apache/cassandra · error · RuntimeException

Could not compact segments: " + toCompact

Error message

Could not compact segments: " + toCompact

What it means

The journal Compactor runs on its own thread; if compaction of a set of segments fails with an IOException, it wraps it in a RuntimeException after signalling completion. This is a fail-fast wrapper turning a checked I/O failure into an unchecked one for the scheduler thread.

Source

Thrown at src/java/org/apache/cassandra/journal/Compactor.java:105

            toCompact.subList(limit, toCompact.size()).clear();
        }

        try
        {
            Collection<StaticSegment<K, V>> newSegments = segmentCompactor.compact(toCompact);

            for (StaticSegment<K, V> segment : newSegments)
                toCompact.remove(segment);

            journal.replaceCompactedSegments(toCompact, newSegments);
            for (StaticSegment<K, V> segment : toCompact)
                segment.discard(journal);

            compacted.signalAll();
        }
        catch (IOException e)
        {
            throw new RuntimeException("Could not compact segments: " + toCompact);
        }
    }

    @Override
    public boolean isTerminated()
    {
        return executor.isTerminated();
    }

    @Override
    public void shutdown()
    {
        logger.debug("Shutting down " + executor);
        if (scheduled != null)
            scheduled.cancel(false);
        executor.shutdown();
    }

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Check disk space and filesystem health on the journal directory
2. Inspect the suppressed/cause stack (IOException) for the root cause; ensure logging captures it
3. Avoid deleting or manipulating segment files while compaction is active
4. Restart the journal/compactor thread after fixing the underlying I/O condition

Example fix

// before
throw new RuntimeException("Could not compact segments: " + toCompact);
// after
throw new RuntimeException("Could not compact segments: " + toCompact, e); // preserve cause (recommended upstream fix)

// pre-check: disk space and writability of journal dir
Files.getFileStore(journalDir).getUsableSpace(); journalDir.canWrite();

try { compact(...); } catch (RuntimeException e) { if (e.getCause() instanceof IOException) handleIoFailure(e.getCause()); else throw e; }

When it happens

Trigger: IOException while reading segments, rewriting filtered entries, or writing compacted output during Compactor.run(); disk full, permission errors, or segment files disappearing mid-compaction.

Common situations: Disk exhaustion on the commitlog/journal volume; concurrent recovery/cleanup deleting segments; filesystem errors (I/O errors, read-only mounts).

Understand the failure class

Background: "failed to write file", "Could not save figure", "Error saving remote file" — file write failed: causes and fixes across languages and libraries — this error's family across 38 libraries.

Related errors

• JournalReadError(descriptor, file, e)
• Unsupported disk access mode for compaction_read_disk_access
• concurrent_compactors should be strictly greater than 0, but
• Invalid value of compaction_throughput:
• compaction_throughput: is too large; it should be less than

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/54c59ffec1a64df2. Report an issue: GitHub.