nodetool compactionhistory¶

Displays the history of completed compaction operations.

Synopsis¶

nodetool [connection_options] compactionhistory

Description¶

nodetool compactionhistory shows information about previously completed compactions. This helps analyze compaction patterns, identify problematic tables, and understand historical compaction behavior.

Output Format¶

Compaction History:
id                                   keyspace_name columnfamily_name compacted_at            bytes_in    bytes_out   rows_merged
abc12345-def6-7890-abcd-ef1234567890 my_keyspace   my_table          2024-01-15T10:30:00     1073741824  536870912   {1:150, 2:50}
def67890-abcd-1234-ef56-7890abcdef12 my_keyspace   my_table          2024-01-15T09:15:00     2147483648  1073741824  {1:300, 2:100}

Output Fields¶

Field	Description
`id`	Unique compaction ID
`keyspace_name`	Keyspace containing the table
`columnfamily_name`	Table name
`compacted_at`	Timestamp of compaction completion
`bytes_in`	Total bytes read from input SSTables
`bytes_out`	Total bytes written to output SSTable
`rows_merged`	Distribution of rows merged per SSTable

Examples¶

View All Compaction History¶

nodetool compactionhistory

Filter by Keyspace¶

nodetool compactionhistory | grep my_keyspace

Filter by Table¶

nodetool compactionhistory | grep my_table

Recent Compactions Only¶

nodetool compactionhistory | head -20

Understanding the Output¶

Bytes In vs Bytes Out¶

bytes_in: 1073741824  bytes_out: 536870912

bytes_in > bytes_out: Data was compacted (tombstones removed, duplicates merged)
bytes_in ≈ bytes_out: Little compaction benefit (fresh data)
High ratio indicates effective compaction

Rows Merged¶

rows_merged: {1:150, 2:50}

1:150 - 150 rows appeared in only 1 SSTable (no merging needed)
2:50 - 50 rows appeared in 2 SSTables (were merged)

Higher numbers in the merge count indicate more overwrites or deletions being resolved.

Analysis Use Cases¶

Check Compaction Efficiency¶

# Calculate compression ratio for recent compactions
nodetool compactionhistory | awk 'NR>2 {
    if ($5 > 0) {
        ratio = $6 / $5
        printf "%s.%s: %.2f\n", $2, $3, ratio
    }
}' | head -10

Ratio close to 1.0 = little benefit; lower ratio = good compaction.

Find Tables with Most Compactions¶

nodetool compactionhistory | awk 'NR>2 {print $2"."$3}' | sort | uniq -c | sort -rn | head -10

Check Compaction Timestamps¶

# See when compactions occurred
nodetool compactionhistory | awk 'NR>2 {print $4}' | cut -dT -f1 | sort | uniq -c

Troubleshooting with Compaction History¶

Frequent Small Compactions¶

If seeing many small compactions:

nodetool compactionhistory | awk 'NR>2 && $5 < 100000000 {print}' | wc -l

Many small compactions may indicate: - High write rate - STCS with small sstable_size_in_mb - Need to tune compaction settings

Large Compactions Taking Too Long¶

# Find largest compactions
nodetool compactionhistory | awk 'NR>2 {print $5, $2"."$3}' | sort -rn | head -5

Consider: - LCS for more predictable compaction sizes - Increasing compaction throughput - Adding more compaction threads

No Recent Compactions¶

If compactionhistory shows old entries only:

# Check if compactions are running
nodetool compactionstats

# Check if auto-compaction is enabled
nodetool statusautocompaction my_keyspace my_table

Monitoring Compaction Patterns¶

Daily Compaction Volume¶

#!/bin/bash
# compaction_daily_stats.sh

echo "Date,Compactions,BytesIn,BytesOut"
nodetool compactionhistory | awk 'NR>2 {
    date = substr($4, 1, 10)
    counts[date]++
    bytes_in[date] += $5
    bytes_out[date] += $6
}
END {
    for (d in counts) {
        printf "%s,%d,%.2fGB,%.2fGB\n", d, counts[d], bytes_in[d]/1073741824, bytes_out[d]/1073741824
    }
}' | sort

Table-Level Analysis¶

#!/bin/bash
# Analyze compaction efficiency by table

echo "Table,Compactions,AvgRatio"
nodetool compactionhistory | awk 'NR>2 && $5 > 0 {
    table = $2"."$3
    counts[table]++
    ratios[table] += $6/$5
}
END {
    for (t in counts) {
        printf "%s,%d,%.3f\n", t, counts[t], ratios[t]/counts[t]
    }
}' | sort -t, -k2 -rn

History Retention¶

Compaction history is stored in system.compaction_history table:

SELECT * FROM system.compaction_history LIMIT 10;

History Limits

History is kept for a limited time
Older entries are automatically removed
For long-term analysis, export to external monitoring

Command	Relationship
compactionstats	Current compaction status
tablestats	Table statistics including SSTable count
compact	Force compaction
setcompactionthroughput	Control compaction speed