427455640312821154458202477256070485 Keyspace: benchmark: Replication Strategy: org.apache.cassandra.locator.SimpleStrategy Durable Writes: true Options: [replication_factor:3] I am really confused by the Read Count number from nodetool cfstats Really appreciate any hints. -Wei ________________________________ From: Wei Zhu <wz1...@yahoo.com> To: Cassandr usergroup <user@cassandra.apache.org> Sent: Thursday, November 8, 2012 9:37 PM Subject: read request distribution Hi All, I am doing a benchmark on a Cassandra. I have a three node cluster with RF=3. I generated 6M rows with sequence number from 1 to 6m, so the rows should be evenly distributed among the three nodes disregarding the replicates. I am doing a benchmark with read only requests, I generate read request for randomly generated keys from 1 to 6M. Oddly, nodetool cfstats, reports that one node has only half the requests as the other one and the third node sits in the middle. So the ratio is like 2:3:4. The node with the most read requests actually has the smallest latency and the one with the least read requests reports the largest latency. The difference is pretty big, the fastest is almost double the slowest. All three nodes have the exactly the same hardware and the data size on each node are the same since the RF is three and all of them have the complete data. I am using Hector as client and the random read request are in millions. I can't think of a reasonable explanation. Can someone please shed some lights? Thanks. -Wei
