Re: [Xen-devel] libvirt, libxl and QDISKs

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 26 Apr 2013, Ian Campbell wrote:
> On Fri, 2013-04-26 at 11:10 +0100, Stefano Stabellini wrote:
> > On Fri, 26 Apr 2013, Jim Fehlig wrote:
> > >     if (l_disk->driverName) {
> > >         if (STREQ(l_disk->driverName, "tap") ||
> > >             STREQ(l_disk->driverName, "tap2")) {
> > >             switch (l_disk->format) {
> > >             case VIR_STORAGE_FILE_QCOW:
> > >                 x_disk->format = LIBXL_DISK_FORMAT_QCOW;
> > >                 x_disk->backend = LIBXL_DISK_BACKEND_QDISK;
> > >                 break;
> > >             case VIR_STORAGE_FILE_QCOW2:
> > >                 x_disk->format = LIBXL_DISK_FORMAT_QCOW2;
> > >                 x_disk->backend = LIBXL_DISK_BACKEND_QDISK;
> > >                 break;
> > >             case VIR_STORAGE_FILE_VHD:
> > >                 x_disk->format = LIBXL_DISK_FORMAT_VHD;
> > >                 x_disk->backend = LIBXL_DISK_BACKEND_TAP;
> > >                 break;
> > >             case VIR_STORAGE_FILE_NONE:
> > >                 /* No subtype specified, default to raw/tap */
> > >             case VIR_STORAGE_FILE_RAW:
> > >                 x_disk->format = LIBXL_DISK_FORMAT_RAW;
> > >                 x_disk->backend = LIBXL_DISK_BACKEND_TAP;
> > >                 break;
> > >             default:
> > >                 virReportError(VIR_ERR_INTERNAL_ERROR,
> > >                                _("libxenlight does not support disk
> > > driver %s"),
> > >                               
> > > virStorageFileFormatTypeToString(l_disk->format));
> > >                 return -1;
> > >             }
> > >         } else if (STREQ(l_disk->driverName, "file")) {
> > >             x_disk->format = LIBXL_DISK_FORMAT_RAW;
> > >             x_disk->backend = LIBXL_DISK_BACKEND_TAP;
> > >         } else if (STREQ(l_disk->driverName, "phy")) {
> > >             x_disk->format = LIBXL_DISK_FORMAT_RAW;
> > >             x_disk->backend = LIBXL_DISK_BACKEND_PHY;
> > >         } else {
> > >             virReportError(VIR_ERR_INTERNAL_ERROR,
> > >                            _("libxenlight does not support disk driver 
> > > %s"),
> > >                            l_disk->driverName);
> > >             return -1;
> > >         }
> > >     } else {
> > >         /*
> > >          * If driverName is not specified, default to raw as per
> > >          * xl-disk-configuration.txt in the xen documentation and let
> > >          * libxl pick a suitable backend.
> > >          */
> > >         x_disk->format = LIBXL_DISK_FORMAT_RAW;
> > >         x_disk->backend = LIBXL_DISK_BACKEND_UNKNOWN;
> > >     }
> > 
> > It looks like the defaults are the same of libxl.
> > 
> > However the mapping of RAW to TAP (libxl does the same) has always been
> > a bit dubious to me: now that upstream QEMU is used with HVM guests too
> > by libxl, there is no reason to use blktap over QEMU for raw files any
> > more.
> 
> There are two TAP+RAW in the above, one is inside an if (driver="tap")
> which seems reasonable, are you talking about the one in the
> driver="file"?

Yes

> I think it would be better for libvirt in the file and phy cases to just
> say format=raw and leave libxl to pick a backend capable of providing
> this.

I agree

> Changing what libxl does is a separate question, but I think you are
> right that a case could be made for preferring qdisk in the default
> case.

Indeed

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