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[ybyn <1864857@xxxxxxxxxxxxxxxxxx>]

Public bug reported:

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

** Affects: nova
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1864857

Title:
  cinder volumes  lvm iscsi  use io=native default?

Status in OpenStack Compute (nova):
  New

Bug description:
  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

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