环境
- Red Hat Enterprise Linux 6
- EXT4 Filesystem
问题
- Does EXT4 support asynchronous I/O or direct I/O?
- If EXT4 supports these features, how are they used?
决议
- EXT4 file system supports both the asynchronous I/O and the Direct I/O features.
- By default, asynchronous I/O is enabled.
- If Direct I/O is needed, the application should use the O_DIRECT flag while doing the 'open()' system call.
- If Direct I/O has to be used with an existing EXT4 filesystem, the 'sync' mount option should be used both while mounting manually, and when mounting via the /etc/fstab file for boot time activation.
NOTE: If both Direct I/O (DIO) and the 'sync' options are used, file performance will be decreased because the file system will over-ride the use of the kernel page cache. This increases the I/O calls to the underlying disk, which is hundreds of times slower than accessing data via the primary L1-L2 caches. It is best to understand the consequences/drawbacks of DIO prior to implementing it.
