How to flush data to disk reliably?

[Grzegorz Kulewski <kangur@polcom.net>]

Hi,

I am writing an app that has log files. These log files must be reliably 
and permanently flushed to disk on some points. These log files can be:
a. normal files on some (local) filesytem,
b. some block device (disk/partition).

I am asking how to flush the data from these logs to disk. I know of 
several methods:
1. open with O_SYNC,
2. sync(2),
3. fsync,
4. fdatasync,
5. msync (if they are mmaped).

Which of these are best and most reliable for (a/b) and for (IDE/SCSI)? 
What are differences between them? Maybe some other method? Are there any 
other precautions that I should be aware of? What about write caches? Are 
write barriers implemented (on IDE and SATA/SCSI) or should I turn caches 
off?

I am using Linux 2.6. (But I will be glad to hear about differences 
between 2.4 and 2.6 in this aspect if there are any.)


Thanks in advance,

Grzegorz Kulewski