How are the O_SYNC and O_DIRECT flags in open(2) different/alike?
The use and effects of the O_SYNC and O_DIRECT flags is very confusing and appears to vary somewhat among platforms. From the Linux man page (see an example here), O_DIRECT provides synchronous I/O, minimizes cache effects and requires you to handle block size alignment yourself. O_SYNC just guarantees synchronous I/O. Although both guarantee that data is written into the hard disk's cache, I believe that direct I/O operations are supposed to be faster than plain synchronous I/O since they bypass the page cache (Though FreeBSD's man page for open(2) states that the cache is bypassed when O_SYNC is used. See here).
What exactly are the differences between the O_DIRECT and O_SYNC flags? Some implementations suggest using O_SYNC | O_DIRECT. Why?
Answer
O_DIRECT alone only promises that the kernel will avoid copying data from user space to kernel space, and will instead write it directly via DMA (Direct memory access; if possible). Data does not go into caches. There is no strict guarantee that the function will return only after all data has been transferred.
O_SYNC guarantees that the call will not return before all data has been transferred to the disk (as far as the OS can tell). This still does not guarantee that the data isn't somewhere in the harddisk write cache, but it is as much as the OS can guarantee.
O_DIRECT|O_SYNC is the combination of these, i.e. "DMA + guarantee".