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TiFlash v6.2 Upgrade Guide

This document describes the functional changes in TiFlash modules you need to pay attention to when you upgrade TiFlash from earlier versions to v6.2, and recommended actions for you to take.

To learn the standard upgrade process, see the following documents:

Note

FastScan is introduced in v6.2.0 as an experimental feature and becomes generally available (GA) in v7.0.0. It provides more efficient query performance at the cost of strong data consistency.
It is not recommended that you upgrade TiDB that includes TiFlash across major versions, for example, from v4.x to v6.x. Instead, you need to upgrade from v4.x to v5.x first, and then to v6.x.
v4.x is near the end of its life cycle. It is recommended that you upgrade to v5.x or later as soon as possible. For more information, see TiDB Release Support Policy.
PingCAP does not provide bug fixes for non-LTS versions, such as v6.0. It is recommended that you upgrade to v6.1 and later LTS versions whenever possible.
To upgrade TiFlash from versions earlier than v5.3.0 to v5.3.0 or later, you should stop TiFlash and then upgrade it. The following steps help you upgrade TiFlash without interrupting other components:
- Stop the TiFlash instance: tiup cluster stop <cluster-name> -R tiflash
- Upgrade the TiDB cluster without restarting it (only updating the files): tiup cluster upgrade <cluster-name> <version> --offline, such as tiup cluster upgrade <cluster-name> v5.3.0 --offline
- Reload the TiDB cluster: tiup cluster reload <cluster-name>. After the reload, the TiFlash instance is started and you do not need to manually start it.

From 5.x or v6.0 to v6.1

When you upgrade TiFlash from v5.x or v6.0 to v6.1, pay attention to the functional changes in TiFlash Proxy and dynamic pruning.

TiFlash Proxy

TiFlash Proxy is upgraded in v6.1.0 (aligned with TiKV v6.0.0). The new version has upgraded the RocksDB version. After you upgrade TiFlash to v6.1, the data format is converted to the new version automatically.

In regular upgrades, the data conversion does not involve any risks. However, if you need to downgrade TiFlash from v6.1 to any earlier version in special scenarios (for example, testing or verification scenarios), the earlier version might fail to parse the new RocksDB configuration. As as result, TiFlash will fail to restart. It is recommended that you fully test and verify the upgrade process and prepare an emergency plan.

Workaround for downgrading TiFlash in testing or other special scenarios

You can forcibly scale in the target TiFlash node and then replicate data from TiKV again. For detailed steps, see Scale in a TiFlash cluster.

Dynamic pruning

If you do not enable dynamic pruning mode and will not use it in the future, you can skip this section.

Newly installed TiDB v6.1.0: Dynamic pruning is enabled by default.
TiDB v6.0 and earlier: Dynamic pruning is disabled by default. The setting of dynamic pruning after an upgrade inherits that of the previous version. That is, dynamic pruning will not be enabled (or disabled) automatically after an upgrade.
After an upgrade, to enable dynamic pruning, set tidb_partition_prune_mode to dynamic and manually update GlobalStats of partitioned tables. For details, see Dynamic pruning mode.

From v5.x or v6.0 to v6.2

In TiDB v6.2, TiFlash upgrades its data storage format to the V3 version. Therefore, when you upgrade TiFlash from v5.x or v6.0 to v6.2, besides functional changes in TiFlash Proxy and Dynamic pruning, you also need to pay attention to the functional change in PageStorage.

PageStorage

By default, TiFlash v6.2.0 uses PageStorage V3 version format_version = 4. This new data format significantly reduces the peak write I/O traffic. In scenarios with high update traffic and high concurrency or heavy queries, it effectively relieves excessive CPU usage caused by TiFlash data GC. Meanwhile, compared with the earlier storage format, the V3 version significantly reduces space amplification and resource consumption.

After an upgrade to v6.2.0, as new data is written to the existing TiFlash nodes, earlier data will be gradually converted to the new format.
However, earlier data cannot be completely converted to the new format during the upgrade, because the conversion consumes a certain amount of system overhead (services are not affected, but you still need to pay attention). After the upgrade, it is recommended that you run the Compact command to convert the data to the new format. The steps are as follows:
1. Run the following command for each table containing TiFlash replicas:
```
ALTER TABLE <table_name> COMPACT tiflash replica;
```
2. Restart the TiFlash node.

You can check whether tables still use the old data format on Grafana: TiFlash-Summary > Storage Pool > Storage Pool Run Mode.

Only V2: Number of tables using PageStorage V2 (including partitions)
Only V3: Number of tables using PageStorage V3 (including partitions)
Mix Mode: Number of tables with data format converted from PageStorage V2 to PageStorage V3 (including partitions)

Workaround for downgrading TiFlash in testing or other special scenarios

You can forcibly scale in the target TiFlash node and then replicate data from TiKV again. For detailed steps, see Scale in a TiFlash cluster.

From v6.1 to v6.2

When you upgrade TiFlash from v6.1 to v6.2, pay attention to the change in data storage format. For details, see PageStorage.