On Tue, Sep 30, 2014 at 01:51:21PM +1000, Andrew Beekhof wrote: > > On 30 Sep 2014, at 6:22 am, Lars Ellenberg <lars.ellenb...@linbit.com> wrote: > > > On Wed, Sep 10, 2014 at 11:50:58AM +0200, Lars Ellenberg wrote: > >> > >> Hi Andrew (and others). > >> > >> For a certain use case (yes, I'm talking about DRBD "peer-fencing" on > >> loss of replication link), it would be nice to be able to say: > >> > >> update some_attribute=some_attribute+1 where some_attribute >= 0 > >> > >> delete some_attribute where some_attribute=0 > >> > >> Ok, that's not the classic cmpxchg(), more of an atomic_add(); > >> or similar enough. With hopefully just a single cib roundrip. > >> > >> > >> Let me rephrase: > >> Update attribute "this_is_pink" (for node-X with ID attr-ID): > >> > >> fail if said attr-ID exists elsewhere (not as the intended attribute > >> at the intended place in the xml tree) > >> (this comes for free already, I think) > >> > >> if it does not exist at all, assume it was present with current value 0 > >> > >> if the current (or assumed current) value is >= 0, add 1 > >> > >> if the current value is < 0, fail > >> > >> (optionally: return new value? old value?) > > > > Did anyone read this? > > Yep, but it requires a non-trivial answer so it got deferred :) > > Its a reasonable request, we've spoken about something similar in the past > and its clear that at some point attrd needs to grow some extra capabilities. > Exactly when it will bubble up to the top of the todo list is less certain, > though I would happily coach someone with the necessary motivation. > > The other thing to mention is that currently the only part that wont work is > "if the current value is < 0, fail". > Setting value="value++" will do the rest.
Nice. > So my question would be... how important is the 'lt 0' case? > > Actually, come to think of it, it's not a bad default behaviour. > Certainly failing value++ if value=-INFINITY would be logically consistent > with the existing code. > Would that be sufficient? > I need to think about that some more. I may need to actually try this out and try to implement my scenario. -- : Lars Ellenberg : LINBIT | Your Way to High Availability : DRBD/HA support and consulting http://www.linbit.com DRBD® and LINBIT® are registered trademarks of LINBIT, Austria. _______________________________________________ Pacemaker mailing list: Pacemaker@oss.clusterlabs.org http://oss.clusterlabs.org/mailman/listinfo/pacemaker Project Home: http://www.clusterlabs.org Getting started: http://www.clusterlabs.org/doc/Cluster_from_Scratch.pdf Bugs: http://bugs.clusterlabs.org
