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From S. Brüseke - proIO GmbH <s.brues...@proio.com>
Subject AW: [DISCUSS] XenServer and HA: the way forward
Date Tue, 05 May 2015 08:04:05 GMT
Please correct me if I am wrong, but does that mean that in CS 4.3 a crash of the pool master
will disable the cluster for CS?!?

As a technician I feel more save to add a new host to a cluster using XenCenter because of
shared storage, NICs and other stuff. CS then needs to discover the new host and add it to
its system.
If a pool master fails than CS should discover the new pool master like XenCenter is doing
this.

Mit freundlichen Grüßen / With kind regards,

Swen Brüseke

-----Ursprüngliche Nachricht-----
Von: Koushik Das [mailto:koushik.das@citrix.com] 
Gesendet: Dienstag, 5. Mai 2015 08:12
An: <dev@cloudstack.apache.org>
Betreff: Re: [DISCUSS] XenServer and HA: the way forward

The below is the proposal for switching to XenServer HA.

http://mail-archives.apache.org/mod_mbox/cloudstack-dev/201403.mbox/%3C83F77FF1BD50194AB65AFA5479D082A71E4E8D@SJCPEX01CL02.citrite.net%3E


On 04-May-2015, at 9:03 PM, Tim Mackey <tmackey@gmail.com> wrote:

> Thanks for starting this thread Remi.
> 
> From my perspective the pros of simply enabling XenServer HA are:
> 
> - automatic election of pool master in the event of hardware failure
> - automatic fencing of a host in the event of dom0 corruption
> - automatic fencing of a host in the event of heartbeat failure
> 
> The risks of simply enabling XenServer HA are:
> 
> - additional code to detect a newly elected pool master
> - acceptance of the fact an admin can force a new pool master from 
> XenServer CLI
> - requirement for pool size to be greater than 2 (pool size of 2 
> results in semi-deterministic fencing which isn't user obvious)
> - understanding that storage heartbeat can be shorter than storage 
> timeout (aggressive fencing)
> - understanding that HA plans are computed even when no VMs are 
> protected (performance decrease)
> 
> One question we'll want to decide on is who is the primary actor when 
> it comes to creating the pool since that will define the first pool master.
> During my demo build using 4.4 at CCCEU I expected to add pool members 
> through the CS UI, but found that adding them in XenServer was required.
> This left me in an indeterminate state wrt pool members.
> 
> I vote that if a host is added to CS and it *is* already a member of a 
> pool, that the pool be imported as a cluster and any future membership 
> changes happen using CS APIs.  If a host is added which isn't a member 
> of a pool, then the user be asked if they wish to add it to an 
> existing cluster (and behind the scenes add it to a pool), or create a 
> new cluster and add it to that cluster.  This would be a change to the "add host" semantics.
> Once the host is added, we can enable XenServer HA on the pool if it 
> satisfies the requirements for XenServer HA (has shared storage and 
> three or more pool members).
> 
> There are some details we'd want to take care of, but this flow makes 
> sense to me, and we could use it even with upgrades.
> 
> -tim
> 
> On Mon, May 4, 2015 at 6:04 AM, Remi Bergsma <remi@remi.nl> wrote:
> 
>> Hi all,
>> 
>> Since CloudStack 4.4 the implementation of HA in CloudStack was 
>> changed to use the XenHA feature of XenServer. As of 4.4, it is 
>> expected to have XenHA enabled for the pool (not for the VMs!) and so 
>> XenServer will be the one to elect a new pool master, whereas 
>> CloudStack did it before. Also, XenHA takes care of fencing the box 
>> instead of CloudStack should storage be unavailable. To be exact, 
>> they both try to fence but XenHA is usually faster.
>> 
>> To be 100% clear: HA on VMs is in all cases done by CloudStack. It's 
>> just that without a pool master, no VMs will be recovered anyway. 
>> This brought some headaches to me, as first of all I didn't know. We 
>> probably need to document this somewhere. This is important, because 
>> without XenHA turned on you'll not get a new pool master (a behaviour change).
>> 
>> Personally, I don't like the fact that we have "two captains" in case 
>> something goes wrong. But, some say they like this behaviour. I'm OK 
>> with both, as long as one can choose whatever suits their needs best.
>> 
>> In Austin I talked to several people about this. We came up with the 
>> idea to have CloudStack check whether XenHA is on or not. If it is, 
>> it does the current 4.4+ behaviour (XenHA selects new pool master). 
>> When it is not, we do the CloudStack 4.3 behaviour where CloudStack is fully in control.
>> 
>> I also talked to Tim Mackey and he wants to help implement this, but 
>> he doesn't have much time. The idea is to have someone else join in 
>> to code the change and then Tim will be able to help out on a 
>> regularly basis should we need in depth knowledge of XenServer or its 
>> implementation in CloudStack.
>> 
>> Before we kick this off, I'd like to discuss and agree that this is 
>> the way forward. Also, if you're interested in joining this effort 
>> let me know and I'll kick it off.
>> 
>> Regards,
>> Remi
>> 




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