By default, `gh-ost` would like you to connect to a replica, from where it figures out the master by itself. This wiring is required should your master execute using `binlog_format=STATEMENT`.
If, for some reason, you do not wish `gh-ost` to connect to a replica, you may connect it directly to the master and approve this via `--allow-on-master`.
When your migration issues a column rename (`change column old_name new_name ...`) `gh-ost` analyzes the statement to try an associate the old column name with new column name. Otherwise the new structure may also look like some column was dropped and another was added.
`gh-ost` will print out what it thinks the _rename_ implied, but will not issue the migration unless you provide with `--approve-renamed-columns`.
If you think `gh-ost` is mistaken and that there's actually no _rename_ involved, you may pass `--skip-renamed-columns` instead. This will cause `gh-ost` to disassociate the column values; data will not be copied between those columns.
`gh-ost` infers the identity of the master server by crawling up the replication topology. You may explicitly tell `gh-ost` the identity of the master host via `--assume-master-host=the.master.com`. This is useful in:
- master-master topologies (together with `--allow-master-master`), where `gh-ost` can arbitrarily pick one of the co-master and you prefer that it picks a specific one
- _tungsten replicator_ topologies (together with `--tungsten`), where `gh-ost` is unable to crawl and detect the master
If you happen to _know_ your servers use RBR (Row Based Replication, i.e. `binlog_format=ROW`), you may specify `--assume-rbr`. This skips a verification step where `gh-ost` would issue a `STOP SLAVE; START SLAVE`.
Skipping this step means `gh-ost` would not need the `SUPER` privilege in order to operate.
`--critical-load` defines a threshold that, when met, `gh-ost` panics and bails out. The default behavior is to bail out immediately when meeting this threshold.
This may sometimes lead to migrations bailing out on a very short spike, that, while in itself is impacting production and is worth investigating, isn't reason enough to kill a 10 hour migration.
When `--critical-load-interval-millis` is specified (e.g. `--critical-load-interval-millis=2500`), `gh-ost` gives a second chance: when it meets `critical-load` threshold, it doesn't bail out. Instead, it starts a timer (in this example: `2.5` seconds) and re-checks `critical-load` when the timer expires. If `critical-load` is met again, `gh-ost` panics and bails out. If not, execution continues.
This is somewhat similar to a Nagios `n`-times test, where `n` in our case is always `2`.
**Danger**: this flag will _silently_ discard any foreign keys existing on your table.
At this time (10-2016) `gh-ost` does not support foreign keys on migrated tables (it bails out when it notices a FK on the migrated table). However, it is able to support _dropping_ of foreign keys via this flag. If you're trying to get rid of foreign keys in your environment, this is a useful flag.
`gh-ost` reads event from the binary log and applies them onto the _ghost_ table. It does so in batched writes: grouping multiple events to apply in a single transaction. This gives better write throughput as we don't need to sync the transaction log to disk for each event.
The `--dml-batch-size` flag controls the size of the batched write. Allowed values are `1 - 100`, where `1` means no batching (every event from the binary log is applied onto the _ghost_ table on its own transaction). Default value is `10`.
Why is this behavior configurable? Different workloads have different characteristics. Some workloads have very large writes, such that aggregating even `50` writes into a transaction makes for a significant transaction size. On other workloads write rate is high such that one just can't allow for a hundred more syncs to disk per second. The default value of `10` is a modest compromise that should probably work very well for most workloads. Your mileage may vary.
Noteworthy is that setting `--dml-batch-size` to higher value _does not_ mean `gh-ost` blocks or waits on writes. The batch size is an upper limit on transaction size, not a minimal one. If `gh-ost` doesn't have "enough" events in the pipe, it does not wait on the binary log, it just writes what it already has. This conveniently suggests that if write load is light enough for `gh-ost` to only see a few events in the binary log at a given time, then it is also light neough for `gh-ost` to apply a fraction of the batch size.
A `gh-ost` execution need to copy whatever rows you have in your existing table onto the ghost table. This can, and often be, a large number. Exactly what that number is?
`gh-ost` initially estimates the number of rows in your table by issuing an `explain select * from your_table`. This will use statistics on your table and return with a rough estimate. How rough? It might go as low as half or as high as double the actual number of rows in your table. This is the same method as used in [`pt-online-schema-change`](https://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html).
`gh-ost` also supports the `--exact-rowcount` flag. When this flag is given, two things happen:
- An initial, authoritative `select count(*) from your_table`.
This query may take a long time to complete, but is performed before we begin the massive operations.
- A continuous update to the estimate as we make progress applying events.
We heuristically update the number of rows based on the queries we process from the binlogs.
While the ongoing estimated number of rows is still heuristic, it's almost exact, such that the reported [ETA](understanding-output.md) or percentage progress is typically accurate to the second throughout a multiple-hour operation.
`gh-ost` maintains two tables while migrating: the _ghost_ table (which is synced from your original table and finally replaces it) and a changelog table, which is used internally for bookkeeping. By default, it panics and aborts if it sees those tables upon startup. Provide `--initially-drop-ghost-table` and `--initially-drop-old-table` to let `gh-ost` know it's OK to drop them beforehand.
We think `gh-ost` should not take chances or make assumptions about the user's tables. Dropping tables can be a dangerous, locking operation. We let the user explicitly approve such operations.
On a replication topology, this is perhaps the most important migration throttling factor: the maximum lag allowed for migration to work. If lag exceeds this value, migration throttles.
When using [Connect to replica, migrate on master](cheatsheet.md), this lag is primarily tested on the very replica `gh-ost` operates on. Lag is measured by checking the heartbeat events injected by `gh-ost` itself on the utility changelog table. That is, to measure this replica's lag, `gh-ost` doesn't need to issue `show slave status` nor have any external heartbeat mechanism.
When `--throttle-control-replicas` is provided, throttling also considers lag on specified hosts. Lag measurements on listed hosts is done by querying `gh-ost`'s _changelog_ table, where `gh-ost` injects a heartbeat.
Typically `gh-ost` is used to migrate tables on a master. If you wish to only perform the migration in full on a replica, connect `gh-ost` to said replica and pass `--migrate-on-replica`. `gh-ost` will briefly connect to the master but other issue no changes on the master. Migration will be fully executed on the replica, while making sure to maintain a small replication lag.
By default `gh-ost` verifies no foreign keys exist on the migrated table. On servers with large number of tables this check can take a long time. If you're absolutely certain no foreign keys exist (table does not referenece other table nor is referenced by other tables) and wish to save the check time, provide with `--skip-foreign-key-checks`.
Issue the migration on a replica; do not modify data on master. Useful for validating, testing and benchmarking. See [testing-on-replica](testing-on-replica.md)
Provide a command delimited list of replicas; `gh-ost` will throttle when any of the given replicas lag beyond `--max-lag-millis`. The list can be queried and updated dynamically via [interactive commands](interactive-commands.md)
### throttle-http
Provide a HTTP endpoint; `gh-ost` will issue `HEAD` requests on given URL and throttle whenever response status code is not `200`. The URL can be queried and updated dynamically via [interactive commands](interactive-commands.md). Empty URL disables the HTTP check.
Makes the _old_ table include a timestamp value. The _old_ table is what the original table is renamed to at the end of a successful migration. For example, if the table is `gh_ost_test`, then the _old_ table would normally be `_gh_ost_test_del`. With `--timestamp-old-table` it would be, for example, `_gh_ost_test_20170221103147_del`.
