[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 1/2] xl: neuter vcpu-set --ignore-host.
On Thu, Sep 26, 2013 at 05:25:17PM +0100, George Dunlap wrote: > On 26/09/13 13:48, Konrad Rzeszutek Wilk wrote: > >On Thu, Sep 26, 2013 at 10:06:31AM +0100, Ian Campbell wrote: > >>On Wed, 2013-09-25 at 16:40 -0400, Konrad Rzeszutek Wilk wrote: > >>>When Xen 4.3 was released we had a discussion whether we should > >>>allow the vcpu-set command to allow the user to set more than > >>>physical CPUs for a guest (it didn't). The author brought up: > >>> - Xend used to do it, > >>IMHO xend is buggy here. If it were being maintained I encourage a patch > >>to file this particular sharp edge off. > >> > >>> - If a user wants to do it, let them do it, > >>We do, we have an option for those who know what they are doing to use > >>in the tiny minority of cases where they need to do this. > >> > >>> - The original author of the change did not realize the > >>> side-effect his patch caused this and had no intention of changing it. > >>a happy accident then. > >> > >>> - The user can already boot a massively overcommitted guest by > >>> having a large 'vcpus=' value in the guest config and we allow > >>> that. > >>IMHO this is an xl bug, I'd be happy to see a patch to fix this and > >>require and override here too. > >I think I posted one some time ago, but I don't recall anybody > >commenting on it. Will repost it. > >>>Since we were close to the release we added --ignore-host parameter > >>>as a mechanism for a user to still set more vCPUs that the physical > >>>machine as a stop-gate. > >>> > >>>This patch keeps said option but neuters the check so that we > >>>can overcommit. In other words - by default the user is > >>>allowed to set as many vCPUs as they would like. > >>and why would a naive user want to do this? non-naive users can use the > >>option if this is what they really want, and are probably grateful for > >>the catch if they didn't intend to overcommit, which is almost always > >>even for expert users. > >> > >>This change need far better rationalisation than "because xend did it" > >>and "because we can". IMHO. > >I am going to defer to George here. His viewpoint (I am going to > >probably mangle it up) was that - if the user wants to do, let him/her > >do it without us putting obstacles. > > > >And I think Ian Jackson was ambivalent here and was deferring to George. > > So I've gone back and read the original thread, and what I actually > said was: > > "So I think the right thing to do long-term is to make it possible > to do in xl. Having a "seatbelt" restriction by default that can be > overridden would be OK with me, but I think a warning message when > vcpus > pcpus would suffice." > > And my summary of mine and IanC's positions at the time (which IanC > did not dispute) was: > > "We both agree that "vcpus > pcpus" is a bad configuration. I think > ideally we should support it (because administrators should be > allowed to shoot themselves in the foot) and Ian[C] seems to be > making the case that we shouldn't support it." > > IanJ, as I understood him, agreed with me that it should be *possible*. > > As IanC points out, it is possible -- you just have to add "--ignore-host". > > So given what all of us think, keeping the "seatbelt" is probably > the best compromise. IanC is happy that a hapless user will not > accidentally shoot his own foot, and IanJ and I are happy that a > skilled user can shoot her own foot if she really wants to. Excellent. Let me prep a patch with said seatbelt option. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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