Posted May 14, 2012 23:56 UTC (Mon) by koverstreet (subscriber, #4296)
Parent article: A bcache update
> Another interesting problem that comes with writeback caching is the
> implementation of barrier operations. Filesystems use barriers (implemented as
> synchronous "force to media" operations in contemporary kernels) to ensure that
> the on-disk filesystem structure is consistent at all times. If bcache does not
> recognize and implement those barriers, it runs the risk of wrecking the
> filesystem's careful ordering of operations and corrupting things on the
> backing device. Unfortunately, bcache does indeed lack such support at the
> moment, leading to a strong recommendation to mount filesystems with barriers
> disabled for now.
I need to delete that from the wiki, it's out of date - it's only old style
(pre 2.6.38) barriers that were a problem, because they implied much stricter
ordering. Cache flushes/FUA are much easier, and are handled just fine.