integrate tiflash faq into tidb-faq.md and add aliases; add one lightning faq (#3069) (#3080)

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

Author: ti-srebot

TOC.md

@@ -406,7 +406,6 @@
     + TiFlash
       + [Overview](/tiflash/tiflash-overview.md)
       + [Use TiFlash](/tiflash/use-tiflash.md)
-      + [FAQ](/tiflash/tiflash-faq.md)
   + TiUP
     + [Overview](/tiup/tiup-overview.md)
     + [Manage TiUP Components](/tiup/manage-tiup-component.md)

faq/tidb-faq.md

@@ -2,7 +2,7 @@
 title: TiDB FAQ
 summary: Learn about the most frequently asked questions (FAQs) relating to TiDB.
 category: faq
-aliases: ['/docs/stable/faq/tidb-faq/','/docs/v4.0/faq/tidb-faq/','/docs/stable/faq/tidb/']
+aliases: ['/docs/stable/faq/tidb-faq/','/docs/v4.0/faq/tidb-faq/','/docs/stable/faq/tidb/','/docs/stable/tiflash/tiflash-faq/','/docs/stable/reference/tiflash/faq/','/tidb/v4.0/tiflash-faq']
 ---
 
 # TiDB FAQ
@@ -730,6 +730,28 @@ The memory usage of TiKV mainly comes from the block-cache of RocksDB, which is
 
 No. TiDB (or data created from the transactional API) relies on a specific key format. It is not compatible with data created from RawKV API (or data from other RawKV-based services).
 
+### Manage the TiFlash server
+
+#### Does TiFlash support direct writes?
+
+Currently, TiFlash does not support direct writes. You can only write data to TiKV, and then replicate the data to TiFlash.
+
+#### How can I estimate the storage resources if I want to add TiFlash to an existing cluster?
+
+You can evaluate which tables might require acceleration. The size of a single replica of these tables data is roughly equal to the storage resources required by two replicas of TiFlash. Note that you need to take into account the free space required.
+
+#### How can data in TiFlash be highly available?
+
+TiFlash restores data through TiKV. As long as the corresponding Regions in TiKV are available, TiFlash can restore data from these Regions.
+
+#### How many replicas are recommended for TiFlash?
+
+If you need highly available TiFlash services (rather than highly available data), it is recommended to set up two replicas for TiFlash. If you allow TiKV replicas to provide analytical services when TiFlash is down, you can set up a single TiFlash replica.
+
+#### Should I use TiSpark or TiDB server for a query?
+
+It is recommended to use TiDB server if you query a single table mainly using filtering and aggregation, because the TiDB server has better performance on the columnar storage. It is recommended to use TiSpark if you query a table mainly using joins.
+
 ### TiDB test
 
 #### What is the performance test result for TiDB using Sysbench?

troubleshoot-tidb-lightning.md

@@ -141,3 +141,11 @@ See the [Checkpoints control](/tidb-lightning/tidb-lightning-checkpoints.md#chec
 3. Ensure the entire cluster is using the same and latest version of `tzdata` (version 2018i or above).
 
     On CentOS, run `yum info tzdata` to check the installed version and whether there is an update. Run `yum upgrade tzdata` to upgrade the package.
+
+## `[Error 8025: entry too large, the max entry size is 6291456]`
+
+**Cause**: A single row of key-value pairs generated by TiDB Lightning exceeds the limit set by TiDB.
+
+**Solution**:
+
+Currently, the limitation of TiDB cannot be bypassed. You can only ignore this table to ensure the successful import of other tables.