Simply disabling the write cache does not guarantee the order of
writes. For one, with tagged I/O and the use of the SIMPLE_Q tag
qualifier, commands may be completed in any order. If you want
some semblance of order, either disable the write cache or use the
FUA bit in all writes, and use the ORDERED tag qualifier. Even
when using these options, it is not clear that the drive cannot
reorder writes "slightly" to make track writes more efficient
(e.g. two separate commands to write sequential sectors on the
same track may be written in reverse order).
ORDERED sounds like the trick... I assume this is some kind of
write-barrier? If so, then I assume it has some kind of strict
temporal ordering, even between command issues to the drive.
If so, that would be idea if we can have the fs communicate this all
the way down to the device layer, making it work for soft-raid and LVM
be a little harder perhaps.
--cw
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