This topic describes things to consider when your applications require direct I/O.
Some applications, such as database management systems, use their own cache management systems. For such applications, SAN File System provides a direct I/O mode, which allows these applications to bypass the data cache. In this mode, SAN File System performs direct writes to disk and does not cache data. This also allows distributed applications on different AIX® or Linux™ client machines to write data to the same file at the same time. Using the direct I/O mode makes files act like raw devices. This gives database systems direct control over their I/O operations, while still providing the advantages of SAN File System, such as the FlashCopy® feature and file-level backup and restore processing. Applications need to be aware of, and configured for, direct I/O.
UNIX-based clients use existing operating-system interfaces to use direct I/O. That is, you must set the O_DIRECT flag to open a file in direct I/O mode.
However, a process can use direct I/O on a file that is being used in cache mode by a process on another client machine. Similarly, a process can use a file in cached mode that is being used in direct I/O mode by a process on another client machine.
Parent topic: Planning the client configuration