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