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I'm using dsbulk 1.6.0 to unload data from cassandra 3.11.3.

Each unload results in wildly different counts of rows. Here are results from 3 invocations of unload, on the same cluster, connecting to the same cassandra host. The table being unloaded is only ever appended, data is never deleted, so a decrease in unloaded rows should not occur. There are 3 cassandra databases in the cluster, and a replication factor of 3, so all data should be present on the chosen host. Furthermore, these were executed in quick succession, the number of added rows would be in the hundreds (if there were any) not in the tens of thousands.

Run 1:

│ total | failed | rows/s | p50ms | p99ms | p999ms
│ 10,937 | 7 | 97 | 15,935.46 | 20,937.97 | 20,937.97
│ Operation UNLOAD_20201024-084213-097267 completed with 7 errors in 1 minute and 51 seconds.

Run 2:

│ total | failed | rows/s | p50ms | p99ms | p999ms
│ 60,558 | 3 | 266 | 12,551.34 | 21,609.05 | 21,609.05
│ Operation UNLOAD_20201025-084208-749105 completed with 3 errors in 3 minutes and 47 seconds.

Run 3:

│ total | failed | rows/s | p50ms | p99ms | p999ms
│ 45,404 | 4 | 211 | 16,664.92 | 30,870.08 | 30,870.08
│ Operation UNLOAD_20201026-084206-791305 completed with 4 errors in 3 minutes and 35 seconds.

It would appear that Run 1 is missing the majority of the data. Run 2 may be closer to complete and Run 3 is missing significant data.

I'm invoking unload as follows:

dsbulk unload -h $CASSANDRA_IP -k $KEYSPACE -t $CASSANDRA_TABLE > $DATA_FILE

I'm assuming this isn't expected behaviour for dsbulk. How do I configure it to reliably unload a complete table without errors?

Tim
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1 Answers1

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Data could be missing from host if host wasn't reachable when the data was written, and hints weren't replayed, and you don't run repairs periodically. And because DSBulk reads by default with consistency level LOCAL_ONE, different hosts will provide different views (the host that you're providing is just a contact point - after that the cluster topology will be discovered, and DSBulk will select replica based on the load balancing policy).

You can enforce that DSBulk read the data with another consistency level by using -cl command line option (doc). You can compare results with using LOCAL_QUORUM or ALL - in these modes Cassandra will also "fix" the inconsistencies as they will be discovered, although this would be much slower & will add the load onto the nodes because of the repaired data writes.

Alex Ott
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