linux-ext4 - Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV

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Date:	Thu, 08 Mar 2012 15:42:52 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Boaz Harrosh <bharrosh@...asas.com>, "Ted Ts'o" <tytso@....edu>,
	Zach Brown <zab@...bo.net>, Eric Sandeen <sandeen@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] Don't do page stablization if !CONFIG_BLKDEV_INTEGRITY

Chris Mason <chris.mason@...cle.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
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