[ovirt-users] VirtIO-SCSI and viodiskcache custom property

[Matthias Leopold]

Am 2018-01-20 um 19:54 schrieb Yaniv Kaul:
> 
> 
> On Jan 19, 2018 3:29 PM, "Matthias Leopold" 
> <matthias.leopold at meduniwien.ac.at 
> <mailto:matthias.leopold at meduniwien.ac.at>> wrote:
> 
>     Hi,
> 
>     is there a reason why the viodiskcache custom property isn't honored
>     when using VirtIO-SCSI?
> 
>     On a Cinder (Ceph) disk "viodiskcache=writeback" is ignored with
>     VirtIO-SCSI and honored when using VirtIO.
> 
>     On an iSCSI disk "viodiskcache=writeback" is ignored with
>     VirtIO-SCSI and the VM can't be started when using VirtIO with
>     "unsupported configuration: native I/O needs either no disk cache or
>     directsync cache mode, QEMU will fallback to aio=threads"
> 
>     We actually want to use "viodiskcache=writeback" with Cinder (Ceph)
>     disks.
> 
> 
> That's because on block storage we use native io and not threads. I 
> assume the hook needs to change to use native io in this case.
> Y.

Thank you, but i still don't quite get what's missing.

We want to use the combination of Cinder (Ceph) + cache=writeback + 
VirtIO-SCSI. Would this be possible if aio=native was used (which is not 
configurable)? With iSCSI storage you still get cache=none when using 
VirtIO-SCSI and "viodiskcache=writeback" (and aio=native). Should i file 
a bug report for the Cinder disk situation?

Thanks again
Matthias