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Expand Up @@ -5,7 +5,7 @@ summary: Learn about the new features, compatibility changes, improvements, and

# TiDB 8.4.0 Release Notes

Release date: xx xx, 2024
Release date: October 30, 2024

TiDB version: 8.4.0

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<tr>
<td rowspan="4">Scalability and Performance</td>
<td><a href="https://docs.pingcap.com/tidb/v8.4/system-variables#tidb_enable_instance_plan_cache-new-in-v840">Instance-level execution plan cache</a> (experimental)**tw@Oreoxmt 1569**</td>
<td>Instance-level execution plan cache allows all sessions within the same TiDB instance to share the execution plan cache. This feature reduces SQL compilation time by caching more execution plans in memory, decreasing overall SQL execution time. It improves OLTP performance and throughput while providing better control over memory usage and enhancing database stability.</td>
<td>Instance-level plan cache allows all sessions within the same TiDB instance to share the plan cache. Compared with session-level plan cache, this feature reduces SQL compilation time by caching more execution plans in memory, decreasing overall SQL execution time. It improves OLTP performance and throughput while providing better control over memory usage and enhancing database stability.</td>
</tr>
<tr>
<td><a href="https://docs.pingcap.com/tidb/v8.4/partitioned-table#global-indexes">Global indexes for partitioned tables (GA)</a>**tw@hfxsd 1961**</td>
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### Performance

* Introduce parallel batching modes for TSO requests, reducing TSO retrieval latency [#54960](https://github.com/pingcap/tidb/issues/54960) @[MyonKeminta](https://github.com/MyonKeminta) **tw@qiancai** <!--1893-->
* Introduce parallel batching modes for TSO requests, reducing TSO retrieval latency [#54960](https://github.com/pingcap/tidb/issues/54960) [#8432](https://github.com/tikv/pd/issues/8432) @[MyonKeminta](https://github.com/MyonKeminta) **tw@qiancai** <!--1893-->

Before v8.4.0, when requesting [TSO](/tso.md) from PD, TiDB collects multiple TSO requests during a specific period and processes them in batches serially to decrease the number of Remote Procedure Call (RPC) requests and reduce PD workload. In latency-sensitive scenarios, however, the performance of this serial batching mode is not ideal.

In v8.4.0, TiDB introduces parallel batching modes for TSO requests with different concurrency capabilities. Parallel modes reduce TSO retrieval latency but might increase the PD workload. To set a parallel RPC mode for retrieving TSO, configure the [tidb_tso_client_rpc_mode](/system-variables.md#tidb_tso_client_rpc_mode-new-in-v840) system variable.
In v8.4.0, TiDB introduces parallel batching modes for TSO requests with different concurrency capabilities. Parallel modes reduce TSO retrieval latency but might increase the PD workload. To set a parallel RPC mode for retrieving TSO, configure the [`tidb_tso_client_rpc_mode`](/system-variables.md#tidb_tso_client_rpc_mode-new-in-v840) system variable.

For more information, see [documentation](/system-variables.md#tidb_tso_client_rpc_mode-new-in-v840).

Expand All @@ -116,7 +116,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
- Eliminates redundancy, caching more execution plans with the same memory consumption.
- Allocates a fixed-size memory on the instance, limiting memory usage more effectively.

In v8.4.0, instance-level execution plan cache only supports caching query execution plans and is disabled by default. You can enable this feature using [`tidb_enable_instance_plan_cache`](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840) and set its maximum memory usage using [`tidb_instance_plan_cache_max_size`](/system-variables.md#tidb_instance_plan_cache_max_size-new-in-v840). Before enabling this feature, disable [Prepared execution plan cache](/sql-prepared-plan-cache.md) and [Non-prepared execution plan cache](/sql-non-prepared-plan-cache.md).
In v8.4.0, instance-level execution plan cache only supports caching query execution plans and is disabled by default. You can enable this feature using [`tidb_enable_instance_plan_cache`](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840) and set its maximum memory usage using [`tidb_instance_plan_cache_max_size`](/system-variables.md#tidb_instance_plan_cache_max_size-new-in-v840). Before enabling this feature, disable [Prepared execution plan cache](/sql-prepared-plan-cache.md) and [Non-prepared execution plan cache](/sql-non-prepared-plan-cache.md).

