Re: new knob to tweak caching mode for backends

[Olaf Hering <olaf@aepfle.de>]

On Tue, May 27, Stefano Stabellini wrote:

> So I think that as long as we keep the default behaviour sane, we can
> expose whatever advanced options we like.

Its not about changing defaults, but rather provide knobs for the new
features. Now that the backend is qdisk (no idea when this became the
default) qemu is in charge. qemu offers a cache= option, and backends
may honour the value.
libvirt seems to have a 1:1 mapping for the qemu backend, I have to
check if libvirt does any processing or if it passes cache= as is.

Now from libirt->libxl->qemu there is no way to pass cache= because
libxl got no cache= knob while qdisk became the default. Again, I have
to check what the libxl driver in libvirt does if cache= specified, if
its ignored or handled as error.


So with this in mind, will a patch be accepted to add some sort of
"char *cache" member to struct libxl_disk_device? Should it accept all
values libvirt and qemu knows about, or just the few values qdisk can
actually handle (nocache|write-back|directsync)?


In addition to that: the direct-io-safe knob does not only enable AIO it
also changes from write-back to directsync. Qemu has a knob aio=native
and cache= for directsync. Looks like direct-io-safe merges both. Is
that intentional? If libxl gets a cache= option, should it be possible
to specify "direct-io-safe,cache=write-back"?


Olaf