Re: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Dec 14, 2012 at 02:33:34PM +1100, Dave Chinner wrote:
> On Thu, Dec 13, 2012 at 06:10:49PM -0800, Darrick J. Wong wrote:
> > On Thu, Dec 13, 2012 at 05:48:06PM -0800, Andy Lutomirski wrote:
> > > On 12/13/2012 12:08 AM, Darrick J. Wong wrote:
> > > > Several complaints have been received regarding long file write latencies when
> > > > memory pages must be held stable during writeback.  Since it might not be
> > > > acceptable to stall programs for the entire duration of a page write (which may
> > > > take many milliseconds even on good hardware), enable a second strategy wherein
> > > > pages are snapshotted as part of submit_bio; the snapshot can be held stable
> > > > while writes continue.
> > > > 
> > > > This provides a band-aid to provide stable page writes on jbd without needing
> > > > to backport the fixed locking scheme in jbd2.  A mount option is added to ext4
> > > > to allow administrators to enable it there.
> > > 
> > > I'm a bit confused as to what it has to do with ext3.  Wouldn't this be
> > > useful as a mount option everywhere, though?
> > 
> > ext3 requires snapshots; the rest are ok with either strategy.
> > 
> > *If* snapshotting is generally liked, then yes I'll go redo it as a vfs mount
> > option.
> 
> It's copying every single IO, right? If so, then please don't
> propagate any further than is necessary to fix the broken
> filesystems...

Yup.  I wasn't intending this flag for general service, though I /am/ curious
to hear if anyone sees any substantive performance difference with snapshots.

--D
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com