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my minimalistic configuration is composed of 1 storage + 2 XenServer in a pool.

My questions:

1) Will HA work in this configuration?
2) What is better for HA, connection with the storage by iSCSI or NFS ?
3) Could you somebody explain me what is iSCSI multipathing and how could I use it?


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1) HA should work fine with this configuration

2) There's advantages to having a direct iSCSI connection to the storage, however then you're depending heavily on the xen machine filesystem and you won't be able to have the same iSCSI volume mounted on both machines if you're not using a clustered file system (CFS, GFS, the likes...).

NFS on the other hand can be a bit hard to maintain but easier in the end, if I were you I would go for NFS for starters and see if that fits your needs, and then maybe do an engineering project on how to do a clustered filesystem with iSCSI volumes in the future

3) iSCSI multipathing is a technology in which you have more than one "path" to access your iSCSI storage (two controllers, two ethernet cards, two different cables, so more than one way to get there), with multipathing you define all the ways you can get to that storage and your machine will then create a virtual disk device which will be a representation of the iSCSI volume through any of the possible ways to access it, if for some reason that preferred path goes down multipath will automatically try to pick another one until it gets out of options.

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+1. Nice answer. – Carlos Garcia Jan 28 '11 at 10:53

It kind of depends on how you implement HA. If you're going to have fallback on the virtual machine level, I would not use NFS. Storing virtual machine images on NFS has performance problems. The best way to store a virtual machine image is on a block device like a logical volume.

For networked virtual machine storage, you could export your storage space with iSCSI, make it into a volume group, use clustered LVM on the nodes to create LV's and have those LV's available on both xen nodes.

Or you can research some kind of cloud solution, like Ubuntu Cloud. Unfortunately, I can't give you any details about that, though...

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