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From Yiping Zhang <>
Subject Re: XenServer cluster size
Date Wed, 09 Mar 2016 17:38:53 GMT
Hi, Tim:

Thanks for very detailed reply.

These are Gen8 HP blades and all my new servers will be Gen9.  That’s why I’d like to
combine them into one maxed out cluster.  I have only two guest VLAN’s and roughly 400 VM
instances for this 10 hosts cluster.  So I think performance wise I should be OK.


On 3/8/16, 4:28 PM, "Tim Mackey" <> wrote:

>Here's the detailed answer ....
>From the XenServer perspective, there are a number of factors which go into
>how various configuration limits are arrived at. Most of the time, they
>aren't hard limits (for example I know of users with more than 16 hosts in
>a pool). What the XenServer team do is for a given metric they determine
>the point at which overall scalability is reduced to a target threshold.
>That then becomes the "configuration limit" for a given release, and we
>retest with every version.
>In the case of the "hosts per pool" limit, we need to ensure that all
>operations we have can be performed without impairment with a given number
>of hosts in a pool. We've kept the same maximum number of hosts in a pool
>for a very long time (close to ten years so far), and that's a direct
>reflection of how much we've increased individual host scalability.
>From a CloudStack perspective, there have been a number of serious scale
>limits which have pushed XenServer. Hundreds of VLANs is one example that
>Ahmad cites, but its also a case of the number of VMs and needing to manage
>all those VM objects.  iirc, the eight host recommendation came from some
>large deployment requirements. If you don't have a need for 100s of VLANs
>per pool, or aren't running 100s of VMs per host, you likely will be able
>to get more than eight hosts per pool.
>From an operations perspective, I would look closely at your pool size and
>ask the question of why you want to such a large pool.  I'd argue having
>two pools of five hosts is more efficient in CloudStack than a single pool
>of ten hosts, plus if something should happen to one pool, the remaining
>pool will continue to be available.  CloudStack is very efficient at
>managing resource pools, so many of the reasons traditional server admins
>cite for wanting large pool sizes aren't as relevant in CloudStack.
>Of particular note is how you scale. With a ten host pool size, that's your
>scalability block size, so as you grow you'll want to increase capacity in
>chunks of ten hosts. With a smaller pool size, you'd be able to add
>capacity in much smaller chunks.
>On Tue, Mar 8, 2016 at 5:24 PM, Ahmad Emneina <> wrote:
>> IIRC, its just a recommendation. I think it stemmed from performance
>> impact, due to numerous VLAN's present, in environments with lots of
>> tenants.
>> On Tue, Mar 8, 2016 at 2:10 PM, Yiping Zhang <> wrote:
>> > Hi, all:
>> >
>> > The CloudStack doc recommends that for XenServer, do not put more than 8
>> > hosts in a cluster, while the Citrix XenServer doc says that XenServer
>> 6.5
>> > can natively support 16 hosts in a cluster (resource pool).
>> >
>> > I am wondering why CloudStack is recommending a smaller cluster size than
>> > that XenServer can natively support?  If I create a cluster with 10
>> > XenServers, what could go wrong for me ?  Has any one tried with CS
>> cluster
>> > with >8 XenServer hosts ?
>> >
>> > My environment is CS 4.5.1 (soon to be upgraded to 4.8.0) on RHEL 6.7 and
>> > XenServer 6.5, using NetApp volumes for both primary and secondary
>> storages.
>> >
>> > Yiping
>> >
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