James Brown created CASSANDRA-17473:
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             Summary: sstables changing in snapshots
                 Key: CASSANDRA-17473
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-17473
             Project: Cassandra
          Issue Type: Bug
            Reporter: James Brown


We use cassandra snapshots and {tar} to make full backups of our cassandra 
clusters. Sometimes, tar fails with a message like

{{tar: 
data/addresses/addresses-eb0196100b7d11ec852b1541747d640a/snapshots/backup20220318183708/nb-167-big-Data.db:
 file changed as we read it}}

This is kind of strange, since we're reading from a snapshot.

The (very simplified) relevant snippet looks roughly like

{{nice nodetool "${JMX_ARGS[@]}" snapshot -t "$TAG" "${KEYSPACES[@]}"
tar --hard-dereference -czpf data/*/*/snapshots/"$TAG"/}}

This happens maybe 1% of the time when taking backups.

There are no concurrent snapshots going on, but there are concurrent 
compactions and repairs, of course. If it matters, this cluster _is_ running 
incremental repairs.

This is on Cassandra 4.0.3.

It seems wrong to me that an sstable could ever be written to while it's in a 
snapshot.



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