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:
> On Thu, May 31, 2007 at 08:26:45AM +0200, Jens Axboe wrote:
> > 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.
> 
> That sounds like a good idea - we can leave the existing
> WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
> behaviour that only guarantees ordering. The filesystem can then
> choose which to use where appropriate....

Precisely. The current definition of barriers are what Chris and I came
up with many years ago, when solving the problem for reiserfs
originally. It is by no means the only feasible approach.

I'll add a WRITE_ORDERED command to the #barrier branch, it already
contains the empty-bio barrier support I posted yesterday (well a
slightly modified and cleaned up version).

-- 
Jens Axboe