nova/releasenotes/notes/bug-1841363-fallback-to-thr...

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---
fixes:
- |
Since Libvirt v.1.12.0 and the introduction of the `libvirt issue`_ ,
there is a fact that if we set cache mode whose write semantic is not
O_DIRECT (i.e. "unsafe", "writeback" or "writethrough"), there will
be a problem with the volume drivers (i.e. LibvirtISCSIVolumeDriver,
LibvirtNFSVolumeDriver and so on), which designate native io explicitly.
When the driver_cache (default is none) has been configured as neither
"none" nor "directsync", the libvirt driver will ensure the driver_io
to be "threads" to avoid an instance spawning failure.
.. _`libvirt issue`: https://bugzilla.redhat.com/show_bug.cgi?id=1086704