PolarDB:X-Engine FAQ

What is X-Engine?

X-Engine is a storage engine developed by Alibaba Cloud, built on log-structured merge-tree (LSM tree) architecture and the Zstandard (ZSTD) compression algorithm. Unlike InnoDB's B-tree structure, X-Engine optimizes for sequential writes and high compression, making it well-suited for archiving and append-heavy workloads.

For an overview of X-Engine features and architecture, see X-Engine.

Version requirements

X-Engine is supported on PolarDB for MySQL 8.0.1 and 8.0.2. MySQL 5.6 and 5.7 clusters do not support X-Engine.

Certain features require specific revision versions:

How much storage can X-Engine save?

Compared with InnoDB, X-Engine reduces storage usage by up to 70%, depending on your data type. Compression is enabled by default.

To check the estimated compression ratio for a specific table, run the following query. This example queries the compression ratio for the primary index of table test1 in database test:

SELECT
    TABLE_NAME,
    SUBTABLE_NAME,
    SUM(EXTENTS*APRROX_CMP_RATIO)/SUM(EXTENTS)
FROM
    INFORMATION_SCHEMA.XENGINE_SUBTABLE
WHERE TABLE_NAME = 'test.test1' AND SUBTABLE_NAME = 'PRIMARY' AND LEVEL in (1, 2);

The query targets levels 1 and 2 because X-Engine organizes table data in LSM tree-based indexes, and most data resides at those levels. Level 0 holds data freshly flushed from memory and is not compressed. The result is a weighted average compression ratio across the two levels.

To query the compression ratio for a secondary index, replace 'PRIMARY' with the index name in the SUBTABLE_NAME filter.

XENGINE_SUBTABLE columns used in this query:

This query requires an Enterprise Edition cluster running one of the following revision versions:

PolarDB for MySQL 8.0.1, revision 8.0.1.1.40 or later

PolarDB for MySQL 8.0.2, revision 8.0.2.2.22 or later

Why is the compression ratio low right after a bulk data import?

X-Engine uses background asynchronous compression — data is reorganized and compressed gradually without affecting running workloads. Most reorganization and compression operations happen during off-peak hours.

After a bulk import, the data appears close to its original uncompressed size. Storage space is released progressively as background compression completes.

What are the performance trade-offs?

X-Engine's LSM tree structure delivers stronger write performance than read performance. Because data compression is always on, X-Engine scores 10–20% lower than InnoDB in Sysbench benchmarks.

See Performance comparison between X-Engine Edition and Cluster Edition in PolarDB for MySQL 8.0.1 for detailed benchmark results.

How does X-Engine compare to RocksDB and TokuDB?

X-Engine achieves a similar compression ratio to RocksDB. TokuDB is deprecated, and all TokuDB data on Alibaba Cloud has been migrated to X-Engine.

Compared with RocksDB and MyRocks, X-Engine provides large-scale storage, high elasticity, and uninterrupted schema changes. See Benefits of X-Engine for the full comparison.

What are the advantages of PolarDB X-Engine over ApsaraDB RDS X-Engine?

PolarDB X-Engine runs on a compute-storage decoupled architecture with distributed storage. Key advantages:

• Up to 500 TB of storage space

• Read-only nodes can be added or released independently; up to 16 read-only nodes can share a single copy of data

• High availability does not depend on binary logs, which improves write performance

How is X-Engine billed?

X-Engine uses the same billing model as InnoDB — no additional charges. You pay only for the storage space actually used after compression. For example, if your data is 100 GB in InnoDB and compresses to 30 GB in X-Engine, you are charged for 30 GB.

Does X-Engine have real-world production usage?

X-Engine is used within Alibaba Group and by Alibaba Cloud customers. PB-scale data across more than 3,000 clusters is stored in X-Engine. See Best practices for case studies.

What features does X-Engine support?

What isolation levels does X-Engine support?

X-Engine supports read-committed and repeatable-read isolation. Read-committed is the default.

What are the limits of X-Engine?

As a MySQL storage engine, X-Engine supports only common MySQL features. See Limits for the full list.

How do I create a PolarDB cluster with X-Engine?

On the cluster purchase page in the PolarDB console, set Database Engine to MySQL 8.0.1 or MySQL 8.0.2, then set Storage Engine to InnoDB & X-Engine.

How do I enable X-Engine on an existing cluster?

1. Log on to the PolarDB console and click the target cluster ID.

2. In the left-side navigation pane, choose Settings and Management > Data Lifecycle.

3. On the X-Engine (Warm Data) tab, click Enable.

The cluster must run one of the following revision versions:

• PolarDB for MySQL 8.0.1, revision 8.0.1.1.31 or later

• PolarDB for MySQL 8.0.2, revision 8.0.2.2.12 or later

For version requirements and prerequisites, see Prerequisites.

How do I use X-Engine for a table?

Set ENGINE=xengine when creating a table. For existing tables, run an ALTER TABLE statement to change the storage engine. See Usage notes for syntax and limitations.

How do I adjust the X-Engine memory allocation?

1. Log on to the PolarDB console and click the target cluster ID.

2. Choose Settings and Management > Data Lifecycle.

3. On the X-Engine (Warm Data) tab, adjust the memory resource proportion.

See Adjust the proportion of memory resources for details.

How do I enable partitioned tables for X-Engine?

Partitioned tables for X-Engine are in the canary release phase. To apply:

1. Go to Quota Center.

2. Find the quota with ID polardb_mysql_xengine_partition.

3. Click Apply in the Actions column.

For setup guidance, join the DingTalk group (group ID: 24490017825) to get technical support.

How do I check storage space used by X-Engine tables?

Connect to your PolarDB cluster using the primary cluster endpoint, then run the following query. This example checks storage usage for table test1 in database test:

SELECT
    SUM(TOTAL_EXTENT_COUNT)/512 AS SIZE_GB,
    SUM(USED_EXTENT_COUNT)/512 AS USED_SIZE_GB,
    SUM(FREE_EXTENT_COUNT)/512 AS FREE_SIZE_GB
FROM
    INFORMATION_SCHEMA.XENGINE_TABLE_SPACE
WHERE TABLE_SPACE_ID IN (
    SELECT TABLE_SPACE_ID
    FROM INFORMATION_SCHEMA.XENGINE_SUBTABLE
    WHERE TABLE_NAME = 'test.test1');

Each extent is 2 MB. Dividing by 512 converts the extent count to gigabytes.

The query uses two system tables:

XENGINE_TABLE_SPACE — calculates storage size from physical files:

XENGINE_SUBTABLE — calculates storage size by LSM level:

Related topics

• X-Engine overview

• Enable X-Engine

• Usage notes

• X-Engine best practices