Xenserver and Storage
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Think priorities. It will impact some users (because the updater in XOA), not dramatic for the business but it's better to avoid that. So the cost to have it is negligible (already using virt). And I don't have the resource to make the service app HA (because live migration is free…)
edit: in the end, if I follow your arguments, virtualization is also useless for SMBs.
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@olivier said in Xenserver and Storage:
edit: in the end, if I follow your arguments, virtualization is also useless for SMBs.
Nope, not in the least. This would imply a misunderstanding of the purpose of virtualization. Virtualization is free and makes things safer. Everyone benefits from virtualization, every time.
HA is not free, adds its own risks (that are very high) and provides uncommon benefits. Most shops are hurt by HA, not helped by it.
Same logic, totally different results.
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I'm not speaking about HA right now, I'm speaking about live migration
HA is another beast, I agree it should be used only after thinking benefits/problems.
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It's simply good business, look at the cost of downtime, look at the cost of HA. Then look at the risks without HA and the risks with HA. Put them together in a normal cost/risk analysis and the result is almost always that HA doesn't deliver something of value enough to overcome its costs. And as it adds a lot of risk (not as much as it mitigates) it is far, very far, from a clear win even in the risk portion.
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@olivier said in Xenserver and Storage:
I'm not speaking about HA right now, I'm speaking about live migration
They are essentially one and the same. The technology to do one does the other. If you have HA, you can live migrate. If you can live migrate, you can't necessarily do HA. I'm giving you the advantage by lumping them together since the cost of one gives you both.
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HA is automated and more "dangerous". Live migration is a manual process. That was the context I meant.
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If I'm willing to do live migration without HA, you get even more options, technically, making live migration easier and no shared storage needed at all.
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@olivier said in Xenserver and Storage:
HA is automated and more "dangerous". Live migration is a manual process. That was the context I meant.
Makes sense. I've seen live migration beliefs take down big banks because someone thought it was safe and did it without a greenzone and the hypervisors (ESXi) died from trying to do it.
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If live migrations were free, carried no risks, and took no effort, of course they would be a great benefit. But free and riskless they are not. That's what causes problems.
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I’ve said it before, but I let the VMs do this. It’s less complex (when automated). Gluster does the replication in the VMs (for the very few that need it). Everything else is either stateless with floating IPs and rp/lb or it’s at the application layer.
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@stacksofplates said in Xenserver and Storage:
I’ve said it before, but I let the VMs do this. It’s less complex (when automated). Gluster does the replication in the VMs (for the very few that need it). Everything else is either stateless with floating IPs and rp/lb or it’s at the application layer.
You are using Gluster inside of the "HA VMs"? So a file server cluster, for example, are VMs on top of local storage, but with Gluster inside of the VMs, so the VMs can be shut down and then fired up on top of any platform including physical or a different hypervisor, or just left off during work because there are other cluster members available, to remove the need for a live migration?
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@scottalanmiller said in Xenserver and Storage:
@stacksofplates said in Xenserver and Storage:
I’ve said it before, but I let the VMs do this. It’s less complex (when automated). Gluster does the replication in the VMs (for the very few that need it). Everything else is either stateless with floating IPs and rp/lb or it’s at the application layer.
You are using Gluster inside of the "HA VMs"? So a file server cluster, for example, are VMs on top of local storage, but with Gluster inside of the VMs, so the VMs can be shut down and then fired up on top of any platform including physical or a different hypervisor, or just left off during work because there are other cluster members available, to remove the need for a live migration?
Correct. Either scenario. So if we lose a host it’s rekickstarted (which is like 10 mins), the template is added, and then Ansible will recreate the guests and run the provisioning against them. The provisioning joins the VM back into the cluster and data starts replicating to it.
If they aren’t using Gluster they just run whatever provisioning is needed.
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It’s mainly stuff like repos, misc apps, stupid stuff. Any super important data goes on the Isilon.
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@olivier said in Xenserver and Storage:
Real life usage
So we decided to take a look with some benchmarks, and despite choosing in priority something safe/flexible, we had pretty nice performances, as you can see in our multiple benchmarks.Your benchmarks leave a lot to be desired. I don't see working set size. Testing the performance of local DRAM (What gluster does). This isn't very real world....
