Thanks for taking the time to reply.
In my lab environment, I just altered the table to simulate LCS. I'll
update with my findings tomorrow.
INITIAL: compaction = {'class':
'org.apache.cassandra.db.compaction.UnifiedCompactionStrategy',
'max_sstables_to_compact': '64', 'min_sstable_size': '100MiB',
'scaling_parameters': 'T4', 'sstable_growth': '0.3333333333333333',
'target_sstable_size': '1GiB'}
----
NEW: COMPACTION = compaction = {'base_shard_count': '8', 'class':
'org.apache.cassandra.db.compaction.UnifiedCompactionStrategy',
'scaling_parameters': 'L10', 'target_sstable_size': '256MiB'}
On Tue, Mar 10, 2026 at 2:42 PM Patrick McFadin <[email protected]> wrote:
> It looks like you are comparing Cassandra 2 LCS to Cassandra 5 UCS with
> T4, which is a tiered/STCS-like layout. If so, I would not treat the
> storage increase as an expected Cassandra 5 baseline. Since the compression
> ratio is nearly identical, I’d retest with UCS L10 for a more
> apples-to-apples comparison with LCS before drawing conclusions about disk
> usage.
>
> Patrick
>
>
> On Mon, Mar 9, 2026 at 7:39 AM dbms-tech <[email protected]> wrote:
>
>> Testing a C5 upgrade from C5 in our lab environment as PoC. I migrated a
>> 1.3B rows table from C2 to C5.
>> On C2, table was 7.4 TB large. On C5, it is 9.5 TB.
>>
>> I verified the following ...
>> ==============================
>> - There are no stale snapshots in C5.
>> - C5 size above is exclusively for the table's sstables. No other files
>> reside in the /data directory.
>> - tablestats shows 'SSTable Compression Ratio: 0.25316' for C5 and 0.25
>> for C2.
>> - I adjusted C5 table to use: chunk_length_in_kb': '64', 'class':
>> 'org.apache.cassandra.io.compress.ZstdCompressor', 'compression_level': '10
>> to obtain the above compression ratio.
>> - C5 uses default UCS while C2 uses LTS.
>> - C2 used DeflateCompressor.
>> - Compactions are optimal with near zero pending compactions.
>> - Cluster is fully & regularly repaired using Reaper.
>>
>> Inquiries ...
>> - Is C5 (UCS) expected to utilize this much more storage compared to C2
>> (LCS)?
>> - Other than increasing compression level and chunk_size, what else can
>> be done to match C2 storage?
>>
>> Thanks in advance.
>>
>> ==============
>> C5
>> ==============
>> CREATE TABLE xxx.yyy (
>>
>> ddd bigint PRIMARY KEY,
>> xxx boolean,
>> yyy text,
>> zzz text,
>> ddd bigint
>> ) WITH additional_write_policy = '99p'
>> AND allow_auto_snapshot = true
>> AND bloom_filter_fp_chance = 0.01
>> AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}
>> AND cdc = false
>> AND comment = ''
>> AND compaction = {'class':
>> 'org.apache.cassandra.db.compaction.UnifiedCompactionStrategy',
>> 'max_sstables_to_compact': '64', 'min_sstable_size': '100MiB',
>> 'scaling_parameters': 'T4', 'sstable_growth': '0.3333333333333333',
>> 'target_sstable_size': '1GiB'}
>> AND compression = {'chunk_length_in_kb': '64', 'class':
>> 'org.apache.cassandra.io.compress.ZstdCompressor', 'compression_level':
>> '10'}
>> AND memtable = 'default'
>> AND crc_check_chance = 1.0
>> AND default_time_to_live = 0
>> AND extensions = {}
>> AND gc_grace_seconds = 864000
>> AND incremental_backups = true
>> AND max_index_interval = 2048
>> AND memtable_flush_period_in_ms = 0
>> AND min_index_interval = 128
>> AND read_repair = 'BLOCKING'
>> AND speculative_retry = '99p';
>>
>> =================
>> C5
>> ==================
>> nodetool tablestats xxx.yyy
>> Total number of tables: 1
>> ----------------
>> Keyspace: xxx
>> Read Count: 764664474
>> Read Latency: 1.092679410718903 ms
>> Write Count: 127141592
>> Write Latency: 0.02513343241761516 ms
>> Pending Flushes: 0
>> Table: document
>> SSTable count: 783
>> Old SSTable count: 0
>> Max SSTable size: 4.664GiB
>> Space used (live): 1208104887289
>> Space used (total): 1208104887289
>> Space used by snapshots (total): 0
>> Off heap memory used (total): 1324105435
>> SSTable Compression Ratio: 0.25316
>> Number of partitions (estimate): 126389507
>> Memtable cell count: 23023
>> Memtable data size: 424380268
>> Memtable off heap memory used: 450102571
>> Memtable switch count: 1242
>> Speculative retries: 4016043
>> Local read count: 725885015
>> Local read latency: 1.153 ms
>> Local write count: 24986436
>> Local write latency: 0.052 ms
>> Local read/write ratio: 29.05116
>> Pending flushes: 0
>> Percent repaired: 0.0
>> Bytes repaired: 0B
>> Bytes unrepaired: 4.328TiB
>> Bytes pending repair: 0B
>> Bloom filter false positives: 4722014
>> Bloom filter false ratio: 0.01052
>> Bloom filter space used: 289491392
>> Bloom filter off heap memory used: 289485128
>> Index summary off heap memory used: 0
>> Compression metadata off heap memory used: 584517736
>> Compacted partition minimum bytes: 21
>> Compacted partition maximum bytes: 10090808
>> Compacted partition mean bytes: 21423
>> Average live cells per slice (last five minutes): 1.0
>> Maximum live cells per slice (last five minutes): 1
>> Average tombstones per slice (last five minutes): 1.0
>> Maximum tombstones per slice (last five minutes): 1
>> Droppable tombstone ratio: 0.01064
>> Top partitions by size (last update:
>> 2026-03-06T16:13:07Z):
>>
>>
>> --
>>
>> ----------------------------------------
>> Thank you
>>
>>
>>
--
----------------------------------------
Thank you
