update TiDB 6MB limit

best-practices/tidb-best-practices.md

@@ -53,7 +53,7 @@ TiDB provides complete distributed transactions and the model has some optimizat
     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 process is thus stuck. To avoid this problem, the transaction size is limited:
 
     - A transaction is limited to 5,000 SQL statements (by default)
-    - Each Key-Value entry is no more than 6 MB
+    - Each Key-Value entry is no more than 6 MB (by default)
     - The total size of Key-Value entries is no more than 10 GB.
 
     You can find similar limits in [Google Cloud Spanner](https://cloud.google.com/spanner/quotas).

error-codes.md

@@ -88,9 +88,9 @@ TiDB is compatible with the error codes in MySQL, and in most cases returns the
 
 * Error Number: 8025
 
-    The single Key-Value pair being written is too large. The largest single Key-Value pair supported in TiDB is 6 MB.
+    The single Key-Value pair being written is too large. The largest single Key-Value pair supported in TiDB is 6 MB by default.
 
-    If a pair exceeds this limit, you need to manually deal with this row of data to meet the 6 MB limit.
+    If a pair exceeds this limit, you need to properly adjust the [`txn-entry-size-limit`](/tidb-configuration-file.md#txn-entry-size-limit-new-in-v50) configuration value to relax the limit.
 
 * Error Number: 8026
 

faq/migration-tidb-faq.md

@@ -138,7 +138,7 @@ The total read capacity has no limit. You can increase the read capacity by addi
 
 ### The error message `transaction too large` is displayed
 
-Due to the limitation of the underlying storage engine, each key-value entry (one row) in TiDB should be no more than 6MB.
+Due to the limitation of the underlying storage engine, each key-value entry (one row) in TiDB should be no more than 6MB. You can adjust the [`txn-entry-size-limit`](/tidb-configuration-file.md#txn-entry-size-limit-new-in-v50) configuration value up to 120MB.
 
 Distributed transactions need two-phase commit and the bottom layer performs the Raft replication. If a transaction is very large, the commit process would be quite slow and the write conflict is more likely to occur. 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, modify the value of `txn-total-size-limit` in the TiDB configuration file. The maximum value of this configuration item is up to 10G. The actual limitation is also affected by the physical memory of the machine.
 

storage-engine/titan-overview.md

@@ -27,7 +27,7 @@ Titan is suitable for the scenarios where a huge volume of data is written to th
 
 The prerequisites for enabling Titan are as follows:
 
-- The average size of values is large, or the size of all large values accounts for much of the total value size. Currently, the size of a value greater than 1 KB is considered as a large value. In some situations, this number (1 KB) can be 512 B. Note that a single value written to TiKV cannot exceed 6 MB due to the limitation of the TiKV Raft layer.
+- The average size of values is large, or the size of all large values accounts for much of the total value size. Currently, the size of a value greater than 1 KB is considered as a large value. In some situations, this number (1 KB) can be 512 B. Note that a single value written to TiKV cannot exceed 8 MB due to the limitation of the TiKV Raft layer. You can adjust the [`raft-entry-max-size`](/tikv-configuration-file.md#raft-entry-max-size) configuration value to relax the limit.
 - No range query will be performed or you do not need a high performance of range query. Because the data stored in Titan is not well-ordered, its performance of range query is poorer than that of RocksDB, especially for the query of a large range. According PingCAP's internal test, Titan's range query performance is 40% to a few times lower than that of RocksDB.
 - Sufficient disk space, because Titan reduces write amplification at the cost of disk space. In addition, Titan compresses values one by one, and its compression rate is lower than that of RocksDB. RocksDB compresses blocks one by one. Therefore, Titan consumes more storage space than RocksDB, which is expected and normal. In some situations, Titan's storage consumption can be twice that of RocksDB.
 

tidb-limitations.md

@@ -51,7 +51,7 @@ This document describes the common usage limitations of TiDB, including the maxi
 
 | Type       | Upper limit   |
 |:----------|:----------|
-| Size       | 6 MB       |
+| Size       | 6 MB by default. You can adjust the size limit via the [`txn-entry-size-limit`](/tidb-configuration-file.md#txn-entry-size-limit-new-in-v50) configuration item. |
 
 ## Limitation on a single column