Both Percona and Maria allow MySQL to be configured to listen on an extra port when their thread pool is enable.
* https://www.percona.com/doc/percona-server/5.7/performance/threadpool.html
* https://mariadb.com/kb/en/the-mariadb-library/thread-pool-in-mariadb-51-53/
This is valuable because if the table has a lot of traffic (read or write load), gh-ost can end up starving the thread pool as incomming connections are immediately blocked.
By using gh-ost on the extra port, MySQL locking will still behave the same, but MySQL will keep a dedicated thread for each gh-ost connection.
When doing this, it's important to inspect the extra-max-connections variable. Both Percona and Maria default to 1, so gh-ost may easily exceed with its threads.
An example local run using this
```
$ mysql -S /tmp/mysql_sandbox20393.sock -e "select @@global.port, @@global.extra_port"
+---------------+---------------------+
| @@global.port | @@global.extra_port |
+---------------+---------------------+
| 20393 | 30393 |
+---------------+---------------------+
./bin/gh-ost \
--initially-drop-ghost-table \
--initially-drop-old-table \
--assume-rbr \
--port="20395" \
--assume-master-host="127.0.0.1:30393" \
--max-load=Threads_running=25 \
--critical-load=Threads_running=1000 \
--chunk-size=1000 \
--max-lag-millis=1500 \
--user="gh-ost" \
--password="gh-ost" \
--database="test" \
--table="mytable" \
--verbose \
--alter="ADD mynewcol decimal(11,2) DEFAULT 0.0 NOT NULL" \
--exact-rowcount \
--concurrent-rowcount \
--default-retries=120 \
--panic-flag-file=/tmp/ghost.panic.flag \
--postpone-cut-over-flag-file=/tmp/ghost.postpone.flag \
--execute
```
usage example:
```
gh-ost ... --force-table-names=migration_tbl
```
The force-table-names flag will receive a table name and use that name
when creating the _del, _gho, _ghc tables.
For instance for that example above, the tables would be:
_migration_tbl_del
_migration_tbl_gho
_migration_tbl_ghc
[fixes#399]
when throttling on user command there really is no need for injecting heartbeat. The user commanded, therefore gh-ost complies and trusts the reasoning for throttling. What this will allow is complete quiet time. This, in turn, will allow such features as relocating via orchestrator/pseudo-gtid at time of throttling
this optional flag gives --critical-load a second chance. When configured to positive value, meeting with critical-load spawns a timer. When this timer expires a second check for critical-load is made; if still met, gh-ost bails out.
By default the interval is zero, in which case gh-ost bails out immediately on meeting critical load.
This flag makes migration silently and happily discard any existing foreign keys on migrated table. This is useful for intentional dropping of foreign keys, as gh-ost does not otherwise have support for foreign key migration.
At some time in the future gh-ost may support foreign key migration, at which time this flag will be removed
