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:

quoted

On 12/13/2012 12:08 AM, Darrick J. Wong wrote:

quoted

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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com