53 lines
3.3 KiB
Markdown
53 lines
3.3 KiB
Markdown
# Requirements and limitations
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### Requirements
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- 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).
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- If you are using a replica, the table must have an identical schema between the master and replica.
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- `gh-ost` requires an account with these privileges:
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- `ALTER, CREATE, DELETE, DROP, INDEX, INSERT, LOCK TABLES, SELECT, TRIGGER, UPDATE` on the database (schema) where your migrated table is, or of course on `*.*`
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- either:
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- `SUPER, REPLICATION SLAVE` on `*.*`, or:
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- `REPLICATION CLIENT, REPLICATION SLAVE` on `*.*`
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The `SUPER` privilege is required for `STOP SLAVE`, `START SLAVE` operations. These are used on:
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- Switching your `binlog_format` to `ROW`, in the case where it is _not_ `ROW` and you explicitly specified `--switch-to-rbr`
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- If your replication is already in RBR (`binlog_format=ROW`) you can specify `--assume-rbr` to avoid the `STOP SLAVE/START SLAVE` operations, hence no need for `SUPER`.
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- Running `--test-on-replica`: before the cut-over phase, `gh-ost` stops replication so that you can compare the two tables and satisfy that the migration is sound.
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### Limitations
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- Foreign keys not supported. They may be supported in the future, to some extent.
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- Triggers are not supported. They may be supported in the future.
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- MySQL 5.7 generated columns are not supported. They may be supported in the future.
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- MySQL 5.7 `JSON` columns are not supported. They are likely to be supported shortly.
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- The two _before_ & _after_ tables must share a `PRIMARY KEY` or other `UNIQUE KEY`. This key will be used by `gh-ost` to iterate through the table rows when copying. [Read more](shared-key.md)
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- The migration key must not include columns with NULL values. This means either:
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1. The columns are `NOT NULL`, or
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2. The columns are nullable but don't contain any NULL values.
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- by default, `gh-ost` will not run if the only `UNIQUE KEY` includes nullable columns.
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- You may override this via `--allow-nullable-unique-key` but make sure there are no actual `NULL` values in those columns. Existing NULL values can't guarantee data integrity on the migrated table.
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- It is not allowed to migrate a table where another table exists with same name and different upper/lower case.
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- For example, you may not migrate `MyTable` if another table called `MYtable` exists in the same schema.
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- Amazon RDS works, but has it's own [limitations](rds.md).
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- Google Cloud SQL is currently not supported
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- Multisource is not supported when migrating via replica. It _should_ work (but never tested) when connecting directly to master (`--allow-on-master`)
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- Master-master setup is only supported in active-passive setup. Active-active (where table is being written to on both masters concurrently) is unsupported. It may be supported in the future.
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- If you have en `enum` field as part of your migration key (typically the `PRIMARY KEY`), migration performance will be degraded and potentially bad. [Read more](https://github.com/github/gh-ost/pull/277#issuecomment-254811520)
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- Migrating a `FEDERATED` table is unsupported and is irrelevant to the problem `gh-ost` tackles.
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