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@storageninja It's all explained here:
FIO is used to make the benchmarks, on a Debian 9 VM. It's done on a 10GiB file (enough to avoid caching). Throughput is fetched from XenServer VDI RRDs values, which is pretty accurate and close to the "reality". IOPS are fetched from FIO directly. You can find some FIO examples in this Sam's blog post.
4k for IOPS and 4M used for throughput.
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Ok, so let me check my understanding here.
VSAN would be a management VSA running on each host, with the local storage assigned to it in 2GB VHD chunks, presumably the VSA would aggregate these chunks. The VSAs will then keep both local SRs perfectly in sync via a dedicated direct link between the hosts, they then allow me to present the total space to the hosts as an iSCSI SR on which I can place the VM VHDs (so it'll be VHDs in VHDs on the host's storage).
If one host goes down, then the HA feature will auto migrate the VM to the running host.
So what happens if it's just the dedicated link that dies? Will all my VMs be running on both hosts on my network (causing a ton of issues)? And how does this setup cope with the data getting out sync if a host fails?
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I don't know about VSAN, but for XOSAN, it will be:
- a virtual shared SR is exposed to XenServer (for XenServer, it's a bit like a NFS shared SR)
- data is chunked (how depends on XOSAN mode, replicated or disperse) on various nodes
- if one host is down with its VM, XS will boot those VMs to other hosts
- each node decide to stop or not their write operations if they can meet the quorum. Eg on 3 hosts, it means 2 VMs can still communicate: it's OK. The isolated host (running but cut of from the rest) will be read only. Luckily, XenServer HA knows it, the host will stop and its VM started elsewhere.
You won't have split brain scenario (data written "independently" on various sides). If quorum is not met: go back to read only. Data integrity is more important than being able to write.
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@jrc said in Xenserver and Storage:
VSAN would be a management VSA ...
So what happens if it's just the dedicated link that dies? Will all my VMs be running on both hosts on my network (causing a ton of issues)? And how does this setup cope with the data getting out sync if a host fails?
I hate all these terms. LOL. VSAN is just a normal SAN, but virtualized. You can just use SAN and that, hopefully, answers all questions alone.
VSA is a really weird acronym that is used to mean a virtualized NAS. So VSAN is not a VSA as one is SAN and one is NAS. Neither term needs to exist, because they are still SAN and NAS.
How do SANs normally cope with losing connectivity to each other?
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@scottalanmiller said in Xenserver and Storage:
@jrc said in Xenserver and Storage:
VSAN would be a management VSA ...
So what happens if it's just the dedicated link that dies? Will all my VMs be running on both hosts on my network (causing a ton of issues)? And how does this setup cope with the data getting out sync if a host fails?
I hate all these terms. LOL. VSAN is just a normal SAN, but virtualized. You can just use SAN and that, hopefully, answers all questions alone.
It does not, and yes I hate these acronyms as well.
How do SANs normally cope with losing connectivity to each other?
I've no clue as I have only ever worked with the one I have, an it is a single unit multipathed to my 2 hosts. So are you saying that real SANs also sync data between themselves?? So the 2 VMs (what I called VSAs) are like 2 physical SANs?
Look I get that VSAN = SAN in all functionality once setup. It's the setup, and the possible ramifications of said setup that I am unclear on.
I am trying to work out the best path from my current, fragile setup, to one that is more reliable and fault tolerant.
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@jrc said in Xenserver and Storage:
I've no clue as I have only ever worked with the one I have, an it is a single unit multipathed to my 2 hosts. So are you saying that real SANs also sync data between themselves??
Of course, if you want high availability. That's the only path to HA. Any storage device that you want protection against system failure needs a synced unit that you can fail over to. That's why we say SANs generally only make sense when you have zero or two. A single unit is a single point of failure and most SANs aren't as reliable as normal servers, so generally a really fragile single point of failure.
So if you don't care about HA, you don't need two SANs or two VSANs. They are literally the same things. If you do want HA, you need at least two of either. VSAN can have the benefit of being RLS, which a hardware SAN cannot, so VSAN has the possibility of being way safer, even if you have only one. Fewer points of failure.
