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

[Andy Lutomirski]

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?

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

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.

--Andy
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