Remove support for MySQL 5.6 (end-of-life Feb 2021)

.github/workflows/replica-tests.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        version: [mysql-5.6.43,mysql-5.7.25,mysql-8.0.16]
+        version: [mysql-5.7.25,mysql-8.0.16]
 
     steps:
     - uses: actions/checkout@v2

doc/requirements-and-limitations.md

@@ -2,7 +2,7 @@
 
 ### Requirements
 
-- `gh-ost` currently requires MySQL versions 5.6 and greater.
+- `gh-ost` currently requires MySQL versions 5.7 and greater.
 
 - You will need to have one server serving Row Based Replication (RBR) format binary logs. Right now `FULL` row image is supported. `MINIMAL` to be supported in the near future. `gh-ost` prefers to work with replicas. You may [still have your master configured with Statement Based Replication](migrating-with-sbr.md) (SBR).
 

doc/throttle.md

@@ -38,7 +38,7 @@ Note that you may dynamically change both `--max-lag-millis` and the `throttle-c
 
   `--max-load='Threads_running=100,Threads_connected=500'`
 
-  Metrics must be valid, numeric [status variables](http://dev.mysql.com/doc/refman/5.6/en/server-status-variables.html)
+  Metrics must be valid, numeric [status variables](https://dev.mysql.com/doc/refman/5.7/en/server-status-variables.html)
 
 #### Throttle query
 
@@ -97,7 +97,7 @@ Copy: 0/2915 0.0%; Applied: 0; Backlog: 0/100; Elapsed: 42s(copy), 42s(total); s
 
 Throttling time is limited by the availability of the binary logs. When throttling begins, `gh-ost` suspends reading the binary logs, and expects to resume reading from same binary log where it paused.
 
-Your availability of binary logs is typically determined by the [expire_logs_days](https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_expire_logs_days) variable. If you have `expire_logs_days = 10` (or check `select @@global.expire_logs_days`), then you should be able to throttle for up to `10` days.
+Your availability of binary logs is typically determined by the [expire_logs_days](https://dev.mysql.com/doc/refman/5.7/en/replication-options-binary-log.html#sysvar_expire_logs_days) variable. If you have `expire_logs_days = 10` (or check `select @@global.expire_logs_days`), then you should be able to throttle for up to `10` days.
 
 Having said that, throttling for so long is far fetching, in that the `gh-ost` process itself must be kept alive during that time; and the amount of binary logs to process once it resumes will potentially take days to replay.
 

doc/why-triggerless.md

@@ -7,7 +7,7 @@ Existing MySQL schema migration tools:
 - [LHM](https://github.com/soundcloud/lhm)?target=https://github.com
 - [oak-online-alter-table](https://github.com/shlomi-noach/openarkkit)?target=https://github.com
 
-are all using [triggers](http://dev.mysql.com/doc/refman/5.6/en/triggers.html) to propagate live changes on your table onto a ghost/shadow table that is slowly being synchronized. The tools not all work the same: while most use a synchronous approach (all changes applied on the ghost table), the Facebook tool uses an asynchronous approach (changes are appended to a changelog table, later reviewed and applied on ghost table).
+are all using [triggers](https://dev.mysql.com/doc/refman/5.7/en/triggers.html) to propagate live changes on your table onto a ghost/shadow table that is slowly being synchronized. The tools not all work the same: while most use a synchronous approach (all changes applied on the ghost table), the Facebook tool uses an asynchronous approach (changes are appended to a changelog table, later reviewed and applied on ghost table).
 
 Use of triggers simplifies a lot of the flow in doing a live table migration, but also poses some limitations or difficulties. Here are reasons why we choose to [design a triggerless solution](triggerless-design.md) to schema migrations.