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Re: [PATCH 0/2] block: propagate discard alignment from format drivers t

From: Denis V. Lunev
Subject: Re: [PATCH 0/2] block: propagate discard alignment from format drivers to the guest
Date: Fri, 3 Jul 2020 20:36:33 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0

On 6/11/20 8:16 PM, Denis V. Lunev wrote:
> Nowaday SCSI drivers in guests are able to align UNMAP requests before
> sending to the device. Right now QEMU provides an ability to set
> this via "discard_granularity" property of the block device which could
> be used by management layer.
> Though, in particular, from the point of QEMU, there is
> pdiscard_granularity on the format driver level, f.e. on QCOW2 or iSCSI.
> It would be beneficial to pass this value as a default for this
> property.
> Technically this should reduce the amount of use less UNMAP requests
> from the guest to the host. Basic test confirms this. Fedora 31 guest
> during 'fstrim /' on 32 Gb disk has issued 401/415 requests with/without
> proper alignment to QEMU.
> Changes from v2:
> - 172 iotest fixed
> Changes from v1:
> - fixed typos in description
> - added machine type compatibility layer as suggested by Kevin
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Eduardo Habkost <ehabkost@redhat.com>
> CC: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> CC: John Snow <jsnow@redhat.com>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Fam Zheng <fam@euphon.net>
ping v3

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