[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Jens Axboe <jens axboe oracle com>]

On Thu, Jul 05 2007, Tejun Heo wrote:
> Hello, Jens.
> 
> Jens Axboe wrote:
> > On Mon, May 28 2007, Neil Brown wrote:
> >> I think the implementation priorities here are:
> >>
> >> 1/ implement a zero-length BIO_RW_BARRIER option.
> >> 2/ Use it (or otherwise) to make all dm and md modules handle
> >>    barriers (and loop?).
> >> 3/ Devise and implement appropriate fall-backs with-in the block layer
> >>    so that  -EOPNOTSUP is never returned.
> >> 4/ Remove unneeded cruft from filesystems (and elsewhere).
> > 
> > This is the start of 1/ above. It's very lightly tested, it's verified
> > to DTRT here at least and not crash :-)
> > 
> > It gets rid of the ->issue_flush_fn() queue callback, all the driver
> > knowledge resides in ->prepare_flush_fn() anyways. blkdev_issue_flush()
> > then just reuses the empty-bio approach to queue an empty barrier, this
> > should work equally well for stacked and non-stacked devices.
> > 
> > While this patch isn't complete yet, it's clearly the right direction to
> > go.
> 
> Finally took a brief look. :-) I think the sequencing for zero-length
> barrier can be better done by pre-setting QUEUE_ORDSEQ_BAR in
> start_ordered() rather than short circuiting the request after it's
> issued.  What do you think?

Yeah, that might be cleaner and should achieve the same effect. I'll
test!

-- 
Jens Axboe