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 <d...@openvz.org> > CC: Kevin Wolf <kw...@redhat.com> > CC: Max Reitz <mre...@redhat.com> > CC: Eduardo Habkost <ehabk...@redhat.com> > CC: Marcel Apfelbaum <marcel.apfelb...@gmail.com> > CC: John Snow <js...@redhat.com> > CC: Paolo Bonzini <pbonz...@redhat.com> > CC: Fam Zheng <f...@euphon.net> > > -- Eduardo
