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

[Darrick J. Wong]

On Fri, Dec 14, 2012 at 05:12:37PM -0800, Andy Lutomirski wrote:
> On Thu, Dec 13, 2012 at 6:10 PM, Darrick J. Wong
> <darrick.wong@xxxxxxxxxx> 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.
> >
> >> 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
> 
> It survived.  I hit at least one mm bug, but I really don't think it's
> a problem with your code.  (I have not tried this workload on Linux
> 3.7 at all before.  It normally runs on 3.5.)  The box in question is

Would you mind sending along the bug report so I can make sure?

> ext4 on LVM on dm-crypt on (hardware) RAID 5 on hpsa, which should not
> need stable pages.
> 
> The majority of the data written (that wasn't unlinked before it was
> dropped from cache) was checksummed when written and verified later.
> Most of this data was written using mmap.  This workload hammers the
> vm concurrently in several threads, and it frequently stalls when
> stable pages are enabled, so it's probably exercising the code
> decently well.

Did you observe any change in performance?

> Feel free to add Tested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

Will do!  Thanks for the testing!

--D
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