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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 11:27:39 GMT
In my opinion the way we should go is "keep it simple". We really should consider to drop support
for XenServer 6.0.2 here and not making things more complicated and provide more than 1 option.
Of course it depends on how many CS installations are still using XenServer 6.0.2.!
Can somebody give more information on this?

Mit freundlichen Grüßen / With kind regards,

Swen Brüseke

-----Ursprüngliche Nachricht-----
Von: Remi Bergsma [mailto:remi@remi.nl] 
Gesendet: Dienstag, 5. Mai 2015 12:36
An: dev@cloudstack.apache.org
Betreff: Re: [DISCUSS] XenServer and HA: the way forward

Hi all,

Thanks for pointing me to the proposal, Koushik. Too bad no one responded to such a major
change.

When I put my "Operations" hat on, I see several issues. First of all, there was no mention
of this change in the release notes. Not as a new feature, nor as a bug that was fixed. How
do we expect people that operate CloudStack will know about this? It's not even in the recommended
install instructions for a new cloud today. In my experience, when I talk to people about
this, I find that almost no one knows. We as a community can do better than this!

For XenServer 6.5 and 6.2 one can enable XenHA for the pool only, but for XenServer 6.0.2
this is a different story because as far as I know one can only enable HA as a whole (HA on
pool + HA on VMs). And this is only if you have the paid version, which we happen to have.
But I don't think it is a solution, as this leads to corruption when XenServer and CloudStack
try to recover the same VM at the same time (Trust me, I've been there). Why do we even "support"
6.0.2 one could ask?

I still do have some XenServer 6.0.2 clusters running.. If the pool master crashes I need
to manually appoint a new one. I don't like manual work and if I had known before I wouldn't
have upgraded before this was resolved. Or do I miss something here?

If we want to drop support for older XenServer versions, then let's vote for it and be very
clear about it. Dropping XenServer 6.0.2 comes a bit too early if  you ask me.

Let's discuss how to proceed. I still feel the best solution is to add a switch between both
HA methods so one can choose which one suits best, and for older XenServer versions we will
restore the HA feature that way.

Any thoughts?

Regards,
Remi

Op di 5 mei 2015 om 08:13 schreef Koushik Das <koushik.das@citrix.com>:

> The below is the proposal for switching to XenServer HA.
>
>
> http://mail-archives.apache.org/mod_mbox/cloudstack-dev/201403.mbox/%3
> C83F77FF1BD50194AB65AFA5479D082A71E4E8D@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
> >>
>
>



- proIO GmbH -
Geschäftsführer: Swen Brüseke
Sitz der Gesellschaft: Frankfurt am Main

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