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Date:   Thu, 7 Dec 2023 20:15:18 -0700
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Subject: Re: [PATCH] virtio_blk: set the default scheduler to none
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To:     Keith Busch <kbusch@kernel.org>, Ming Lei <ming.lei@redhat.com>
Cc:     Li Feng <fengli@smartx.com>, "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>
References: <20231207043118.118158-1-fengli@smartx.com>
 <ZXJ4xNawrSRem2qe@fedora> <ZXKDFdzXN4xQAuBm@kbusch-mbp>
From:   Jens Axboe <axboe@kernel.dk>
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On 12/7/23 7:44 PM, Keith Busch wrote:
> 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.

Indeed, any kind of scheduling or merging should not need to happen
here, but rather at the lower end. But there could be an argument to be
made for having fewer commands coming out of virtio-blk, however. Would
be interesting to test.

-- 
Jens Axboe