On Wed, 6 Jul 2011, Ian Jackson wrote:
> +static int disk_try_backend(disk_try_backend_args *a,
> + libxl_disk_backend backend) {
> + /* returns 0 (ie, DISK_BACKEND_UNKNOWN) on failure, or
> + * backend on success */
> + libxl_ctx *ctx = libxl__gc_owner(a->gc);
> + switch (backend) {
> +
> + case LIBXL_DISK_BACKEND_PHY:
> + if (!(a->disk->format == LIBXL_DISK_FORMAT_RAW ||
> + a->disk->format == LIBXL_DISK_FORMAT_EMPTY)) {
> + LIBXL__LOG(ctx, LIBXL__LOG_DEBUG, "Disk vdev=%s, backend phy"
> + " unsuitable due to format %s",
> + a->disk->vdev,
> + libxl_disk_format_to_string(a->disk->format));
> + return 0;
> + }
> + if (a->disk->format != LIBXL_DISK_FORMAT_EMPTY &&
> + !S_ISBLK(a->stab.st_mode)) {
> + LIBXL__LOG(ctx, LIBXL__LOG_DEBUG, "Disk vdev=%s, backend phy"
> + " unsuitable as phys path not a block device",
> + a->disk->vdev);
> + return ERROR_INVAL;
> + }
> +
> + return backend;
> +
> + case LIBXL_DISK_BACKEND_TAP:
> + if (!libxl__blktap_enabled(a->gc)) {
> + LIBXL__LOG(ctx, LIBXL__LOG_DEBUG, "Disk vdev=%s, backend tap"
> + " unsuitable because blktap not available",
> + a->disk->vdev);
> + return 0;
> + }
> + if (a->disk->format == LIBXL_DISK_FORMAT_EMPTY ||
> + (S_ISREG(a->stab.st_mode) && !a->stab.st_size)) {
> + LIBXL__LOG(ctx, LIBXL__LOG_DEBUG, "Disk vdev=%s, backend tap"
> + " unsuitable because empty devices not supported",
> + a->disk->vdev);
> + return 0;
> + }
> + return backend;
> +
TAP should only be used for raw or vhd formats, not for qcow or qcow2.
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