Re: Shared image files and block script performance

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-09-29 at 15:18 -0600, Mike Latimer wrote:
> Hi Ian,
> 
> On Tuesday, September 29, 2015 10:25:32 AM Ian Campbell wrote:
> > On Mon, 2015-09-28 at 17:14 -0600, Mike Latimer wrote:
> > > Any better options or ideas?
> > 
> > Is part of the problem that shell is a terrible choice for this kind of
> > check?
> 
> There is some truth to that...  ;)
> 
> > Would shelling out to a helper utility allow this to be written in
> > something better?
> 
> A helper utility would be useful, however, I'm seeing a huge amount of gain 
> with nothing more than a little code motion. Specifically, if shared_list is 
> generated within the check_sharing function, the (potentially) large list of 
> devices is not too painful to work with.
> 
> For example, the attached patch works well in my environment, and removes the 
> exponential slowdown. The main change is that $devmm becomes a comma delimited 
> list of devices (major:minor) to check against the vbd's found in xenstore. A 
> few minor changes are required along the way, but nothing significant. The 
> comma delimited list might become problematic at very large numbers (hundreds) 
> of a single shared device, but I don't think it will be a problem in practice. 
> Even if it has limitations, this approach offers significant improvements in 
> performance.
> 
> I'll continue to test this patch here, but I'm interested in your opinion.

If you have a patch which improves things then I think you should just go
ahead formally submit it (http://wiki.xen.org/wiki/Submitting_Xen_Patches).

Ian.