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Date: Thu, 7 Dec 2023 19:44:37 -0700
From: Keith Busch <kbusch@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Li Feng <fengli@smartx.com>, Jens Axboe <axboe@kernel.dk>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jason Wang <jasowang@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Xuan Zhuo <xuanzhuo@linux.alibaba.com>,
	"open list:BLOCK LAYER" <linux-block@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:VIRTIO BLOCK AND SCSI DRIVERS" <virtualization@lists.linux.dev>
Subject: Re: [PATCH] virtio_blk: set the default scheduler to none
Message-ID: <ZXKDFdzXN4xQAuBm@kbusch-mbp>
References: <20231207043118.118158-1-fengli@smartx.com>
 <ZXJ4xNawrSRem2qe@fedora>
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In-Reply-To: <ZXJ4xNawrSRem2qe@fedora>

On Fri, Dec 08, 2023 at 10:00:36AM +0800, Ming Lei wrote:
> On Thu, Dec 07, 2023 at 12:31:05PM +0800, Li Feng wrote:
> > virtio-blk is generally used in cloud computing scenarios, where the
> > performance of virtual disks is very important. The mq-deadline scheduler
> > has a big performance drop compared to none with single queue. In my tests,
> > mq-deadline 4k readread iops were 270k compared to 450k for none. So here
> > the default scheduler of virtio-blk is set to "none".
> 
> The test result shows you may not test HDD. backing of virtio-blk.
> 
> none can lose IO merge capability more or less, so probably sequential IO perf
> drops in case of HDD backing.

More of a curiosity, as I don't immediately even have an HDD to test
with! Isn't it more useful for the host providing the backing HDD use an
appropriate IO scheduler? virtio-blk has similiarities with a stacking
block driver, and we usually don't need to stack IO schedulers.