| Message ID | 20200611171608.22052-1-den@openvz.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | block: propagate discard alignment from format drivers to the guest | expand |
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> > > Sorry for missed v3 tag in the subject :(
On 6/11/20 8:21 PM, Denis V. Lunev wrote: > 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> >> >> > Sorry for missed v3 tag in the subject :( ping
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> > >
On 6/19/20 7:20 PM, Eduardo Habkost wrote: > 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? First of all, I think that this should be very rare case. Though nothing bad is expected. The quest will see old value, i.e. one negotiated at guest startup. Let us assume that block backend has been changed and discard alignment is - less than set. In this case the guest will continue to send larger than possible requests, i.e. some blocks will not be discarded as it could happen. This will happen until the guest restarts and see smaller alignment. First re-trim will discard all non-discarded so far blocks - greater than set. The code will work like now, i.e. some extra requests will be sent. Den
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 v2
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
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>