Re: [PATCH 03/11] block: deprecate barrier and replace blk_queue_ordered() with blk_queue_flush()

 On 08/14/2010 02:42 AM, hch@lst.de wrote:

On Fri, Aug 13, 2010 at 06:07:13PM -0700, Jeremy Fitzhardinge wrote:

quoted

 On 08/12/2010 05:41 AM, Tejun Heo wrote:

quoted

Barrier is deemed too heavy and will soon be replaced by FLUSH/FUA
requests.  Deprecate barrier.  All REQ_HARDBARRIERs are failed with
-EOPNOTSUPP and blk_queue_ordered() is replaced with simpler
blk_queue_flush().

blk_queue_flush() takes combinations of REQ_FLUSH and FUA.  If a
device has write cache and can flush it, it should set REQ_FLUSH.  If
the device can handle FUA writes, it should also set REQ_FUA.

Christoph, do these two patches (parts 2 and 3) make xen-blkfront
correct WRT barriers/flushing as far as your concerned?

If all your backends handle a zero-length BLKIF_OP_WRITE_BARRIER request
it is a fully correct, but rather suboptimal implementation.  To get
all the benefit of the new non-draining barriers you'll need a new
If all your backends handle a zero-length BLKIF_OP_FLUSH request that
only flushes the cache, but has no ordering side effects.

Is the effect of the flush that, once complete, any previously completed
write is guaranteed to be on durable storage, but it is not guaranteed
to have any effect on pending writes?  If so, does it flush writes that
were completed before the flush is issued, or writes that complete
before the flush completes?

  Note that
the quite suboptimal here means not as good as the new barrier
implementation, but it shouldn't be notiably worse than the old one
for Xen.

OK, thanks.  We can do some testing on that and see if there's a benefit
to adding a flush operation with the appropriate semantics.

    J