On 24/02/2010 21:32, Neil Brown wrote: [...]
If you open /dev/md0 with O_DIRECT there is no page cache involved and so no setting of Dirty flags. So you could engineer a situation with O_DIRECT that writes different data to the two devices, but you would have to try fairly hard.
Hang on. O_DIRECT sets off all sort of alarm bells for me, not that I understand it properly. Of course there's O_DIRECT on files too. Linus Torvalds is quite outspoken about it: http://kerneltrap.org/node/7563
Could we be seeing mismatches because applications are opening their files with O_DIRECT in a (perhaps misguided) attempt to get better performance?
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