Caches¶

The caches virtual table provides statistics for all Cassandra caches, enabling monitoring of cache efficiency and capacity.

Overview¶

Cassandra maintains several caches to reduce disk I/O and improve read performance. The caches table shows hit rates, sizes, and capacity for each cache.

SELECT name, capacity_bytes, size_bytes, entry_count, hit_ratio
FROM system_views.caches;

Schema¶

VIRTUAL TABLE system_views.caches (
    name text PRIMARY KEY,
    capacity_bytes bigint,
    entry_count int,
    hit_count bigint,
    hit_ratio double,
    recent_hit_rate_per_second bigint,
    recent_request_rate_per_second bigint,
    request_count bigint,
    size_bytes bigint
)

Column	Type	Description
`name`	text	Cache name
`capacity_bytes`	bigint	Maximum configured cache size
`size_bytes`	bigint	Current memory used by cache
`entry_count`	int	Number of entries in cache
`hit_count`	bigint	Total cache hits since startup
`request_count`	bigint	Total cache requests since startup
`hit_ratio`	double	Hit rate (0.0-1.0)
`recent_hit_rate_per_second`	bigint	Recent hits per second
`recent_request_rate_per_second`	bigint	Recent requests per second

Cache Types¶

KeyCache¶

Caches partition index locations to avoid index file lookups.

Aspect	Description
Purpose	Maps partition keys to SSTable positions
Impact	Reduces disk seeks for partition lookups
Size	Configured via `key_cache_size_in_mb`
Target hit ratio	> 0.85 for active partitions

Configuration:

# cassandra.yaml
key_cache_size_in_mb: 100        # Cache size (auto if empty)
key_cache_save_period: 14400     # Save interval (seconds)
key_cache_keys_to_save: 0        # 0 = save all

RowCache¶

Caches entire rows (off-heap). Disabled by default due to memory overhead.

Aspect	Description
Purpose	Stores complete row data
Impact	Eliminates disk I/O for cached rows
Size	Per-table via `caching` option
Target hit ratio	> 0.90 if enabled

Enable per table:

ALTER TABLE my_table WITH caching = {
    'keys': 'ALL',
    'rows_per_partition': '100'
};

Row Cache Considerations

Row cache is suitable only for: - Small, frequently-read rows - Rarely-updated data - Sufficient off-heap memory

Unsuitable for: - Large rows - Frequently-updated data - Memory-constrained nodes

CounterCache¶

Caches counter values for faster counter reads.

Aspect	Description
Purpose	Stores recent counter values
Impact	Reduces counter read latency
Size	Configured via `counter_cache_size_in_mb`
Target hit ratio	> 0.90 for active counters

Configuration:

# cassandra.yaml
counter_cache_size_in_mb: 50
counter_cache_save_period: 7200

ChunkCache¶

Caches compressed SSTable chunks (Cassandra 4.0+).

Aspect	Description
Purpose	Stores decompressed SSTable data
Impact	Reduces decompression overhead
Size	Auto-sized based on available memory
Target hit ratio	Varies by workload

Monitoring Queries¶

Cache Overview¶

SELECT
    name,
    size_bytes / 1048576 AS size_mb,
    capacity_bytes / 1048576 AS capacity_mb,
    entry_count,
    hit_ratio,
    recent_hit_rate_per_second AS hits_per_sec
FROM system_views.caches;

Cache Efficiency¶

-- Cache efficiency metrics
SELECT name, hit_ratio, size_bytes, capacity_bytes
FROM system_views.caches;

Interpreting hit_ratio:

hit_ratio	Status
≥ 0.90	Excellent
0.80–0.89	Good
0.60–0.79	Fair
< 0.60	Poor

Cache Pressure¶

-- Check cache fill levels
SELECT name, size_bytes, capacity_bytes
FROM system_views.caches;

Cache is at pressure when size_bytes / capacity_bytes > 0.95.

Alerting Rules¶

Low Hit Ratio Alert¶

-- Key cache hit ratio degradation
SELECT name, hit_ratio
FROM system_views.caches
WHERE name = 'KeyCache' AND hit_ratio < 0.80;

Thresholds:

Cache	Warning	Critical
KeyCache	< 0.85	< 0.70
RowCache	< 0.90	< 0.80
CounterCache	< 0.85	< 0.70

Cache Full Alert¶

-- Cache sizes (alert when size_bytes approaches capacity_bytes)
SELECT name, size_bytes, capacity_bytes
FROM system_views.caches;

Alert when size_bytes / capacity_bytes > 0.98.

Troubleshooting¶

Low Key Cache Hit Ratio¶

Symptoms: - KeyCache hit_ratio < 0.80 - Increased read latency

Common Causes: 1. Cache too small for working set 2. Access patterns with poor locality 3. Many unique partitions accessed once

Resolution:

# Increase key cache size
key_cache_size_in_mb: 200  # Increase from default

Monitor after change:

SELECT name, hit_ratio, entry_count
FROM system_views.caches
WHERE name = 'KeyCache';

Cache Thrashing¶

Symptoms: - Cache at capacity - Hit ratio declining over time - High eviction rate (not directly visible, inferred from stagnant hit_ratio with high request_rate)

Resolution: 1. Increase cache size if memory available 2. Review access patterns for cache-unfriendly queries 3. Consider if caching is appropriate for workload

Counter Cache Misses¶

Symptoms: - CounterCache hit_ratio low - Counter read latency high