That is basically our set up. We'll be holding all data on all nodes.

My problem was more on how the cache would behave. I thought it might go this way:

1. No cache hit

Read from 3 nodes to verify results are correct and then return. Write result into RowCache.

2. Cache hit

Read from Cache directly and return.

If now the value gets updated it would be found in the RowCache and either invalidated (hence case 1 on next read) or updated (hence case 2 on next read). However I couldn't find any information on this.

If this was the case it would mean that each node would only have to hold 1/5 of my data in Cache (you're right about the DC clone so 1/5 of data instead of 1/10). If however 3 nodes have to be read each time and all 3 fill up the row cache with the same data that would make my cache requirements bigger.



On 10/10/13 14:06, Ken Hancock wrote:
If you're hitting 3/5 nodes, it sounds like you've set your replication factor to 5. Is that what you're doing so you can have a 2-node outtage?

For a 5-node cluster, RF=5, each node will have 100% of your data (a second DC is just a clone), so with a 3GB off-heap it means that 3GB / <total data size in GB> total would be cacheable in the row cache.

On the other hand, if you're doing RF=3, each node will have 60% of your data instead of 100% so the effective percentage of rows that are cache goes up by 66%.

Great quick & dirty caclulator: http://www.ecyrd.com/cassandracalculator/

On Thu, Oct 10, 2013 at 6:40 AM, Artur Kronenberg <artur.kronenberg@openmarket.com> wrote:

I was reading through configuration tips for cassandra and decided to use row-cache in order to optimize the read performance on my cluster.

I have a cluster of 10 nodes, each of them opeartion with 3 GB off-heap using cassandra 2.4.1. I am doing local quorum reads, which means that I will hit 3 nodes out of 5 because I split my 10 nodes into two data-centres.

I was under the impression that since each node gets a certain range of reads my total amount of off-heap would be 10 * 3 GB = 30 GB. However is this still correct with quorum reads? How does cassandra handle row-cache hits in combination with quorum reads?

-- artur

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