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

[Andy Lutomirski]

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
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.

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

--Andy
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