linux-kernel - Re: [dm-devel] [PATCH 0/3] add resync speed control for dm-raid1

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Message-ID: <20121210132123.5a4ab015@notabene.brown>
Date:	Mon, 10 Dec 2012 13:21:23 +1100
From:	NeilBrown <neilb@...e.de>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	gzhao@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH 0/3] add resync speed control for dm-raid1

On Thu, 22 Nov 2012 14:27:50 +0800 Guangliang Zhao <gzhao@...e.com> wrote:

> Hi,
> 
> These patches are used to add resync speed control for dm-raid1. The
> second and third patch provide support for user-space tool dmsetup.
> 
> Guangliang Zhao (3):
>   dm raid1: add resync speed control for dm-raid1
>   dm raid1: add interface to set resync speed
>   dm raid1: add interface to get resync speed
> 
>  drivers/md/dm-raid1.c |   93 +++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 90 insertions(+), 3 deletions(-)
> 

The problem with this approach is that it slows down resync even when there
is no other IO happening.
If that is deemed to be acceptable, then the patch set seems fine, though I
would probably make the default a lot higher so as not to change current
default behaviour for anyone.

If it isn't acceptable, then you either need to monitor the number of
requests going to the underlying devices - like md does - or monitor the
number of requests coming in to the dm-raid1 target - which is probably
easier with dm.

i.e. only impose the rate limit if there have been any requests for the
dm-raid1 target in the last 'RESYNC_JIFFIES'.

What do you think of that?

NeilBrown

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