[important]cinder volumes lvm iscsi use io=native default?

[ybyn]

Description
===========
when i'm using cinder volumes , and use lvm as backend, i want to enhance my disk performance in my case, i boot my guest with volumes, and when change the cachemode to block=writethrough or blcok=writeback, then nova-compute will rasie: `libvirtError: unsupported configuration: native I/O needs either no disk cache or directsync cache mode, QEMU will fallback to aio=threads`
i found that in `nova/virt/libvirt/volume/iscsi.py：`
the code is:
 `def get_config(self, connection_info, disk_info):
    """Returns xml for libvirt."""
    conf = super(LibvirtISCSIVolumeDriver,
                 self).get_config(connection_info, disk_info)
    conf.source_type = "block"
    conf.source_path = connection_info['data']['device_path']
    conf.driver_io = "native"
    return conf`

why here specify the driver_io to native , do i have chance to change it back to `threads` in the conf?

Steps to reproduce
==================
nova.conf:
disk_cachemodes = file=default,block=writethrough (or writeback the same)
restart nova-compute

Expected result
===============
guest can normally started

Actual result
=============
I/O needs either no disk cache or directsync cache mode, QEMU will fallback to aio=threads`

Environment
===========
1. Exact version of OpenStack you are running. See the following
  nova-common/stable 2:17.0.13
stable queens build from source.

2. Which hypervisor did you use?
   libvirt-qemu x86_64

2. Which storage type did you use?
  lvm

3. Which networking type did you use?
   flat linux-bridge

[Arthur Dayne (palagend)]

Please see the review https://review.opendev.org/#/c/682772/

[Arthur Dayne (palagend)]

Related bug is https://bugs.launchpad.net/nova/+bug/1841363