18.12.2024 Problem:
The client reported a critical issue with the Cassandra cluster after adding a new data center and a rack containing three nodes. Despite bringing the new data center online, no data was being transferred from the source data center. Additionally, attempts to run a repair operation on the nodes were unsuccessful, which prevented the new nodes from synchronizing with the existing cluster data.
Process:
1. Initial Analysis:
The expert reviewed the initial setup and configurations of the new data center and rack nodes.
The client provided Cassandra status outputs and system log files for both the source and new data centers. The status revealed that, despite the nodes being marked as “Up” and “Normal,” the new data center was not receiving any data from the source.
2. Expert’s Preliminary Recommendation:
The expert identified a potential oversight in the repair process and suggested running a full nodetool rebuild operation on each node in the new data center.
The command syntax provided was nodetool rebuild -- name_of_existing_data_center, emphasizing that specifying the source data center was essential. Without this specification, the rebuild operation might appear successful without actually transferring data.
3. Client Action:
The client ran the nodetool rebuild -dc dc1 command on all three nodes in the new data center and confirmed the rebuild process was in progress.
To monitor data transfer, they executed the nodetool netstats command on one of the nodes, observing that the system was indeed receiving data files from the source data center.
4. Further Clarifications:
The client observed discrepancies in the nodetool status output, which did not reflect the data transfer progress. Additionally, they noted “Mismatch (Blocking): 12” in the output and requested an explanation.
The expert explained:
– “Mismatch (Blocking)” represents the number of blocking read repair operations performed since the server restarted.
– “Mismatch (Background)” represents the number of background read repair operations.
The expert also clarified that nodetool status is not designed to provide real-time data transfer metrics, which can result in temporary delays in displaying accurate data.
Solution:
The issue was successfully resolved by running the nodetool rebuild -dc dc1 command on each node in the new data center. This command initiated a controlled data transfer from the existing data center to the new one, bringing the added nodes up-to-date. The monitoring output from nodetool netstats indicated that the necessary files were being received, verifying successful data synchronization.
Conclusion:
By executing the correct rebuild command with appropriate parameters, the client achieved a stable and synchronized Cassandra cluster across the old and new data centers. The clarification provided on repair mismatches and nodetool status usage enabled the client to better monitor future synchronization tasks. This case highlights the importance of precise command execution in Cassandra cluster management, particularly during data center expansions.