XenServer hyperconverged
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If you shutdown all XOSAN VMs, your SR is unreachable. When you start it, as soon the minimal number of nodes is up, it will work again (even if there is auto healing, it will be slower but work in R/W)
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If you replace one node, you can do it live (we have a "replace" button in the UI!). The new node will replace the old one and get all the missing pieces via the heal process.
In the end, nothing to do to make it work again
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What if you shut down host 1 now, and host 2 an hour from now, then power one host 1? Will the system just run now with the old data?
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@Dashrender Can you be more specific on the total number of nodes from the start?
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@olivier I think this is a 2 node setup that @Dashrender is discussing.
Can the system scale to more than 2 nodes?
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@olivier said in XenServer hyperconverged:
@Dashrender Can you be more specific on the total number of nodes from the start?
Let's assume your picture, 6 total nodes, but only 2 copies of data. So assume node1 in orange is shutdown, then 1 hour later node2 orange is shutdown. What happens?
How about a 2 node setup? i.e. no other witnesses to the configuration.
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@dustinb3403 said in XenServer hyperconverged:
@olivier I think this is a 2 node setup that @Dashrender is discussing.
Can the system scale to more than 2 nodes?
This IS meant to scale (up to max pool size, 16 hosts, which is more a XenServer limit than ours
)@Dashrender The picture is not clear maybe: each "disk" icon is not a disk, but a XenServer host. So you have 6 hosts there. When you lost "enough" hosts (2 replicated hosts in the 6 setup, in the same "mirror"), it will be in read only. As soon one of this 2 is back online, R/W is back.
On a 2 node setup, there is an arbiter VM that acts like the witness. If you lose the host with the 2x VMs (one arbiter and one "normal"), you'll go in read only. No split brain possible.
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In this picture
https://i.imgur.com/VKOVnkU.pnghow many systems have the data on it? only 2?
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@olivier said in XenServer hyperconverged:
@dustinb3403 said in XenServer hyperconverged:
@olivier I think this is a 2 node setup that @Dashrender is discussing.
Can the system scale to more than 2 nodes?
This IS meant to scale (up to max pool size, 16 hosts, which is more a XenServer limit than ours
)@Dashrender The picture is not clear maybe: each "disk" icon is not a disk, but a XenServer host. So you have 6 hosts there. When you lost "enough" hosts (2 replicated hosts in the 6 setup, in the same "mirror"), it will be in read only. As soon one of this 2 is back online, R/W is back.
On a 2 node setup, there is an arbiter VM that acts like the witness. If you lose the host with the 2x VMs (one arbiter and one "normal"), you'll go in read only. No split brain possible.
The question is (i think) if you lost all hosts in the orange group, would XOSAN and the operating VM's in the entire pool still be functional Read/Write until those servers are brought back online?
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@olivier So you just can't make any writes during this period of node failure/recovery? Are the writes cached? If so how much can be cached and for how long?
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@dashrender said in XenServer hyperconverged:
In this picture
https://i.imgur.com/VKOVnkU.pnghow many systems have the data on it? only 2?
This is a part of distributed-replicated setup. In this "branch", you have 2 XS hosts with 100GiB data on each, in "RAID1"-like.
The others branches (not in the picture you displayed) are like a RAID0 on top.
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The question is (i think) if you lost all hosts in the orange group, would XOSAN and the operating VM's in the entire pool still be functional Read/Write until those servers are brought back online?
Because data are spread on all subvolumes (think like RAID10), you'll be in read only on the whole thing.
You can avoid that if you decide to NOT stripe files on all subvolumes (which is the default behavior in Gluster by the way), but it's NOT a good thing for VMs (because heal time would be horrible, and subvolumes won't be balanced)
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so @olivier on each host in an XOSAN pool, is there a dedicated witness VM?
If so that witness acts as the arbitrator for that host. Meaning if the VM goes offline the available storage, ram and CPU for that host is unavailable.
It doesn't mean that there is an individual VM running on that host that wouldn't be able to move to either of the other 2 servers in the 3 server pool.
Am I correct in thinking that the Orange, Yellow and Pink boxes are individual XS servers, presenting 100GB each to the pool?
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@dustinb3403 I suppose you speak about the 2 XS host setup. It's only in this case you need an arbiter, and it will be on one of the 2 host (to avoid split brain).
If only your arbiter VM is down, quorum is still met (2 on 3). If you lose more than 1/3 of nodes (arbiter or not), you are on read only (protecting against split brains).
And no, you are not correct: each disk picture is a XenServer host. There is a XOSAN VM controller on each host (and an extra arbiter ONLY in 2 hosts scenario, if you grow the XOSAN from 2 to 4, no more arbiter, it's automatically removed)
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I asked to modify the picture to draw a host with the disk inside to avoid confusion
Thanks for the feedback on that guys 
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I haven't read the docs so this might be a stupid question...
Orange, Yellow & Pink each contain 2 hosts, correct?
You need to always add 2 hosts every time you scale up/out?
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@olivier said in XenServer hyperconverged:
And no, you are not correct: each disk picture is a XenServer host. There is a XOSAN VM controller on each host (and an extra arbiter ONLY in 2 hosts scenario, if you grow the XOSAN from 2 to 4, no more arbiter, it's automatically removed)
OK that is what I assumed in a private chat, that each disk image, inside of each orange, yellow and pink box was it's own server.
I also mentioned that in a PM that each server in the pool would have it's own witness.
The picture is a bit confusing though. (glad you asked for an more clear depiction)
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@fateknollogee said in XenServer hyperconverged:
You need to always add 2 hosts every time you scale up/out?
Good question.
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@fateknollogee If you are in replicated-2, you can add:
- 2 hosts to create a new extra RAID1 (bottom of a RAID0), creating a distributed-replicated (2x2)
- 1 host to go from replicated 2 replicated 3 (data is copied on three hosts, so it's a 1x3)
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@olivier said in XenServer hyperconverged:
@fateknollogee If you are in replicated-2, you can add:
- 2 hosts to create a new extra RAID1 (bottom of a RAID0), creating a distributed-replicated (2x2)
- 1 host to go from replicated 2 replicated 3 (data is copied on three hosts, so it's a 1x3)
So you can scale in any fashion you want.
What are the benefits to scaling with 2 hosts at a time?
What are the benefits to scaling with only a single host at a time?
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@dustinb3403 Scaling at one host: going on "triplicate" (same data on each node, you have 1/3 of total disk capacity, but you can lose 2 hosts and still have data access).
Scaling by adding 2 hosts at the same time: it's adding a subvolume, so more space.