For more information, see [documentation](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840).

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Traffic replay is useful in the following scenarios:

- Validate TiDB version upgrades
- Verify TiDB version upgrades
- Assess change impact
- Validate performance before scaling TiDB
- Test performance limits

For more information, see [documentation](/tiproxy/tiproxy-traffic-replay.md).
For more information, see [documentation](/tiproxy/tiproxy-traffic-replay.md).

### SQL

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- Encrypt using a master key stored on a local disk
- Encrypt using a master key managed by a Key Management Service (KMS)

For more information, see [documentation](/br/br-pitr-manual.md#encrypt-log-backup-data).
For more information, see [documentation](/br/br-pitr-manual.md#encrypt-the-log-backup-data).

* BR requires fewer privileges when restoring backup data in a cloud storage system [#55870](https://github.com/pingcap/tidb/issues/55870) @[Leavrth](https://github.com/Leavrth) **tw@Oreoxmt** <!--1943-->

Expand All @@ -236,7 +236,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
- The [INFORMATION_SCHEMA.PROCESSLIST](/information-schema/information-schema-processlist.md) table adds `TIDB_CPU` and `TIKV_CPU`, showing the cumulative CPU consumption of the SQL statements currently being executed in a session.
- The [slow query Log](/analyze-slow-queries.md) adds the `Tidb_cpu_time` and `Tikv_cpu_time` fields, showing the CPU time of captured SQL statements.

By default, the CPU time consumed by TiKV is displayed. Collecting the CPU time consumed by TiDB brings additional overhead (about 8%), so the CPU time consumed by TiDB only shows the actual value when [Top SQL](/dashboard/top-sql.md) is enabled; otherwise, it always show as `0`.
By default, the CPU time consumed by TiKV is displayed. Collecting the CPU time consumed by TiDB brings additional overhead (about 8%), so the CPU time consumed by TiDB only shows the actual value when [Top SQL](/dashboard/top-sql.md) is enabled; otherwise, it always shows as `0`.

For more information, see [documentation](/information-schema/information-schema-processlist.md) and [documentation](/information-schema/information-schema-slow-query.md).

