linux-kernel - Re: [PATCH] writeback: Per-block device bdi->dirty_writeback_interval and bdi->dirty_expire

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Date:	Thu, 18 Aug 2011 21:55:58 +0530
From:	Kautuk Consul <consul.kautuk@...il.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Artem Bityutskiy <dedekind1@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH] writeback: Per-block device bdi->dirty_writeback_interval
 and bdi->dirty_expire_interval.

Hi Wu,

On Thu, Aug 18, 2011 at 6:43 PM, Wu Fengguang <fengguang.wu@...el.com> wrote:
> Hi Artem,
>
>> Here is a real use-case we had when developing the N900 phone. We had
>> internal flash and external microSD slot. Internal flash is soldered in
>> and cannot be removed by the user. MicroSD, in contrast, can be removed
>> by the user.
>>
>> For the internal flash we wanted long intervals and relaxed limits to
>> gain better performance.
>>
>> For MicroSD we wanted very short intervals and tough limits to make sure
>> that if the user suddenly removes his microSD (users do this all the
>> time) - we do not lose data.
>
> Thinking twice about it, I find that the different requirements for
> interval flash/external microSD can also be solved by this scheme.
>
> Introduce a per-bdi dirty_background_time (and optionally dirty_time)
> as the counterpart of (and works in parallel to) global dirty[_background]_ratio,
> however with unit "milliseconds worth of data".
>
> The per-bdi dirty_background_time will be set low for external microSD
> and high for internal flash. Then you get timely writeouts for microSD
> and reasonably delayed writes for internal flash (controllable by the
> global dirty_expire_centisecs).
>
> The dirty_background_time will actually work more reliable than
> dirty_expire_centisecs because it will checked immediately after the
> application dirties more pages. And the dirty_time could provide
> strong data integrity guarantee -- much stronger than
> dirty_expire_centisecs -- if used.
>
> Does that sound reasonable?
>
> Thanks,
> Fengguang
>

My understanding of your email appears that you are agreeing in
principle that the temporal
aspect of this problem needs to be addressed along with your spatial
pattern analysis technique.

I feel a more generic solution to the problem is required because the
problem faced by Artem can appear
in a different situation for a different application.

I can re-implement my original patch in either centiseconds or
milliseconds as suggested by you.

Kindly advise if my understanding is correct.

Thanks,
Kautuk Consul.
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