Re: [PATCH 0/2] block: propagate discard alignment from format drivers to the guest

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jun 11, 2020 at 08:16:06PM +0300, 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.

I assume the value is visible to the guest.  What is supposed to
happen if live migrating and the block backend is a different one
on the destination?

Also, don't we have mechanisms to change the block backend change
at run time?  What should happen in that case?

> 
> 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>
> 
> 

-- 
Eduardo