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

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

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.

> If this becomes widely used, would it be better to snapshot on
> wait_for_stable_page instead of on io submission?

That really depends on how long you can afford to wait and how much free
memory you have. :)  It's all a big tradeoff between write latency and
consumption of memory pages and bandwidth, and one that I doubt I'm qualified
to make for everyone.

> FWIW, I'm about to pound pretty hard on this whole patchset on a box
> that doesn't need stable pages.  I'll let you know how it goes.

Yay!

--D