[ovirt-users] Behaviour when attaching shared iSCSI storage with existing data

[Sam McLeod]

Hi Yaniv,

Thanks for your detailed reply, it's very much appreciated.

> On 5 Jan 2018, at 8:34 pm, Yaniv Kaul <ykaul at redhat.com> wrote:
> 
> Indeed, greenfield deployment has its advantages.
> 
> The down side to that is juggling iSCSI LUNs, I'll have to migrate VMs on XenServer off one LUN at a time, remove that LUN from XenServer and add it to oVirt as new storage, and continue - but if it's what has to be done, we'll do it.
> 
> The migration of VMs has three parts:
> - VM configuration data (from name to number of CPUs, memory, etc.)

That's not too much of an issue for us, we have a pretty standard set of configuration for performance / sizing.

> - Data - the disks themselves.

This is the big one, for most hosts at least the data is on a dedicated logical volume, for example if it's postgresql, it would be LUKS on top of a logical volume for /var/lib/pgsql etc....

> - Adjusting VM internal data (paths, boot kernel, grub?, etc.)

Everything is currently PVHVM which uses standard grub2, you could literally dd any one of our VMs to a physical disk and boot it in any x86/64 machine.

> The first item could be automated. Unfortunately, it was a bit of a challenge to find a common automation platform. For example, we have great Ansible support, which I could not find for XenServer (but[1], which may be a bit limited). Perhaps if there aren't too many VMs, this could be done manually. If you use Foreman, btw, then it could probably be used for both to provision?
> The 2nd - data movement could be done in at least two-three ways:
> 1. Copy using 'dd' from LUN/LV/raw/? to a raw volume in oVirt.
> 2. (My preferred option), copy using 'dd' from LUN/LV/raw and upload using the oVirt upload API (example in Python[2]). I think that's an easy to implement option and provides the flexibility to copy from pretty much any source to oVirt.

A key thing here would be how quickly the oVirt API can ingest the data, our storage LUNs are 100% SSD each LUN can easily provide at least 1000MB/s and around 2M 4k write IOP/s and 2-4M 4k read IOP/s so we always find hypervisors disk virtualisation mechanisms to be the bottleneck - but adding an API to the mix, especially one that is single threaded (if that does the data stream processing) could be a big performance problem.

> 3. There are ways to convert XVA to qcow2 - I saw some references on the Internet, never tried any.

This is something I was thinking of potentially doing, I can actually export each VM as an OVF/OVA package - since that's very standard I'm assuming oVirt can likely import them and convert to qcow2 or raw/LVM?

> 
> As for the last item, I'm really not sure what changes are needed, if at all. I don't know the disk convention, for example (/dev/sd* for SCSI disk -> virtio-scsi, but are there are other device types?)

Xen's virtual disks are all /dev/xvd[a-z]
Thankfully, we partition everything as LVM and partitions (other than boot I think) are mounted as such.

> 
> I'd be happy to help with any adjustment needed for the Python script below.

Very much appreciated, when I get to the point where I'm happy with the basic architectural design and POC deployment of oVirt - that's when I'll be testing importing VMs / data in various ways and have made note of these scripts.

> 
> Y.
> 
> [1] http://docs.ansible.com/ansible/latest/xenserver_facts_module.html <http://docs.ansible.com/ansible/latest/xenserver_facts_module.html>
> [2] https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/upload_disk.py <https://github.com/oVirt/ovirt-engine-sdk/blob/master/sdk/examples/upload_disk.py>
>  

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