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| Variable name | Change type | Description |
|--------|------------------------------|------|
| [`log_bin`](/system-variables.md#log_bin) | Deleted | In v8.4.0, [TiDB Binlog](https://docs.pingcap.com/tidb/v8.3/tidb-binlog-overview) is removed. This variable indicates whether TiDB Binlog is used, and is deleted starting from v8.4.0. |
| [`sql_log_bin`](/system-variables.md#sql_log_bin) | Deleted | In v8.4.0, [TiDB Binlog](https://docs.pingcap.com/tidb/v8.3/tidb-binlog-overview) is removed. This variable indicates whether to write changes to TiDB Binlog or not, and is deleted starting from v8.4.0. |
| `log_bin` | Deleted | In v8.4.0, [TiDB Binlog](https://docs.pingcap.com/tidb/v8.3/tidb-binlog-overview) is removed. This variable indicates whether TiDB Binlog is used, and is deleted starting from v8.4.0. |
| `sql_log_bin` | Deleted | In v8.4.0, [TiDB Binlog](https://docs.pingcap.com/tidb/v8.3/tidb-binlog-overview) is removed. This variable indicates whether to write changes to TiDB Binlog or not, and is deleted starting from v8.4.0. |
| [`tidb_enable_global_index`](/system-variables.md#tidb_enable_global_index-new-in-v760) | Deprecated | In v8.4.0, this variable is deprecated. Its value will be fixed to the default value `ON`, that is, [global index](/partitioned-table.md#global-indexes) is enabled by default. You only need to add the keyword `GLOBAL` to the corresponding column when executing `CREATE TABLE` or `ALTER TABLE` to create a global index. |
| [`tidb_enable_list_partition`](/system-variables.md#tidb_enable_list_partition-new-in-v50) | Deprecated | In v8.4.0, this variable is deprecated. Its value will be fixed to the default value `ON`, that is, [list partitioning](/partitioned-table.md#list-partitioning) is enabled by default. |
| [`tidb_enable_table_partition`](/system-variables.md#tidb_enable_table_partition) | Deprecated | In v8.4.0, this variable is deprecated. Its value will be fixed to the default value `ON`, that is, [table partitioning](/partitioned-table.md) is enabled by default. |
Expand All @@ -292,6 +292,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
| [`tidb_schema_cache_size`](/system-variables.md#tidb_schema_cache_size-new-in-v800) | Modified | Changes the default value from `0` to `536870912` (512 MiB), indicating that this feature is enabled by default. The minimum value allowed is set to `67108864` (64 MiB). |
| [`tidb_auto_analyze_concurrency`](/system-variables.md#tidb_auto_analyze_concurrency-new-in-v840)| Newly added | Sets the concurrency within a single automatic statistics collection task. Before v8.4.0, this concurrency is fixed at `1`. To speed up statistics collection tasks, you can increase this concurrency based on your cluster's available resources. |
| [`tidb_enable_instance_plan_cache`](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840)| Newly added | Controls whether to enable the Instance Plan Cache feature. |
| [`tidb_enable_stats_owner`](/system-variables.md#tidb_enable_stats_owner-new-in-v840)| Newly added | Controls whether the corresponding TiDB instance can run automatic statistics update tasks. |
| [`tidb_hash_join_version`](/system-variables.md#tidb_hash_join_version-new-in-v840) | Newly added | Controls whether TiDB uses an optimized version of the Hash Join operator. The default value of `legacy` means that the optimized version is not used. If you set it to `optimized`, TiDB uses the optimized version of the Hash Join operator when executing it to improve Hash Join performance. |
| [`tidb_instance_plan_cache_max_size`](/system-variables.md#tidb_instance_plan_cache_max_size-new-in-v840) | Newly added | Sets the maximum memory usage for Instance Plan Cache. |
| [`tidb_instance_plan_cache_reserved_percentage`](/system-variables.md#tidb_instance_plan_cache_reserved_percentage-new-in-v840) | Newly added | Controls the percentage of idle memory reserved for Instance Plan Cache after memory eviction. |
Expand All @@ -305,6 +306,7 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
| -------- | -------- | -------- | -------- |
| TiDB | [`grpc-keepalive-time`](/tidb-configuration-file.md#grpc-keepalive-time) | Modified | Adds the minimum value of `1`. |
| TiDB | [`grpc-keepalive-timeout`](/tidb-configuration-file.md#grpc-keepalive-timeout) | Modified | Before v8.4.0, the data type of this parameter is INT, and the minimum value is `1`. Starting from v8.4.0, the data type is changed to FLOAT64, and the minimum value becomes `0.05`. In scenarios where network jitter occurs frequently, you can reduce the impact of network jitter on performance by setting a smaller value to shorten the retry interval. |
| TiDB | [`tidb_enable_stats_owner`](/tidb-configuration-file.md#tidb_enable_stats_owner-new-in-v840) | Newly added | Controls whether the corresponding TiDB instance can run automatic statistics update tasks. |
| TiKV | [`region-split-keys`](/tikv-configuration-file.md#region-split-keys) | Modified | Changes the default value from `"960000"` to `"2560000"`. |
| TiKV | [`region-split-size`](/tikv-configuration-file.md#region-split-size) | Modified | Changes the default value from `"96MiB"` to `"256MiB"`. |
| TiKV | [`sst-max-size`](/tikv-configuration-file.md#sst-max-size) | Modified | Changes the default value from `"144MiB"` to `"384MiB"`. |
Expand All @@ -325,8 +327,6 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
| BR | [`--master-key`](/br/br-pitr-manual.md#encrypt-the-log-backup-data) | Newly added | Specifies the master key for log backup data. It can be a master key stored on a local disk or a master key managed by a cloud Key Management Service (KMS). |
| BR | [`--master-key-crypter-method`](/br/br-pitr-manual.md#encrypt-the-log-backup-data) | Newly added | Specifies the encryption algorithm based on the master key for log backup data, which can be `aes128-ctr`, `aes192-ctr`, or `aes256-ctr`. The default value is `plaintext`, indicating that data is not encrypted. |

