linux-kernel - Re: [PATCH] MD: make bio mergeable

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Message-ID: <20160428211908.GA3116523@devbig084.prn1.facebook.com>
Date:	Thu, 28 Apr 2016 14:19:10 -0700
From:	Shaohua Li <shli@...com>
To:	Holger Kiehl <Holger.Kiehl@....de>
CC:	<linux-block@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-raid <linux-raid@...r.kernel.org>, <qkrwngud825@...il.com>,
	<Kernel-team@...com>, Ming Lei <ming.lei@...onical.com>,
	Jens Axboe <axboe@...com>, Neil Brown <neilb@...e.de>
Subject: Re: [PATCH] MD: make bio mergeable

On Thu, Apr 28, 2016 at 08:00:22PM +0000, Holger Kiehl wrote:
> Hello,
> 
> On Mon, 25 Apr 2016, Shaohua Li wrote:
> 
> > blk_queue_split marks bio unmergeable, which makes sense for normal bio.
> > But if dispatching the bio to underlayer disk, the blk_queue_split
> > checks are invalid, hence it's possible the bio becomes mergeable.
> > 
> > In the reported bug, this bug causes trim against raid0 performance slash
> > https://bugzilla.kernel.org/show_bug.cgi?id=117051
> > 
> This patch makes a huge difference. On a system with two Samsung 850 Pro
> in a MD Raid0 setup the time for fstrim went down from ~30min to 18sec!
> 
> However, on another system with two Intel P3700 1.6TB NVMe PCIe SSD's
> also setup as one big MD Raid0, the patch does not make any difference
> at all. fstrim takes more then 4 hours!

Does the raid0 cross two partitions or two SSD?

can you post blktrace data in the bugzilloa, I'll track the bug there.

Thanks,
Shaohua