Replicated type

Data are replicated from a node to another.

Pros:

  • fast (must be used for SSDs)
  • relatively flexible

Cons:

  • lower capacity (so higher cost, better for SSDs)
  • a bit more complex to maintain in distributed-replicated (see "RAID 10 like")

2-way replication

This type is pretty simple to understand: everything written on one node is mirrored to another one. It's very similar to RAID 1.

picture replication

If you lose one node, your data are still here. This mode will give you 50% of your total disk space (e.g with 2x nodes of 100GiB, you'll have only 100GiB of space).

3-way replication

Same than 2-way, but data is replicated on 3 nodes in total.

picture triplication

2 nodes can be destroyed without losing your data. This mode will give you 33% of your total disk space (e.g with 3x nodes of 100GiB, you'll have only 100GiB of space).

Building a "RAID 10" like

If you have more than 2 or 3 nodes, it could be interesting to distribute data on multiple replicated nodes. This is called "distributed-replicated" type. Here is an example with 6 nodes:

picture distributed-replicated with 6 nodes

It's very similar to RAID 10. In this example, you'll have 300GiB of data usable.

This is the mode you'll use in a more than 3 nodes setup.

Examples

Here is some examples depending of the number of XenServer hosts.

2 hosts

This is a kind of special mode. On a 2 nodes setup, one node must know what's happening if it can't contact the other node. This is called a split brain scenario. To avoid data loss, it goes on read only. But there is a way to overcome this, with a special node, called the arbiter. It will only require an extra VM using only few disk space.

Thanks to this arbiter, you'll have 3 nodes running on 2 XenServer hosts:

  • if the host with 1 node is down, the other host will continue to provide a working XOSAN
  • if the host with 2 nodes (1 normal and 1 arbiter) id down, the other node will go into read only mode, to avoid split brain scenario.

This way, in all cases, you are protected.

3 hosts

The easiest way is to use 3-way replication. You can lose completely 2 hosts, it will continue to work on the survivor!

4 hosts

The usual deal is to create a "group" of 2 replicated nodes (2x2). In a picture:

2x2 replication

5 hosts

There is no way to use the local disks of 5 nodes in a replicated type. So you'll use 4 hosts in XOSAN, and the 5th would be also able to use the shared XOSAN SR, without participating directly to it.

6 hosts

You have 2 choices:

  1. 2-way replication on 3 replicated nodes (2x3)
  2. 3-way replication on 2 triplicated nodes (3x2)

There is more fault tolerance on mode 2, but less space usable. It's up to you!

2x3 mode

3x2 mode

Growing a replicated XOSAN

You can grow a replicated XOSAN by adding pairs, in other words "RAID 1"-like mirrors to the existing setup, like you would adds mirrored disks in "RAID 10" setup. Examples:

  • on a 2 hosts setup, going for 4 hosts by adding 2 mirrored nodes
  • on a 3 hosts setup using 3-way replication, by adding 3 mirrored nodes

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