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

[Bill Davidsen <davidsen@tmr.com>]

Jens Axboe wrote:
> 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).
>
>   
Wait. Do filesystems expect (depend on) anything but ordering now? Does 
md? Having users of barriers as they currently behave suddenly getting 
SYNC behavior where they expect ORDERED is likely to have a negative 
effect on performance. Or do I misread what is actually guaranteed by 
WRITE_BARRIER now, and a flush is currently happening in all cases?

And will this also be available to user space f/s, since I just proposed 
a project which uses one? :-(
I think the goal is good, more choice is almost always better choice, I 
just want to be sure there won't be big disk performance regressions.

--
bill davidsen <davidsen@tmr.com>
 CTO TMR Associates, Inc
 Doing interesting things with small computers since 1979

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/