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Date: Mon, 27 Mar 2017 16:56:26 +0200
From: Christoph Hellwig <hch@lst.de>
Message-ID: <20170327145625.GA24952@lst.de>
References: <1490690365-21109-1-git-send-email-changpeng.liu@intel.com>
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Subject: Re: [Qemu-devel] [PATCH] virtio-blk: add DISCARD support to
 virtio-blk driver
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To: Changpeng Liu <changpeng.liu@intel.com>
Cc: virtio-dev@lists.oasis-open.org, virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, hch@lst.de, qemu-devel@nongnu.org

On Tue, Mar 28, 2017 at 04:39:25PM +0800, Changpeng Liu wrote:
> Currently virtio-blk driver does not provide discard feature flag, so the
> filesystems which built on top of the block device will not send discard
> command. This is okay for HDD backend, but it will impact the performance
> for SSD backend.
> 
> Add a feature flag VIRTIO_BLK_F_DISCARD and command VIRTIO_BLK_T_DISCARD
> to extend exist virtio-blk protocol. virtio-blk protocol uses a single
> 8 bytes descriptor containing type,reserved and sector, currently Linux
> uses the reserved field as IO priority, here we also re-use the reserved
> field as number of discard sectors.

Do you have a link to the specification for this feature?  At least
virtio-v1.0 does not seem to specify a discard feature.

Note that Linux 4.11 and later have support for multi-range discard
ala ATA TRIM, SCSI UNMAP and NVMe deallocate which might be useful
here, too.

> +		q->limits.discard_zeroes_data = 0;

No need to clear this.  Also hopefully this field goes away for 4.12

> +		blk_queue_max_discard_segments(q, 1);

No need to set this.