FusionIO/DirectFS atomic write support
Starting with 5.5.31 / 10.0.2 XtraDB and Innodb that are included with now supports atomic writes without using doublewrite buffer.
Partial write operations
When Innodb writes to the filesystem, there is in general case no guarantee that write operation will be complete (not partial), in case of poweroff event, of if operating system crashes at the exact moment write is done. Without detection or prevention of partial writes, integrity of the database can be compromised after recovery. Thus, since its inception, Innodb had mechanism to detect and ignore partial writes via innodb_doublewrite parameter (also innodb_checksum can be used to detect partial write).
Atomic write - a faster alternatve to innodb_doublewrite
The innodb_doublewrite parameter does not come without set of problems. Especially on SSD, writing each page twice can have detrimenal effects (known as write leveling). A better solution would be to directly ask filesystem to provide atomic (all or nothing) write guarantee. As of time of writing, it is only directFS filesystem on FusionIO device that provides atomic write functionality. Now, this functionality is also supported by MariaDB's XtraDB and Innodb storage engines with MDEV-4338 . To switch atomic writess atomic writes instead of doublewrite buffer, add
innodb_use_atomic_writes = 1
to the my.cnf config file.
Following happens if innodb_use_atomic_writes is switched ON
- in case innodb_file_flush_method is neither O_DIRECT, ALL_O_DIRECT, or O_DIRECT_NO_FSYNC, it is switched to O_DIRECT
- innodb_use_fallocate is switched ON (files are extended using posix_fallocate rather than writing zeros behind the end of file)
- Whenever Innodb datafile is opened, a special ioctl() is issued to switch on atomic writes. If the call fails, error is logged and returned to the caller. This means in case system tablespace is not located on atomic write capable device/filesystem, innodb will refuse to start.
