Re: block_device usage and incorrect block writes

[Jens Axboe]

On Wed, Jan 24 2007, Chris Frost wrote:
> On Thu, Jan 18, 2007 at 01:13:06PM +1100, Jens Axboe wrote:
> > noop doesn't guarentee that IO will be queued with the device in the
> > order in which they are submitted, and it definitely doesn't guarentee
> > that the device will process them in the order in which they are
> > dispatched. noop being FIFO basically means that it will not sort
> > requests. You can still have reordering if one request gets merged with
> > another, for instance.
> > 
> > The block layer in general provides no guarentees about ordering of
> > requests, unless you use barriers. So if you require ordering across a
> > given write request, it needs to be a write barrier.
> 
> Thank your explaining this aspect of the linux block device layer design.
> Earlier, we tried bio barriers (hard barriers) and found the slow down
> to be too great. After my previous email we looked further down the stack and
> noticed that struct request also has a soft barrier option. For our tests,
> soft barriers perform almost as well as no barriers, and our system
> is ok (for now, at least) with the write reordering that devices can do.
> 
> As our code calls generic_make_request(), it does not have access to the
> created struct request. We have modified block/ll_rw_blk.c:__make_request()
> to 1) not merge requests and to 2) add the REQ_SOFTBARRIER flag to the new
> request's cmd_flags field. Is there a more modular way for a function to
> create a new request with a soft barrier?

It would not be a problem to expose the soft/hard barrier difference at
the bio level as well, so you have direct access to it. I think it would
be quite useful in general.

-- 
Jens Axboe

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