linux-kernel - Re: ext4 write performance regression in 3.6-rc1 on RAID0/5

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Date:	Wed, 22 Aug 2012 14:31:00 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	NeilBrown <neilb@...e.de>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Li Shaohua <shli@...ionio.com>, Theodore Ts'o <tytso@....edu>,
	Marti Raudsepp <marti@...fo.org>,
	Kernel hackers <linux-kernel@...r.kernel.org>,
	ext4 hackers <linux-ext4@...r.kernel.org>, maze@...gle.com,
	"Shi, Alex" <alex.shi@...el.com>, linux-fsdevel@...r.kernel.org,
	linux RAID <linux-raid@...r.kernel.org>
Subject: Re: ext4 write performance regression in 3.6-rc1 on RAID0/5

On Wed, Aug 22, 2012 at 04:00:25PM +1000, NeilBrown wrote:
> On Wed, 22 Aug 2012 11:57:02 +0800 Yuanhan Liu <yuanhan.liu@...ux.intel.com>
> wrote:
> 
> >  
> > -#define NR_STRIPES		256
> > +#define NR_STRIPES		1024
> 
> Changing one magic number into another magic number might help your case, but
> it not really a general solution.

Agreed.

> 
> Possibly making sure that max_nr_stripes is at least some multiple of the
> chunk size might make sense, but I wouldn't want to see a very large multiple.
> 
> I thing the problems with RAID5 are deeper than that.  Hopefully I'll figure
> out exactly what the best fix is soon - I'm trying to look into it.
> 
> I don't think the size of the cache is a big part of the solution.  I think
> correct scheduling of IO is the real answer.

Yes, it should not be. But with less max_nr_stripes, the chance to get a
full strip write is less, and maybe that's the reason why the chance to
block at get_active_strip() is more; and also, the reading is more.

The perfect case would be there are no reading; setting max_nr_stripes
to 32768(the max we get set now), you will find the reading is quite
less(almost zero, please see the iostat I attached in former email).

Anyway, I do agree this should not be the big part of the solution. If
we can handle those stripes faster, I guess 256 would be enough.

Thanks,
Yuanhan Liu
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