ticdc: refine faq doc (#20547) (#20617)

Author: ti-chi-bot

ticdc/ticdc-faq.md

@@ -292,7 +292,15 @@ For more information, refer to [Open protocol Row Changed Event format](/ticdc/t
 
 ## How much PD storage does TiCDC use?
 
-TiCDC uses etcd in PD to store and regularly update the metadata. Because the time interval between the MVCC of etcd and PD's default compaction is one hour, the amount of PD storage that TiCDC uses is proportional to the amount of metadata versions generated within this hour. However, in v4.0.5, v4.0.6, and v4.0.7, TiCDC has a problem of frequent writing, so if there are 1000 tables created or scheduled in an hour, it then takes up all the etcd storage and returns the `etcdserver: mvcc: database space exceeded` error. You need to clean up the etcd storage after getting this error. See [etcd maintenance space-quota](https://etcd.io/docs/v3.4.0/op-guide/maintenance/#space-quota) for details. It is recommended to upgrade your cluster to v4.0.9 or later versions.
+When using TiCDC, you might encounter the `etcdserver: mvcc: database space exceeded` error, which is primarily related to the mechanism that TiCDC uses etcd in PD to store metadata.
+
+etcd uses Multi-Version Concurrency Control (MVCC) to store data, and the default compaction interval in PD is 1 hour. This means that etcd retains multiple versions of all data for 1 hour before compaction.
+
+Before v6.0.0, TiCDC uses etcd in PD to store and update metadata for all tables in a changefeed. Therefore, the PD storage space used by TiCDC is proportional to the number of tables being replicated by the changefeed. When TiCDC is replicating a large number of tables, the etcd storage space could fill up quickly, increasing the probability of the `etcdserver: mvcc: database space exceeded` error.
+
+If you encounter this error, refer to [etcd maintenance space-quota](https://etcd.io/docs/v3.4.0/op-guide/maintenance/#space-quota) to clean up the etcd storage space.
+
+Starting from v6.0.0, TiCDC optimizes its metadata storage mechanism, effectively avoiding the etcd storage space issues caused by the preceding reasons. If your TiCDC version is earlier than v6.0.0, it is recommended to upgrade to v6.0.0 or later versions.
 
 ## Does TiCDC support replicating large transactions? Is there any risk?