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Re: [Qemu-devel] [PATCH] virtio-blk: add DISCARD support to virtio-blk d
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From: |
Christoph Hellwig |
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Subject: |
Re: [Qemu-devel] [PATCH] virtio-blk: add DISCARD support to virtio-blk driver |
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Date: |
Mon, 27 Mar 2017 16:56:26 +0200 |
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User-agent: |
Mutt/1.5.17 (2007-11-01) |
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.