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. Yiping On 3/8/16, 4:28 PM, "Tim Mackey" <tmac...@gmail.com> wrote: >Yiping, > >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. > >-tim > >On Tue, Mar 8, 2016 at 5:24 PM, Ahmad Emneina <aemne...@gmail.com> 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 <yzh...@marketo.com> 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 >> > >>
