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From: Dave Chinner <david@fromorbit.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-block@vger.kernel.org, hch@lst.de
Subject: Re: [PATCH 7/8] blk-wbt: add general throttling mechanism
Date: Thu, 10 Nov 2016 11:00:27 +1100	[thread overview]
Message-ID: <20161110000027.GV28177@dastard> (raw)
In-Reply-To: <85a891d5-0eec-a051-702f-9aac13e13b03@kernel.dk>

On Wed, Nov 09, 2016 at 09:07:08AM -0700, Jens Axboe wrote:
> On 11/09/2016 01:40 AM, Jan Kara wrote:
> >Also I'm not sure why such logic for devices with writeback cache is
> >needed. Sure the disk is fast to accept writes but if that causes long
> >read latencies, we should scale down the writeback limits so that we
> >eventually end up submitting only one write request anyway -
> >effectively the same thing as limit=0 - won't we?
> 
> Basically we want to avoid getting into that situation. The problem with
> write caching is that it takes a while for you to notice that anything
> is wrong, and when you do, you are way down in the hole. That causes the
> first violations to be pretty bad.

Yeah, slow RAID devices with a large BBWC in front of them are
notorious for doing this. You won't notice the actual IO performance
until the write cache is filled (can be GB in size) and by then it's
way too late to fix up with OS level queuing...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  parent reply	other threads:[~2016-11-10  0:00 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-11-01 21:08 [PATCHSET] Throttled buffered writeback Jens Axboe
2016-11-01 21:08 ` [PATCH 1/8] block: add WRITE_BACKGROUND Jens Axboe
2016-11-02 14:55   ` Christoph Hellwig
2016-11-02 16:22     ` Jens Axboe
2016-11-05 22:27   ` Jan Kara
2016-11-01 21:08 ` [PATCH 2/8] writeback: add wbc_to_write_flags() Jens Axboe
2016-11-02 14:56   ` Christoph Hellwig
2016-11-01 21:08 ` [PATCH 3/8] writeback: mark background writeback as such Jens Axboe
2016-11-02 14:56   ` Christoph Hellwig
2016-11-05 22:26   ` Jan Kara
2016-11-01 21:08 ` [PATCH 4/8] writeback: track if we're sleeping on progress in balance_dirty_pages() Jens Axboe
2016-11-02 14:57   ` Christoph Hellwig
2016-11-02 14:59     ` Jens Axboe
2016-11-08 13:02   ` Jan Kara
2016-11-01 21:08 ` [PATCH 5/8] block: add code to track actual device queue depth Jens Axboe
2016-11-02 14:59   ` Christoph Hellwig
2016-11-02 15:02     ` Jens Axboe
2016-11-02 16:40       ` Johannes Thumshirn
2016-11-05 22:37   ` Jan Kara
2016-11-01 21:08 ` [PATCH 6/8] block: add scalable completion tracking of requests Jens Axboe
2016-11-08 13:30   ` Jan Kara
2016-11-08 15:25     ` Jens Axboe
2016-11-09  9:01       ` Jan Kara
2016-11-09 16:09         ` Jens Axboe
2016-11-09 19:52           ` Jens Axboe
2016-11-10 19:38             ` Jan Kara
2016-11-12  5:19               ` Jens Axboe
2016-11-01 21:08 ` [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe
2016-11-08 13:39   ` Jan Kara
2016-11-08 15:41     ` Jens Axboe
2016-11-09  8:40       ` Jan Kara
2016-11-09 16:07         ` Jens Axboe
2016-11-09 19:52           ` Jens Axboe
2016-11-10 19:36             ` Jan Kara
2016-11-10  0:00           ` Dave Chinner [this message]
2016-11-01 21:08 ` [PATCH 8/8] block: hook up writeback throttling Jens Axboe
2016-11-08 13:42   ` Jan Kara
2016-11-08 15:16     ` Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2016-10-26 20:52 [PATCHSET] block: buffered " Jens Axboe
2016-10-26 20:52 ` [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe

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