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Re: [Xen-devel] [PATCH] libxl: don't add cache mode for empty drives

On 04/27/2016 01:38 AM, Roger Pau Monné wrote:
> On Tue, Apr 26, 2016 at 10:35:31PM -0600, Jim Fehlig wrote:
>> qemu commit 91a097e7 forbids specifying the cache mode for empty
>> drives. Attempting to create a domain with an empty qdisk cdrom
>> results in
>> qemu-system-x86_64: -drive if=ide,index=1,readonly=on,media=cdrom,
>>    cache=writeback,id=ide-832: Must specify either driver or file
>> Change libxl to only emit cache mode when a cdrom target is specified.
> What happens then when a cdrom is inserted? I cannot seem to find the code 
> in libxl_cdrom_insert that sets the cache mode.

I cannot find it either. I suppose it would need to be setup via xenstore,
similar to other options like feature_discard. But looking at
$qemu-src/hw/block/xen_disk.c, it seems the XenBlkDev struct has no field to
specify cache mode. Would qemu's xen_disk need to be extended to support cache
mode, followed by a libxl patch to set the cache mode in xenstore?

>  Is the default one used 
> then?

Yes, the default cache mode (which is already writeback AIUI) would be used if
not explicitly specified. Which brings up the option of removing
'cache=writeback' for cdroms altogether. Any opinion on that option?


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