adding The Fine Print documentation

README.md

@@ -54,7 +54,11 @@ More tips:
 - Use `--postpone-cut-over-flag-file` to gain control over cut-over timing
 - Get familiar with the [interactive commands](doc/interactive-commands.md)
 
-Also see [requirements and limitations](doc/requirements-and-limitations.md), [what if?](doc/what-if.md)
+Also see:
+
+- [requirements and limitations](doc/requirements-and-limitations.md)
+- [what if?](doc/what-if.md)
+- [the fine print](doc/the-fine-print.md)
 
 ## What's in a name?
 

doc/the-fine-print.md

@@ -0,0 +1,41 @@
+# The Fine Print: What are You Not Telling Me?
+
+Here are technical considerations you may be interested in. We write here things that are not an obvious [Requirements & Limitations](requirements-and-limitations.md)
+
+# Connecting to replica
+
+`gh-ost` prefers connecting to replica. If your master uses Statement Based Replication, this is a _requirement_.
+
+What does "connect to replica" mean?
+
+- `gh-ost` connects to the replica as a normal client
+- It additionally connects as a replica to the replica (pretends to be a MySQL replica itself)
+- It auto-detects master
+
+`gh-ost` reads the RBR binary logs from the replica, and applies events onto the master as tables are being migrated.
+
+THE FINE PRINT:
+
+- You trust the replica's binary logs to represent events applied on master.
+  If you don't trust the replica, if you suspect there's data drift between replica & master, take notice. If your master is RBR, do instead connect `gh-ost` to master, via `--allow-on-master` (see [cheatsheet](cheatsheet.md)).
+  Our take: we trust replica data; if master dies in production, we promote a replica. Our read serving is based on replica(s).
+
+- Replication needs to run.
+  This is an obvious, but worth stating. You cannot perform a migration with "connect to replica" if your replica lags. `gh-ost` will actually do all it can so that replication does not lag, and avoid critical operations at such time when replication does lag.
+
+# Network usage
+
+`gh-ost` reads binary logs and then applies them onto the migrated server.
+
+THE FINE PRINT:
+
+- `gh-ost` delivers more network traffic than other online-schema-change tools, that let MySQL handle all data transfer internally. This is part of the [triggerless design](triggerless-design.md).
+  Our take: we deal with cross-DC migration traffic and this is working well for us.
+
+# Impersonating as a replica
+
+`gh-ost` impersonates as a replica: connects to a MySQL server, says "oh hey, I'm a replica, please send me binary logs kthx".
+
+THE FINE PRINT:
+
+- `SHOW SLAVE HOSTS` or `SHOW PROCESSLIST` will list down this strange "replica" that you can't really connect to.