### System tables

## Offline package changes

Starting from v8.4.0, the following contents are removed from the `TiDB-community-toolkit` [binary package](/binary-package.md):
Expand Down Expand Up @@ -384,7 +384,7 @@ Starting from v8.4.0, the following contents are removed from the `TiDB-communit

- Increase the default value of Region from 96 MiB to 256 MiB to avoid the extra overhead caused by too many Regions [#17309](https://github.com/tikv/tikv/issues/17309) [LykxSassinator](https://github.com/LykxSassinator) **tw@hfxsd** <!--1925-->
- Support setting memory usage limits for in-memory pessimistic locks in a Region or TiKV instance. When hot write scenarios cause a large number of pessimistic locks, you can increase the memory limits via configuration. This helps avoid CPU and I/O overhead caused by pessimistic locks being written to disk. [#17542](https://github.com/tikv/tikv/issues/17542) @[cfzjywxk](https://github.com/cfzjywxk) **tw@Oreoxmt** <!--1967-->
- Introduce a new `spill-dir` configuration item in Raft Engine, supporting multi-disk storage for Raft logs; when the disk where the home directory (`dir`) is located runs out of space, the Raft Engine automatically writes new logs to `spill-dir`, ensuring continuous operation of the system [#17356](https://github.com/tikv/tikv/issues/17356) [LykxSassinator](https://github.com/LykxSassinator) **tw@hfxsd** <!--1970-->
- Introduce a new `spill-dir` configuration item in Raft Engine, supporting multi-disk storage for Raft logs; when the disk where the home directory (`dir`) is located runs out of space, the Raft Engine automatically writes new logs to `spill-dir`, ensuring continuous operation of the system [#17356](https://github.com/tikv/tikv/issues/17356) @[LykxSassinator](https://github.com/LykxSassinator) **tw@hfxsd** <!--1970-->
- (dup): release-6.5.11.md > Improvements> TiKV - - Optimize the compaction trigger mechanism of RocksDB to accelerate disk space reclamation when handling a large number of DELETE versions [#17269](https://github.com/tikv/tikv/issues/17269) @[AndreMouche](https://github.com/AndreMouche)
- Support dynamically modifying flow-control configurations for write operations [#17395](https://github.com/tikv/tikv/issues/17395) @[glorv](https://github.com/glorv)
- Improve the speed of Region Merge in scenarios with empty tables and small Regions [#17376](https://github.com/tikv/tikv/issues/17376) @[LykxSassinator](https://github.com/LykxSassinator)
Expand All @@ -395,7 +395,6 @@ Starting from v8.4.0, the following contents are removed from the `TiDB-communit
- Support graceful offline of TiKV nodes during data import by TiDB Lightning [#7853](https://github.com/tikv/pd/issues/7853) @[okJiang](https://github.com/okJiang) **tw@qiancai** <!--1852-->
- Rename `scatter-range` to `scatter-range-scheduler` in `pd-ctl` commands [#8379](https://github.com/tikv/pd/issues/8379) @[okJiang](https://github.com/okJiang)
- Add conflict detection for `grant-hot-leader-scheduler` [#4903](https://github.com/tikv/pd/issues/4903) @[lhy1024](https://github.com/lhy1024)
- The TSO client supports sending and receiving multiple TSO gRPC requests in parallel [#8432](https://github.com/tikv/pd/issues/8432) @[MyonKeminta](https://github.com/MyonKeminta)

+ TiFlash <!--tw@qiancai: 2 notes-->

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