Support for Fake Changes

Replication is single threaded in nature, and it’s important from the standpoint of performance to make sure that the queries executed by the replication thread or the events applied should be executed as fast as possible. A single event taking too long to apply is going to cause entire replication to stall, slowing down the rate at which replication catches up. This is especially painful when the slave server is restarted because with cold buffer pool individual events take far too long to complete. The slave is also generally I/O bound because of the difference of workload on master and the slave, and the biggest problem with single replication thread is that it has to read data to execute queries and most of the time is spent reading data then actually updating it.

Concept of Replication Prefetching

The process can be sped up by having prefetch threads to warm the server: replay statements and then rollback at commit. Prefetching works on a simple principle that if the data needed by the slave to apply events is already read then the application of events will be very fast as the data would already be cached. Replication is made up of two independent processes, an I/O thread that receives events from the master and writes to the relay log, and a SQL thread that reads the relay logs and applies those events. If the events in the relay log can be read in advance before the SQL thread reads them then the data that is needed by the SQL thread to apply the event would already be in the buffer pool and hence random disk I/O would be avoided, which would drastically improve the performance of SQL thread.

Prefetching with InnoDB Fake Changes

The way prefetching can be implemented without Support for Fake Changes, in most of the cases is by replaying the statements from the relay log but then manually converting all COMMITs to ROLLBACKs . This has the caveat of introducing the extra locking that is caused by the locks that are taken by the statements which are being replayed. The locks taken by statements executed by the process which is doing the prefetching can also cause lock contention with events that the SQL thread is trying to apply. Another issue with doing rollback is that, when a transaction changes data, old versions of the data are written to the undo log buffer. During the rollback phase InnoDB then has to read old versions of the data corresponding to what it was before the change from the undo log buffer and move it back to the InnoDB data page. In case of large transactions, or a transaction that updates a lot of data, the rollback can be costly and can generate significant amount of I/O.

Keeping in view the need of prefetching and the current caveats the innodb_fake_changes variable was implemented. The innodb_fake_changes variable enables an option for the server-side which allows for prefetching to work in a more performant manner. What enabling this option really does is that InnoDB reads the data needed by the DML queries but does not actually update the records, and hence no undo log record is generated, as nothing has changed, which means that rollback is instantaneous, and InnoDB doesn’t have to do any additional work on rollback. However, the problem of locking contention is not completely solved, when the records are read, SHARED locks are taken on the records, so this can still cause contention with data changes that SQL thread needs to make. Percona Server does have a variable innodb_locking_fake_changes to make fake changes implementation completely lock-less. Because the fake changes implementation is not ready for lock-less operation for all workloads this variable is not safe to use and that is why it is disabled by default.

The innodb_fake_changes option, by enabling rollbacks on COMMITs, enables prefetching tools to use it. It’s by no way a tool that does prefetching of data. It merely provides a feature that is needed by prefetching tools to work in a performant manner. There is no prefetching that is transparently done by the slave when innodb_fake_changes is enabled, i.e., there is no change in slave behavior, there is no separate thread that is started to prefetch events. A separate utility is needed that runs with the session innodb_fake_changes variable enabled and that reads events from the relay log.



This feature is only safe to use with an InnoDB-only server, because it is implemented in InnoDB only. Using it with any other storage engine such as MyISAM will cause data inconsistencies because COMMITs will not be rolled back on those storage engines.

DML operations are supported

Currently only DML operations are supported, i.e. UPDATE, INSERT, REPLACE and DELETE (set deleted flag).

DDL operations are not supported

DDL operations are not supported, i.e. ALTER TABLE and TRUNCATE TABLE. Running the DDL operations with innodb_fake_changes enabled would return an error and the subsequent DML operations may fail (from missing column etc.).

Explicit COMMIT will lead to an error

There are two types of transactions, implicit and explicit. Implicit transactions are ones that are created automatically by InnoDB to wrap around statements that are executed with autocommit enabled. For example, an UPDATE query that is not enclosed by START TRANSACTION and COMMIT, when autocommit is enabled will be automatically treated as a single statement transaction. When MySQL writes events to the binary log, the events corresponding to the implicit transactions are automatically wrapped by BEGIN and COMMIT.

When a session has the innodb_fake_changes option enabled, all the COMMITs will lead to an error, and nothing will be committed, this is in line with the implementation of innodb_fake_changes option, which guarantees that data is not left in an inconsistent state. Hence the option innodb_fake_changes would not be needed to be enabled at the GLOBAL level, rather the option innodb_fake_changes will only be enabled at the SESSION level by the utility that you would use to read and replay the relay logs. Enabling innodb_fake_changes only for the session that is used by the utility will ensure that the utility can safely execute DML queries without the actual data getting modified.

How to use InnoDB Fake Changes

A separate tool would be needed to read the relay log and replay the queries, the only purpose of innodb_fake_changes is to prevent actual data modifications. There are two different tools developed by Facebook that rely on innodb_fake_changes and can be used for the purpose of slave prefetching:

  • One tool is built using python and is named prefetch .
  • Second tool is built in C and is named faker.

Both the tools rely on the Percona Server innodb_fake_changes option.

Any other utility that can read the relay logs and replay them using multiple threads, would achieve what the above two tools achieve. Making sure that data is not modified by the tool would be done by enabling innodb_fake_changes option, but only on the SESSION level.

System Variables

variable innodb_fake_changes
Version Info:

Global, Session





Default Value:


This variable enables the Support for Fake Changes feature.

variable innodb_locking_fake_changes
Version Info:

Global, Session





Default Value:


When this variable is set to OFF, fake transactions will not take any row locks. This feature was implemented because, although fake change transactions downgrade the requested exclusive (X) row locks to shared (S) locks, these S locks prevent X locks from being taken and block the real changes. However, this option is not safe to set to OFF by default, because the fake changes implementation is not ready for lock-less operation for all workloads. Namely, if a real transaction will remove a row that a fake transaction is doing a secondary index maintenance for, the latter will fail. This option is considered experimental and might be removed in the future if lockless operation mode fixes are implemented.

Implementation Details

  • The fake session is used as a prefetch of the replication, it should not affect to later replication SQL execution.
  • The effective unit is each transaction. The behavior is decided at the start of the each one and never changed during the transaction
  • INSERT operations doesn’t use the INSERT BUFFER, it always causes the reading of the page actually for the option. DELETE also doesn’t use the INSERT BUFFER.
  • It never acquires X_LOCK from tables or records, only S_LOCK.
  • The auto increment values behaves as usual.
  • It reserves free pages as usual.
  • Existed only root ~ leaf pages, which are accessed in the DML operation.
  • It will not prefetch allocate/free, split/merge, INODE, XDES or other management pages. The same is for extern pages, i.e. large BLOB s).
  • Foreign key constraints are checked (for causing IO), but passed always.

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