Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV_INTEGRITY

Jeff Moyer
2012-03-08T20:43:00Z

Chris Mason <chris.mason@oracle.com> writes:

> On Thu, Mar 08, 2012 at 12:20:26PM -0800, Boaz Harrosh wrote:
>> I think I understand this one. It's do to the sync nature introduced
>> by page_waiting in mkwrite.
>
> Pages go from dirty to writeback for a few reasons. Background
> writeout, or O_DIRECT or someone running sync
>
> background writeout shouldn't be queueing up so much work that
> synchronous writeout has a 2 second delay.

So now we're back to figuring out how to tell how long I/O will take?
If writeback is issuing random access I/Os to spinning media, you can
bet it might be a while. Today, you could lower nr_requests to some
obscenely small number to improve worst-case latency. I thought there
was some talk about improving the intelligence of writeback in this
regard, but it's a tough problem, especially given that writeback isn't
the only cook in the kitchen.

Cheers,
Jeff