Re: [Xen-devel] libxl - avoid calling block script

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Really adding Ian and Wei.

On Fri, Feb 09, 2018 at 10:55:24AM +0000, Roger Pau Monné wrote:
> On Fri, Feb 09, 2018 at 02:02:42AM +0100, Marek Marczykowski-Górecki wrote:
> > Hi,
> > 
> > I'd like to avoid calling block script to speed up domain startup a
> > little (there may be multiple disks, all already being block devices).
> > Right now I have restored setting physical-device xenstore entry in
> > libxl (by reverting [1]),
> 
> This will work for Linux, but will break for FreeBSD, which expects
> physical-device-path instead of physical-device as the output of
> hotplug scripts.
> 
> > then applying the patch below (it's on 4.8).
> > This works well for my case, but maybe there is some option to have it
> > in vanilla Xen? Right now, this require explicit "script=block" to call
> > the script (for example to setup loop device).
> > 
> > Alternative idea I have is setting disk->script="block" early
> > (in libxl_device_disk_init()?), so default do not change, but it's still
> > possible to change it to NULL and avoid calling the script. The problem
> > is libxl_device_disk_init() is a generated and I don't see how it could
> > be modified... Any hints?
> > 
> > Yet another idea is having some specific value for disk->script,
> > that would avoid calling it, but I find this much less elegant solution.
> 
> So the problem is creation time for domains that have quite a lot of
> disks attached. Adding Ian and Wei who know more about the async
> dispatch system, but I think (at least from a technical PoV) it
> should be possible to parallelize device attachment and thus hotplug
> script execution. Devices are independent from each other.
> 
> Also the Linux hotplug scripts in general seem extremely convoluted,
> I'm not sure whether we could gain some speed there just by
> simplification.
> 
> Thanks, Roger.
> 
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