Expand JOB_TYPE documentation for ADMIN SHOW DDL... output

sql-statements/sql-statement-admin-show-ddl.md

@@ -51,7 +51,10 @@ The `ADMIN SHOW DDL JOBS` statement is used to view all the results in the curre
 - `JOB_ID`: each DDL operation corresponds to a DDL job. `JOB_ID` is globally unique.
 - `DB_NAME`: the name of the database where the DDL operation is performed.
 - `TABLE_NAME`: the name of the table where the DDL operation is performed.
-- `JOB_TYPE`: the type of DDL operation.
+- `JOB_TYPE`: the type of DDL operation. Common job types include the following:
+    - `ingest`: Ingestion with accelerated index backfilling as configured by [`tidb_ddl_enable_fast_reorg`](/system-variables.md#tidb_ddl_enable_fast_reorg-new-in-v630).
+    - `txn`: Basic transactional backfill.
+    - `txn-merge`: Transactional backfill with a temporary index that gets merged with the original index when the backfill is finished.
 - `SCHEMA_STATE`: the current state of the schema object that the DDL operates on. If `JOB_TYPE` is `ADD INDEX`, it is the state of the index; if `JOB_TYPE` is `ADD COLUMN`, it is the state of the column; if `JOB_TYPE` is `CREATE TABLE`, it is the state of the table. Common states include the following:
     - `none`: indicates that it does not exist. Generally, after the `DROP` operation or after the `CREATE` operation fails and rolls back, it will become the `none` state.
     - `delete only`, `write only`, `delete reorganization`, `write reorganization`: these four states are intermediate states. For their specific meanings, see [How the Online DDL Asynchronous Change Works in TiDB](/ddl-introduction.md#how-the-online-ddl-asynchronous-change-works-in-tidb). As the intermediate state conversion is fast, these states are generally not visible during operation. Only when performing `ADD INDEX` operation can the `write reorganization` state be seen, indicating that index data is being added.

system-variables.md

@@ -1330,7 +1330,9 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 
 > **Note:**
 >
-> Before you upgrade TiDB to v6.5.0 or later, it is recommended that you check whether the [`temp-dir`](/tidb-configuration-file.md#temp-dir-new-in-v630) path of TiDB is correctly mounted to an SSD disk. This path is a TiDB configuration item, which takes effect after TiDB is restarted. Therefore, setting this configuration item before upgrading can avoid another restart.
+> * Before you upgrade TiDB to v6.5.0 or later, it is recommended that you check whether the [`temp-dir`](/tidb-configuration-file.md#temp-dir-new-in-v630) path of TiDB is correctly mounted to an SSD disk. This path is a TiDB configuration item, which takes effect after TiDB is restarted. Therefore, setting this configuration item before upgrading can avoid another restart.
+>
+> * Index acceleration requires a [`temp-dir`](/tidb-configuration-file.md#temp-dir-new-in-v630) that is writable and has enough free space. If the `temp-dir` is unusable, TiDB falls back to non-accelerated index building. It is recommended to put the `temp-dir` on a SSD disk.
 
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