nodetool compactionstats

Displays statistics about currently running compactions and pending compaction tasks.

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Synopsis

nodetool [connection_options] compactionstats [-H]

See connection options for connection options.

Description

nodetool compactionstats shows:

• Currently active compaction operations
• Progress of each compaction
• Pending compaction tasks by keyspace
• Compaction throughput

This is essential for monitoring compaction health and identifying backlogs.

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Options

Option	Description
-H, --human-readable	Display sizes in human-readable format
-V, --vtable	Use virtual table format for output

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Output Example

pending tasks: 15
- my_keyspace.users: 8
- my_keyspace.orders: 7
          id                compaction type        keyspace           table       completed           total      unit   progress
          abc123-def4-...        Compaction     my_keyspace           users      1234567890      2345678901     bytes     52.65%
          def456-abc7-...        Compaction     my_keyspace          orders       567890123      1234567890     bytes     46.01%
          ghi789-jkl0-...        Validation     my_keyspace           users        45678901       123456789     bytes     37.00%
Active compaction remaining time :   0h12m34s

Note: The output includes pending tasks broken down by keyspace and table, plus a per-compaction id column (unless -V/--vtable is used).

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Output Fields

Field	Description
pending tasks	Total compaction tasks waiting to run (with per-keyspace.table breakdown)
id	Unique identifier for the active compaction operation
compaction type	Type of operation (Compaction, Validation, Cleanup, etc.)
keyspace	Keyspace being compacted
table	Table being compacted
completed	Bytes processed so far
total	Total bytes to process
unit	Unit of measurement
progress	Percentage complete

Compaction Types

Type	Description
Compaction	Regular SSTable compaction
Validation	Building Merkle tree for repair
Cleanup	Removing data not owned by node
Scrub	Rebuilding corrupted SSTables
Upgrade	Upgrading SSTable format
Index_build	Building secondary index
Anticompaction	Splitting SSTables for incremental repair

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Interpreting Results

Healthy State

pending tasks: 0
Active compaction remaining time :        n/a

No active compactions and no backlog.

Active Compaction

pending tasks: 3
          compaction type        keyspace           table       completed           total      unit   progress
               Compaction     my_keyspace           users      1234567890      2345678901     bytes     52.65%
Active compaction remaining time :   0h05m23s

Normal operation with manageable workload.

Compaction Backlog

pending tasks: 150
          compaction type        keyspace           table       completed           total      unit   progress
               Compaction     my_keyspace           users      1234567890      2345678901     bytes     52.65%
Active compaction remaining time :   2h45m12s

High Pending Tasks

Large pending task count indicates:

• Writes exceeding compaction capacity
• Insufficient compaction throughput
• Need to tune compaction settings

Actions:

• Increase compaction throughput
• Add more concurrent compactors
• Reduce write rate if possible
• Review compaction strategy

Multiple Active Compactions

pending tasks: 10
          compaction type        keyspace           table       completed           total      unit   progress
               Compaction     my_keyspace           users      1234567890      2345678901     bytes     52.65%
               Compaction     my_keyspace          orders       567890123      1234567890     bytes     46.01%
               Compaction      other_ks           events       123456789       234567890     bytes     52.63%
               Compaction      other_ks            logs        234567890       345678901     bytes     67.95%

Multiple concurrent compactors running (controlled by concurrent_compactors).

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When to Use

Monitor Compaction Health

# Periodic check
nodetool compactionstats

# Continuous monitoring
watch -n 5 'nodetool compactionstats'

During High Write Load

Monitor for backlog buildup:

watch -n 2 'nodetool compactionstats | head -5'

During Repair

Validation compactions appear during repair:

nodetool compactionstats | grep Validation

After Adding Data

After bulk loads or migrations:

# Watch compaction catch up
watch -n 10 'nodetool compactionstats'

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Examples

Basic Usage

nodetool compactionstats

Human-Readable Output

nodetool compactionstats -H

Output:

pending tasks: 15
          compaction type        keyspace           table       completed           total      unit   progress
               Compaction     my_keyspace           users           1.2 GB           2.3 GB   bytes     52.65%

Check Pending Tasks Only

nodetool compactionstats | head -1

Monitor Continuously

watch -n 5 'nodetool compactionstats -H'

Compare Across Nodes

for node in node1 node2 node3; do
    echo "=== $node ==="
    ssh "$node" "nodetool compactionstats | head -3"
done

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Managing Compaction Backlog

Increase Throughput

# Check current throughput
nodetool getcompactionthroughput

# Increase throughput (MB/s)
nodetool setcompactionthroughput 256

Add Concurrent Compactors

# Check current setting
nodetool getconcurrentcompactors

# Increase (use cautiously)
nodetool setconcurrentcompactors 4

Concurrent Compactors

More compactors = more parallel I/O. Only increase if disk can handle it.

Stop Non-Critical Compactions

# Stop current compactions (for emergency)
nodetool stop COMPACTION

# Better: Stop specific types
nodetool stop INDEX_BUILD

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Pending Tasks Analysis

Understanding Pending Count

pending tasks: 150

Pending tasks accumulate when:

Cause	Solution
High write rate	Increase throughput/compactors
Large SSTables	Tune compaction strategy
Slow disks	Improve I/O capacity
Repairs running	Normal during repair
Insufficient threads	Increase compactors

Checking Task Distribution

# See which tables have pending compactions
nodetool compactionstats -H

Tables with consistently high pending may need:

• Different compaction strategy
• More frequent cleanup
• Data model review

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Compaction Throughput

Current Setting

nodetool getcompactionthroughput

Adjust Throughput

# Increase to 256 MB/s
nodetool setcompactionthroughput 256

# Unlimited (not recommended)
nodetool setcompactionthroughput 0

Throughput Guidelines

• Default: 64 MB/s
• SSD clusters: 256-512 MB/s
• HDD clusters: 64-128 MB/s
• Monitor disk utilization when increasing

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Warning Signs

Continuous Backlog Growth

# Check every minute
while true; do
    echo "$(date): $(nodetool compactionstats | head -1)"
    sleep 60
done

If pending continuously increases, action required.

Very Long Compactions

Compactions running for hours may indicate:

• Very large SSTables
• Slow disk I/O
• Memory pressure (GC during compaction)

High CPU During Compaction

Check with system tools:

top -p $(pgrep -f CassandraDaemon)

Compression/decompression is CPU-intensive.

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Related Commands

Command	Relationship
getcompactionthroughput	Check throughput setting
setcompactionthroughput	Adjust throughput
compact	Force compaction
tablestats	SSTable counts per table