Messages in this thread | | | Date | Thu, 13 Dec 2012 18:10:49 -0800 | From | "Darrick J. Wong" <> | Subject | Re: [PATCH 4/4] block: Optionally snapshot page contents to provide stable pages during write |
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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
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