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

[Jens Axboe <jens axboe oracle com>]

On Thu, May 31 2007, David Chinner wrote:
> IOWs, there are two parts to the problem:
> 
> 	1 - guaranteeing I/O ordering
> 	2 - guaranteeing blocks are on persistent storage.
> 
> Right now, a single barrier I/O is used to provide both of these
> guarantees. In most cases, all we really need to provide is 1); the
> need for 2) is a much rarer condition but still needs to be
> provided.
> 
> > if I am understanding it correctly, the big win for barriers is that you 
> > do NOT have to stop and wait until the data is on persistant media before 
> > you can continue.
> 
> Yes, if we define a barrier to only guarantee 1), then yes this
> would be a big win (esp. for XFS). But that requires all filesystems
> to handle sync writes differently, and sync_blockdev() needs to
> call blkdev_issue_flush() as well....
> 
> So, what do we do here? Do we define a barrier I/O to only provide
> ordering, or do we define it to also provide persistent storage
> writeback? Whatever we decide, it needs to be documented....

The block layer already has a notion of the two types of barriers, with
a very small amount of tweaking we could expose that. There's absolutely
zero reason we can't easily support both types of barriers.

-- 
Jens Axboe