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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On Wed, Apr 27, 2016 at 5:22 PM, Jim Fehlig <jfehlig@xxxxxxxx> wrote:
> 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?

What's the effective difference between caching modes for read-only
media anyway?

 -George

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