# ScyllaDB Large Partitions Table

This document describes how to work with ScyllaDB’s large partitions table.
The large partitions table can be used to trace large partitions in a cluster.
The table is updated every time a partition is written and/or deleted,and includes a compaction process which flushes MemTables to SSTables.

Large Partitions can cause any of the following symptoms:

* Longer latencies on a single shard (look at the “ScyllaDB Overview Metrics” dashboard of [ScyllaDB Monitoring Stack](https://monitoring.docs.scylladb.com/stable/)).
* Oversized allocation warning messages in the log (e.g. `seastar_memory - oversized allocation: 2842624 bytes, please report`)

If you are experiencing any of the above, search to see if you have large partitions.

Note that large partitions are detected only when they are stored in a single SSTable.
ScyllaDB does not account for data belonging to the same logical partition, but spread across multiple SSTables, as long as any single partition in each SSTable does not cross the large partitions warning threshold.
However, note that over time, compaction, and Size-Tiered Compaction Strategy in particular, may collect the dispersed partition data from several SSTables and store it in a single SSTable, thus crossing the large partitions threshold.

## Viewing - Find Large Partitions

* Search for large partitions.

For example:

```console
SELECT * FROM system.large_partitions;
```

Example output:

```console
keyspace_name | table_name | sstable_name     | partition_size | partition_key                                       | compaction_time     | dead_rows | range_tombstones | rows
--------------+------------+------------------+----------------+-----------------------------------------------------+---------------------+-----------+------------------+--------
       demodb |       tmcr | md-6-big-Data.db |     1188716932 | {key: pk{000400000001}, token:-4069959284402364209} | 2018-07-23 08:10:34 |         0 |                0 |    100
       testdb |       tmcr | md-7-big-Data.db |        1234567 | {key: pk{000400000001}, token:-3169959284402457813} | 2018-07-23 08:10:34 |         0 |                0 | 100101
```

| Parameter        | Description                                                                    |
|------------------|--------------------------------------------------------------------------------|
| keyspace_name    | The name of a keyspace holding the large partition                             |
| table_name       | The name of a table containing the large partition                             |
| sstable_name     | The name of SSTable containing the large partition                             |
| partition_size   | The size of the partition in this sstable                                      |
| partition_key    | The value of a partition key that identifies the large partition               |
| dead_rows        | The number of dead rows in the partition in this sstable                       |
| range_tombstones | The number of range tombstones in the partition in this sstable                |
| rows             | The number of rows including range tombstones in the partition in this sstable |
| compaction_time  | Time when compaction last occurred                                             |

#### NOTE
The large partitions table is **local** per node. Querying the table returns results for the specific node you query.

* Search within all the large partitions for a specific keyspace and or table.

For example we are looking for the keyspace `demodb` and table `tmcr`:

```console
SELECT * FROM system.large_partitions WHERE keyspace_name = 'demodb' AND table_name = 'tmcr';
```

Example output:

```console
keyspace_name | table_name | sstable_name     | partition_size | partition_key                                       | compaction_time     | dead_rows | range_tombstones | rows
--------------+------------+------------------+----------------+-----------------------------------------------------+---------------------+-----------+------------------+------
       demodb |       tmcr | md-6-big-Data.db |     1188716932 | {key: pk{000400000001}, token:-4069959284402364209} | 2018-07-23 08:10:34 |         0 |                0 | 1942
```

<a id="large-partition-table-configure"></a>

## Configure

Configure the detection thresholds of large partitions with the `compaction_large_partition_warning_threshold_mb` parameter (default: 1000MB)
and the `compaction_rows_count_warning_threshold` parameter (default 100000)
in the scylla.yaml configuration file.
Partitions that are bigger than the size threshold and/or hold more than the rows count threshold are reported in the `system.large_partitions` table and generate a warning in the ScyllaDB log (refer to [log](https://docs.scylladb.com/manual/branch-2025.4/getting-started/logging.md)).

For example (set to 500MB / 50000, respectively):

```console
compaction_large_partition_warning_threshold_mb: 500
compaction_rows_count_warning_threshold: 50000
```

## Storing

Large partitions are stored in a system table with the following schema:

```console
DESCRIBE TABLE system.large_partitions;

CREATE TABLE system.large_partitions (
    keyspace_name text,
    table_name text,
    sstable_name text,
    partition_size bigint,
    partition_key text,
    compaction_time timestamp,
    dead_rows bigint,
    range_tombstones bigint,
    rows bigint,
    PRIMARY KEY ((keyspace_name, table_name), sstable_name, partition_size, partition_key)
) WITH CLUSTERING ORDER BY (sstable_name ASC, partition_size DESC, partition_key ASC)
```

## Expiring Data

In order to prevent stale data from appearing, all rows in `system.large_partitions` table are inserted with Time To Live (TTL) equal to 30 days.

[Troubleshoot](https://docs.scylladb.com/manual/branch-2025.4/troubleshooting/index.md)

## Additional Resources

[Large Partitions Hunting](https://docs.scylladb.com/manual/branch-2025.4/troubleshooting/debugging-large-partition.md)