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[Matt Riedemann <mriedem.os@xxxxxxxxx>]

Have you tested this or just guessing that the libvirt driver in nova
isn't doing the right thing? Because multiattach disks are always set to
cache mode "none":

https://github.com/openstack/nova/blob/20bc0136d0665bafdcd379f19389a0a5ea7bf310/nova/virt/libvirt/driver.py#L423-L426

        # Shareable disks like for a multi-attach volume need to have the
        # driver cache disabled.
        if getattr(conf, 'shareable', False):
            conf.driver_cache = 'none'

** Changed in: nova
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1788619

Title:
  disk cachemodes should be restricted with multiattached volumes

Status in Cinder:
  New
Status in OpenStack Compute (nova):
  Invalid

Bug description:
  If using multiattach, the "writeback" and "unsafe" disk_cachemode
  options presumably break the semantics that an application writing to
  a clustered datastore would rely on for data consistency between
  multiple nodes.

  Volumes should not be allowed to attach to multiple instances
  (multiattach) with unsafe cache modes.

  (This may even include writethrough?  I'm not sure.)

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