[Linux-ha-dev] serializing actions of resource scripts

[Andrew Beekhof]

On Nov 19, 2007, at 6:40 PM, Sebastian Reitenbach wrote:

> Hi,
>
> I try to add a bit memory management monitoring of services within  
> the Xen
> resource, as I am usually interested in the services that the Xen  
> resource
> provides and not whether Xen runs or not. I already got a lot of  
> valuable
> feedback from Dejan, the state can be tracked here:
> http://developerbugs.linux-foundation.org/show_bug.cgi?id=1778
>
> The problems right now are:
> 1) how to handle paused domU's
> 2) how to handle memory management
>
> The first is a bit work, but easy, before every action check in which
> condition the domU is, e.g. running/stopped/paused/... and then  
> handle that
> situation accordingly.
>
> The second one is a bit trickier.
> Right now I have the Xen script working this way on startup/stop of  
> the
> domU:
> 1. check how many memory is available
> 2. subtract the reserved memory for the dom0
> 3. check how many domains will after that action running, divide the  
> result
> from step two with that number
> 4. set xm mem-set the memory for all virtual machines
>
> This is a very basic way to give all virtual machines more or less  
> the same
> amount of memory. Nevertheless, in case more than one Xen resource is
> starting or stopping, this behavior is prone to race conditions, and I
> already have seen it failing.
> My workaround was to create orders, with symmetrical=no and score=0  
> for all
> Xen resources, so that only one can start at a time.

clones have an ordered=true option

it hasn't been used much so it may not be perfect - but if you find  
any problems you can be sure i'll fix them promptly

>
> Dejan suggested to add locking to the Xen resource script, but I  
> fear that
> this will lead to new errors, e.g. assume the default-action- 
> timeout=30s
> and you have 4 Xen resources, and all four will start up at the same  
> time,
> then the first, will aquire the lock, the rest is waiting. Maybe  
> everything
> will work for the second Xen resource too. But I assume then the  
> startup of
> 3 and 4 will fail, because the default-action-timeout was hit.
>
> Is it possible serialize actions of a given type of resource?

as above, yes :-)
at least for starts and stops

groups, clones and master/slave all support the ordered option and the  
lrmd takes care of primitives.

>
> e.g. the Xen resource could be marked as serialization needed, that  
> means,
> in case there are multiple Xen resources in a cluster, actions to  
> these
> resources are not allowed to happen at the same time?
> So that not the Xen resource script would be responsible for locking  
> and
> allowing/disallowing actions to itself, but the CRMD is responsible.
> This could be made more fine-grained, in case you say, it is not  
> allowed to
> have action for resource Xen at the same time on the same node, but  
> it is
> allowed to have multiple actions to the Xen resource in the cluster.

that we cant do - at the crm level its cluster-wide or not at all

>
> Then the LRMD could be made responsible to serialize the actions.
>
>
> Does that makes sense to anyone, or maybe is that already possible?
>
> kind regards
> Sebastian
>
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