Atomic Write Support

Partial Write Operations

When Innodb writes to the filesystem, there is generally no guarantee that a given write operation will be complete (not partial) in cases of a poweroff event, or if the operating system crashes at the exact moment a write is being done.

Without detection or prevention of partial writes, the integrity of the database can be compromised after recovery.

innodb_doublewrite--an Imperfect Solution

Since its inception, Innodb has had a mechanism to detect and ignore partial writes via the (also innodb_checksum can be used to detect a partial write).

Doublewrites, controlled by the system variable, comes with its own set of problems. Especially on SSD, writing each page twice can have detrimental effects (write leveling).

Atomic Write - a Faster Alternative to innodb_doublewrite

A better solution is to directly ask the filesystem to provide an atomic (all or nothing) write guarantee. Currently this is only available on .

Enabling Atomic Writes from

When starting, and beyond automatically detects if any of the supported SSD cards are used.

When opening an InnoDB table, there is a check if the tablespace for the table is and if yes, it will automatically enable atomic writes for the table. If atomic writes support is not detected, the doublewrite buffer will be used.

One can disable atomic write support for all cards by setting the variable to OFF in your my.cnf file. It's ON by default.

Enabling Atomic Writes in to

To use atomic writes instead of the doublewrite buffer, add:

to the my.cnf config file.

Note that atomic writes are only supported on in these versions of MariaDB.

About innodb_use_atomic_writes (in to )

The following happens when atomic writes are enabled

• if is neither O_DIRECT, ALL_O_DIRECT, or O_DIRECT_NO_FSYNC, it is switched to O_DIRECT

• is switched ON (files are extended using posix_fallocate rather than writing zeros behind the end of file)

Here is a flowchart showing how atomic writes work inside InnoDB:

Devices that Support Atomic Writes with MariaDB

MariaDB currently supports atomic writes on the following devices:

• . and above.

• . and above.

_{This page is licensed: CC BY-SA / Gnu FDL}