Cassandra Cleanup Operations

Cleanup removes data that a node no longer owns after topology changes. This operation must run on existing nodes after adding new nodes to the cluster.

When Cleanup is Required

Operation	Cleanup Required	Nodes to Clean
Add node (bootstrap)	Yes	All existing nodes
Decommission	No	None
Remove node	No	None
Replace node	No	None
Rebuild	No	None

Why Cleanup is Necessary

When a new node joins:

Existing nodes stream data to the new node
The new node now owns some token ranges
Original nodes retain copies of data they streamed
This data is now redundant—the new node is the owner

Without cleanup:

Disk usage remains elevated on original nodes
Extra data consumes backup storage
Compaction processes unnecessary data

Behavioral Contract

Guarantees

Only data outside the node's current token ranges is removed
Data owned by the node is never deleted
Operation is resumable if interrupted
Does not affect cluster availability

Operation Characteristics

Aspect	Behavior
Blocking	Yes, runs synchronously
I/O impact	High—reads and rewrites SSTables
Duration	Proportional to data volume
Restartability	Safe to restart if interrupted

Failure Semantics

Scenario	Outcome	Recovery
Cleanup completes	Obsolete data removed	None required
Cleanup interrupted	Partial cleanup	Re-run cleanup
Disk fills during cleanup	Operation fails	Free space, re-run
Node restarts during cleanup	Cleanup aborted	Re-run cleanup

Prerequisites

Before running cleanup:

Requirement	Verification
New nodes must show `UN` status	`nodetool status`
Bootstrap must be complete	No nodes in `UJ` state
Sufficient disk space for compaction	> 20% free recommended
No repairs running	`nodetool netstats`

Timing Matters

Run cleanup after all new nodes have completed bootstrap. Running cleanup before bootstrap completes may remove data still being streamed.

Procedure

Basic Cleanup

Run on each existing node (not the new nodes):

# Full cleanup (all keyspaces)
nodetool cleanup

Targeted Cleanup

Clean specific keyspaces:

# Single keyspace
nodetool cleanup my_keyspace

# Multiple keyspaces
nodetool cleanup keyspace1 keyspace2

Clean specific tables:

# Single table
nodetool cleanup my_keyspace my_table

Sequential Execution

Run cleanup one node at a time to limit cluster-wide I/O impact:

# Node 1
ssh node1 "nodetool cleanup"
# Wait for completion

# Node 2
ssh node2 "nodetool cleanup"
# Wait for completion

# Continue for all original nodes...

Parallel Execution (Advanced)

In large clusters with sufficient I/O capacity:

# Run on multiple nodes simultaneously
# Limit to one per rack to avoid overwhelming storage
parallel-ssh -h nodes_rack1.txt "nodetool cleanup"
# Wait
parallel-ssh -h nodes_rack2.txt "nodetool cleanup"

Parallel Cleanup Impact

Parallel cleanup increases cluster-wide I/O load. Monitor latency and throughput during execution.

Monitoring Cleanup

Progress Tracking

# Active compaction tasks (cleanup appears as compaction)
nodetool compactionstats

# Example output during cleanup:
# pending tasks: 3
# compaction type  keyspace  table  completed  total   unit  progress
# Cleanup          users     data   52428800   104857600  bytes  50.00%

Completion Verification

# No cleanup tasks pending
nodetool compactionstats | grep -i cleanup

# Disk usage should decrease
df -h /var/lib/cassandra

# Compare before/after per node
nodetool status

Log Monitoring

# Watch cleanup progress in logs
tail -f /var/log/cassandra/system.log | grep -i cleanup

Performance Tuning

Limit Concurrent Compactions

Reduce cleanup impact on client operations:

# Reduce concurrent compactors temporarily
nodetool setcompactionthroughput 32  # MB/s (default varies)

# After cleanup
nodetool setcompactionthroughput 0  # Reset to unlimited

Schedule During Low Traffic

Cleanup is I/O intensive. Schedule during:

Off-peak hours
Maintenance windows
Low-traffic periods

Disk Space Management

Cleanup may temporarily increase disk usage during SSTable rewriting:

# Verify sufficient space before cleanup
df -h /var/lib/cassandra

# Rule: Need ~20% free space for compaction overhead

Estimating Duration

Cleanup duration depends on:

Data volume on the node
I/O throughput
Percentage of data that moved to new nodes

Approximate Times

Data per Node	Cleanup Duration
100 GB	30 min - 1 hour
500 GB	2-4 hours
1 TB	4-8 hours
2 TB	8-16 hours

Estimation Formula

Duration ≈ (Node data volume × Fraction moved) / I/O throughput

Example:
- Node has 500 GB
- Added 1 node to 4-node cluster (25% data moved)
- I/O throughput: 100 MB/s

Duration ≈ (500 GB × 0.25) / 100 MB/s = 125 GB / 100 MB/s ≈ 21 minutes

Actual times are typically 2-4x this estimate due to overhead.

Troubleshooting

Cleanup Slow

Symptoms: Cleanup takes much longer than expected

Causes:

Cause	Solution
Disk I/O saturated	Reduce `compaction_throughput_mb_per_sec`
Many small SSTables	Run major compaction first
Competing operations	Wait for repairs/streaming to complete

# Check I/O utilization
iostat -x 5

# Check competing tasks
nodetool compactionstats

Cleanup Fails with Disk Full

Symptoms: Cleanup aborts with "No space left on device"

Solution:

# 1. Free disk space
#    - Delete old snapshots
nodetool clearsnapshot

#    - Remove old logs
sudo journalctl --vacuum-time=7d

# 2. Retry cleanup on specific keyspace
nodetool cleanup smallest_keyspace

# 3. Continue with larger keyspaces as space frees

Cleanup Never Completes

Symptoms: Cleanup runs indefinitely