What causes "Failed to read key from %s" in apache/cassandra?

Check the node's system log and disk health (dmesg, smartctl); address any underlying I/O problem first.

apache/cassandra · critical · FSReadError

Failed to read key from

Error message

Failed to read key from %s

What it means

When iterating a SASI on-disk index, KeyFetcher.apply reads the partition key at a given offset from the sstable's secondary index. An IOException during that read is wrapped in an FSReadError carrying 'Failed to read key from <descriptor>', signaling on-disk corruption or an I/O failure.

Solutions

Check the node's system log and disk health (dmesg, smartctl); address any underlying I/O problem first.
Run nodetool scrub on the affected table to drop or repair corrupt SSTables.
Rebuild the SASI index: drop and recreate the index, or run nodetool upgradesstables to rewrite the SSTables.
Restore the affected SSTables from a backup/repair (nodetool repair) if scrub does not resolve it.

Defensive patterns

Strategy: fallback

Try / catch

try {
    return runSasiQuery();
} catch (FSReadError e) {
    alert("SASI on-disk index read failure: " + e.getMessage());
    return fallbackToNonIndexedQuery(); // or failover to a replica
}

Prevention

Monitor disk health and filesystem errors on Cassandra nodes.
Use clean shutdowns and ensure sufficient disk headroom.
Run nodetool scrub / repair periodically and after suspected corruption.

When it happens

Trigger: sstable.keyAtPositionFromSecondaryIndex(offset) throws IOException while fetching keys during a SASI query — truncated/corrupt index or data file, disk error, or file replaced mid-read.

Common situations: Disk corruption or bad sectors on the node; files truncated by an unclean shutdown or missing fsync; out-of-sync SSTable components after a failed compaction; filesystem full or hardware failure.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/bb2bf09c64ab3bc3. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/index/sasi/SSTableIndex.java:190

    private static class DecoratedKeyFetcher implements Function<Long, DecoratedKey>
    {
        private final SSTableReader sstable;

        DecoratedKeyFetcher(SSTableReader reader)
        {
            sstable = reader;
        }

        public DecoratedKey apply(Long offset)
        {
            try
            {
                return sstable.keyAtPositionFromSecondaryIndex(offset);
            }
            catch (IOException e)
            {
                throw new FSReadError(new IOException("Failed to read key from " + sstable.descriptor, e), sstable.getFilename());
            }
        }

        public int hashCode()
        {
            return sstable.descriptor.hashCode();
        }

        public boolean equals(Object other)
        {
            return other instanceof DecoratedKeyFetcher
                    && sstable.descriptor.equals(((DecoratedKeyFetcher) other).sstable.descriptor);
        }
    }
}

View on GitHub (pinned to 88fd0f6a0e)