Re: raid5-cache I/O path improvements

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On Tue, Sep 08, 2015 at 01:34:20PM -0400, Tejun Heo wrote:
> Hmmm... grep tells me that dm and md actually are branching on whether
> the underlying device supports FUA.  This is tricky.  I didn't even
> mean flush_flags to be used directly by upper layers.  For rotational
> devices, doing multiple FUAs compared multiple writes followed by
> REQ_FLUSH is probably a lot worse - the head gets moved multiple times
> likely skipping over data which can be written out while traversing
> and it's not like stalling write pipeline and draining write queue has
> much impact on hard drives.

Well, that's what we'd need to do for the raid cache as well, given
the resulst that Shaohua sees.  Unless you have a good idea for another
way to handle the issue we'll need to support both behaviors there.
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