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rajib76 avatar image
rajib76 asked Erick Ramirez edited

Why are we not seeing key cache hits in our DSE cluster?

We are running our read workload on DSE cassandra but not seeing any key cache hit...

I was thinking that we will see some key cache hit, but it is zero

dse
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steve.lacerda avatar image steve.lacerda ♦ ·
Can you provide the output of nodetool info in your question?
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yukim avatar image
yukim answered

DSE 6.0 and above started to use the new index format for SSTable files:

  • Partition Index (Partition.db file)
  • Row Index (Row.db file)

https://docs.datastax.com/en/dse/6.8/dse-arch/datastax_enterprise/dbInternals/dbIntHowDataWritten.html#StoringdataondiskinSSTables

Using these index files, DSE can look up the records inside SSTable files, so DSE does not need to use key cache at all.

DSE 5.1 and open source Apache Cassandra use Index.db file which just records partition key and its position in the data file, and use key cache to look it up faster.

So if you are using DSE 5.1, then you should see key cache hit, but if you are using DSE 6.0 or above then key cache hit is always 0.

DSE 6.0 and above still show key cache metrics since those can still be used when you are upgrading from DSE 5.1 or open source Apache Cassandra and SStable files are not upgraded yet, but other than that, you should see 0s for ke y cache metrics.

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