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faq/tidb.md: update the wording about transaction size #1876

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faq/tidb.md: update the wording about transaction size
tiancaiamao Feb 26, 2020
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TomShawn Mar 3, 2020
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10 changes: 2 additions & 8 deletions faq/tidb.md
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Expand Up @@ -88,10 +88,6 @@ The character sets of TiDB use UTF-8 by default and currently only support UTF-8

32 characters at most.

#### What is the maximum number of statements in a transaction?

5000 at most.

#### Does TiDB support XA?

No. The JDBC driver of TiDB is MySQL JDBC (Connector/J). When using Atomikos, set the data source to `type="com.mysql.jdbc.jdbc2.optional.MysqlXADataSource"`. TiDB does not support the connection with MySQL JDBC XADataSource. MySQL JDBC XADataSource only works for MySQL (for example, using DML to modify the `redo` log).
Expand Down Expand Up @@ -930,11 +926,9 @@ The total read capacity has no limit. You can increase the read capacity by addi

#### The error message `transaction too large` is displayed

As distributed transactions need to conduct two-phase commit and the bottom layer performs Raft replication, if a transaction is very large, the commit process would be quite slow and the following Raft replication flow is thus struck. To avoid this problem, we limit the transaction size:
Due to the limitation of the underlying storage engine, each key-value entry (one row) in TiDB should be no more than 6MB.

- A transaction is limited to 5000 SQL statements (by default)
- Each Key-Value entry is no more than 6MB
- The total size of Key-Value entry is no more than 100MB
Distributed transactions need two-phase commit and the underlying implementation performs the Raft replication. If a transaction is very large, the commit process would be quite slow and this transaction is more likely to conflict with other transactions. Moreover, the rollback of a failed transaction leads to an unnecessary performance penalty. To avoid these problems, we limit the total size of key-value entries to no more than 100MB in a transaction by default. If you need larger transactions, change `txn-total-size-limit` in the TiDB configuration file. The maximum value of this configuration item is up to `10G`. The actual limitation might also depend on the hardware of the RAM.

There are [similar limits](https://cloud.google.com/spanner/docs/limits) on Google Cloud Spanner.

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