That is actually my original question. All three nodes have the complete data and all of them have the exactly the same hardware/software configuration and client uses RR to distribute the read request among the nodes, why one of them consistently report the much larger latency than the other two?
Thanks for the explanation of QUORUM, it clears a lot of confusion. -Wei ________________________________ From: Tyler Hobbs <ty...@datastax.com> To: user@cassandra.apache.org; Wei Zhu <wz1...@yahoo.com> Sent: Monday, November 12, 2012 12:43 PM Subject: Re: read request distribution Whichever node gets the initial Thrift request from the client is always the coordinator; there's no concept of making another node the coordinator. As far as QUORUM goes, only two nodes need to give a response to meet the consistency level, so Cassandra only sends out two read requests: one data request, and one digest request (unless the request is selected for read repair, in which case it sends a digest request to two other nodes instead of one). If the coordinator happens to be a replica for the requested data, it will usually pick itself for the data request and send the digest query elsewhere. Since all of your nodes hold all of the data, I'm not sure what you're referring to when you say that "it's in charge of the wider columns". Because of dynamic snitch behavior, you should expect to see the node with the highest latencies get the fewest queries, even if the high latency is partially *caused* by it not getting many queries (i.e. cold cache). On Mon, Nov 12, 2012 at 2:29 PM, Wei Zhu <wz1...@yahoo.com> wrote: Thanks Tyler for the information. From the online document: > > >QUORUM Returns the record with the most recent timestamp after a quorum of replicas has responded. > >It's hard to know that digest query will be sent to *one* other replica. When >the node gets the request, does it become the coordinator since all the nodes >have all the data in this setting? Or it will send the query to the "primary" >node (the node who is in charge of token) and let that node be the >coordinator? I would guess the latter is the case, otherwise it can't explain >why the third node is always slower than the other two given the fact it's in >charge of the "wider" columns than the other two. > > >Thanks.-Wei > > > >________________________________ > From: Tyler Hobbs <ty...@datastax.com> >To: user@cassandra.apache.org; Wei Zhu <wz1...@yahoo.com> >Sent: Saturday, November 10, 2012 3:15 PM >Subject: Re: read request distribution > > > >When you read at quorum, a normal read query will be sent to one replica >(possibly the same node that's coordinating) and a digest query will be sent >to *one* other replica, not both. Which replicas get picked for these is >determined by the dynamic snitch, which will favor replicas that are >responding with the lowest latency. That's why you'll see more queries going >to replicas with lower latencies. > >The Read Count number in nodetool cfstats is for local reads, not coordination >of a read request. > > > > >On Fri, Nov 9, 2012 at 8:16 PM, Wei Zhu <wz1...@yahoo.com> wrote: > >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 >> >> >> > > >-- >Tyler Hobbs >DataStax > > > > -- Tyler Hobbs DataStax
