Re: [dm-devel] [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

[Tejun Heo <tj kernel org>]

Hello,

On 08/20/2010 03:22 PM, Christoph Hellwig wrote:
> Many storage devices, especially in the consumer market, come with volatile
> write back caches.  That means the devices signal I/O completion to the
> operating system before data actually has hit the physical medium.

A bit nit picky but flash devices can also have writeback caches and
the term physical medium sounds a bit off for those cases.  Maybe just
saying "non-volatile media" is better?

> Implementation details for filesystems
> --------------------------------------
> 
> Filesystem can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
> worry if the underlying devices need any explicit cache flushing and how
> the Forced Unit Access is implemented.  The REQ_FLUSH and REQ_FUA flags
> may both be set on a single bio.

It may be worthwhile to explain the sequence of operations when
REQ_FLUSH + data + REQ_FUA is executed.  It can be extrapolated from
the previous two descriptions but I think giving examples of different
sequences depending on FLUSH/FUA configuration would be helpful to
help understanding the overall picture of things.

Other than those, looks good to me.

Thanks.

-- 
tejun