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XEN : Can't add more that 4 hard drive or 2TB HDD to guest

Posted on 2013-10-23
Medium Priority
Last Modified: 2013-11-03
I'm having issues assigning more than 4 hard drive to a XEN guest. When SeaBIOS loads, ATA-7 shows up meaning they're being attached as PATA devices. So this should be the reason why I'm limited to just 4 hard drives.

Also I've tried to attach a 2TB hard drive and sea bios does not detect this drive. Hard drive is a WD Cavier Black (Model: WD2002FAEX).

Any one had the same issue as this previously? Thanks.
Question by:dave558
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LVL 13

Assisted Solution

by:Daniel Helgenberger
Daniel Helgenberger earned 2000 total points
ID: 39596741
What is your actual XEN/OS version?
Please see resource limits imposed by XEN:

Virtual Disk Images (VDI) (including CD-ROM) per Virtual Machine 7
Virtual CD-ROM drives per Virtual Machine 1
Virtual Disk Size (NFS) 2TB minus 4GB
Virtual Disk Size (LVM) 2TB minus 4GB

And disk image number note:
This number is not programmatically enforced, but is the supported limit. Customers can configure up to 16 VDIs using the xe Command Line

I think you are talking about boot volumes in your guests?
Check if something like a little smaller than 2TB works.

Go for KVM/libvirt ;)

Author Comment

ID: 39596856
I'm using Xen 4.3 with Debian Wheezy as domain 0.

Also would it make a difference if I use different bios like ovmf? Do I just need to enable during build-time with ./configure --enable-ovmf? Will I need to do anything else?

Also is it worthwhile to have a look at XCP as alternative? What limitations does it have? Also what are the limitation of KVM?

My main requirements are as follows:
* Support for 2TB and above
* Virtual Disk Images of 7 should be sufficient (inc CD-ROM)
* PCI pass through

Thank you.
LVL 13

Accepted Solution

Daniel Helgenberger earned 2000 total points
ID: 39597771
Please avoid boot disks with more than 2TB. These would require GPT and GPT in turn requires UEFI. I do not know for XEN, but in all the hypervisors I have used UEFI support is far from perfect.
Nothing stops you from booting a small image and mounting a large one in your VM.
Maybe I do not get your setup correct; but passing through a whole disk, (opposed to a LUN) is not considered best practice.
Rather create a LVM physical volume from it and pass Logical Volumes on it as "devices", for instance.

Concerning KVM: the only limit per sé is your hypervisor host, but there are "tested" limits:
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Is your cloud always on? With an Always On cloud you won't have to worry about downtime for maintenance or software application code updates, ensuring that your bottom line isn't affected.


Author Comment

ID: 39599169
Yes, I'm consider using LVM to manage volumes as it does offer a lot more flexibility.

At this stage, I'm currently testing passing multiple LVM partitions to a single guest machine and noticed that they been emulated as IDE device thus limiting the number to only four. Regardless of whether devices are virtual files, partitions or physical hard drives, seabios is presenting them as IDE emulated devices.

Guest is booting off a smaller partition but when passing a 2TB hard drive, the OS does not pick up the hard drive due to bios being non-uefi. I would think ovmf would fix this problem but its development is still experimental.

I'll give para-virtualisation a go and see if it does fix both this issue. Thank you.
I might look paravirtual
LVL 13

Expert Comment

by:Daniel Helgenberger
ID: 39599226
Paravirtual should be the prefered way when it comes to host devices; with the right caching algorithm (write back) you should get about 90% of the native performance of the drive while keeping flebiliy.

Assisted Solution

dave558 earned 0 total points
ID: 39610394
Para-virtualisation has fixed the problem with limited number of hard drives due to IDE emulation.

From my understanding, booting will be an issue for 2TB on Windows-based platforms for PC BIOS and require UEFI to boot correctly. With Windows Server 2008, it shouldn't have any issues recognising the hard-drive and you can easily partition drive with GPT.

Hard drive was added as a data drive but was not recognised at all, so I'm suspecting this 2TB limitation is related to missing drivers or something else. This system was compiled by some else and a lot of things where stripped out from it. Testing on another machine did not have the same 2TB limitation.

Author Closing Comment

ID: 39619716
Pointed me in right direction to resolve problem.

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