[dm-devel] Re: [PATCH] Implement barrier support for single device DM devices

[David Chinner]

On Fri, Feb 15, 2008 at 04:07:54PM +0300, Michael Tokarev wrote:
> Alasdair G Kergon wrote:
> > On Fri, Feb 15, 2008 at 01:08:21PM +0100, Andi Kleen wrote:
> >> Implement barrier support for single device DM devices
> >  
> > Thanks.  We've got some (more-invasive) dm patches in the works that
> > attempt to use flushing to emulate barriers where we can't just
> > pass them down like that.
> 
> I wonder if it's worth the effort to try to implement this.
> 
> As far as I understand (*), if a filesystem realizes that the
> underlying block device does not support barriers, it will
> switch to using regular flushes instead

No, typically the filesystems won't issue flushes, either.

> - isn't it the same
> thing as you're trying to do on an MD level?
> 
> Note that a filesystem must understand barriers/flushes on
> underlying block device, since many disk drives don't support
> barriers anyway.
> 
> (*) this is, in fact, an interesting question.  I still can't
> find complete information about this.  For example, how safe
> xfs is if barriers are not supported or turned off?  Is it
> "less safe" than with barriers?  Will it use regular cache
> flushes if barriers are not here?

Try reading at the XFS FAQ:

http://oss.sgi.com/projects/xfs/faq/#wcache

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group