[libvirt] libvirt-libxl driver defaulting to tap disk and not working (on Fedora 18 and rawhide)

[Jim Fehlig]

Ian Campbell wrote:
>>     if (l_disk->driverName) {
>>         ...
>>     } else {
>>         /* No driverName - default to raw/tap?? */
>>         x_disk->format = LIBXL_DISK_FORMAT_RAW;
>>         x_disk->backend = LIBXL_DISK_BACKEND_TAP;
>>     }
>>
>> I recall some discussion about the choice of these defaults, but can't
>> find anything in the xen or libvirt ml archives.  We can adjust the
>> defaults (or defer to libxl?) if it makes sense.  I took a quick peek at
>> newer libxl code (recall this was written against Xen 4.1 libxl) and
>> perhaps 'x_disk->backend = LIBXL_DISK_BACKEND_QDISK' would be a saner
>> default?  Adding Ian to help with this question...
>>     
>
> LIBXL_DISK_BACKEND_UNKNOWN will cause libxl to pick the best available
> (which includes checking if blktap is actually there) backend given the
> format and file type (block device, file etc). That's probably the right
> default.
>   

Ok, thanks.

> libxl__device_disk_set_backend is the guy to look at if you are
> interested how the selection happens.
>   

After taking a peek, agreed that LIBXL_DISK_BACKEND_UNKNOWN is a better
backend default.  I'll leave the format default of LIBXL_DISK_FORMAT_RAW
to be consistent with $xen-src/docs/misc/xl-disk-configuration.txt.

Thanks,
Jim