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Re: [Xen-users] HVM domU on storage driver domain

  • To: xen-users@xxxxxxxxxxxxx
  • From: "Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>
  • Date: Mon, 16 Jan 2017 12:04:30 -0500
  • Delivery-date: Mon, 16 Jan 2017 17:05:05 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>

On 2017-01-16 11:06, G.R. wrote:
Hi all,
I'm trying out the storage driver domain feature (one of the major
motivation to drive me out of my stable but ancient XEN 4.3.2 version,
the other motivation being the DISCARD feature).

I understand that HVM is NOT officially supported, but I played some
tricks to work it around.
The trick is to use file based storage and mount it through NFS and make
the same file accessible through the same path in both dom0 && NAS domU.
I have to disable one sanity check in libxl.c to make it stop
complaining about non-local disk image.

It's worthy to note that I wanted to grant read-only access to dom0 to
avoid any consistency / corruption issue.
I think this should be plausible for BIOS booting purpose. But for some
reason qemu refused to run with read-only permission.
Any solution?

What I get now is a booting HVM domU on storage driver domain.
By booting, I mean:
1. I passed the BIOS bootstrap phase which is supposed to backed by qemu.
2. The windows 7 OS is stuck in the boot screen forever :-)

Really a black joke, huh? The fact is that I have no way to determine
what is going on here.
Is it a performance issue or something has simply gone wrong?
Any suggestion on how to diagnose further?

Some observations:
1. The whole system is pretty idle with the booting win7 + NAS domU.
2. When I get the win7 into recover mode the disk spins up happily.
3. When win7 is booting, lsof reports nothing to the disk image on both
dom0 && NAS, as if the disk image is not being accessed.
4. In recover mode, I can see qemu accessing the disk image on dom0 from
5. The NAS is freeNas 9.10 with FreeBSD 10 kernel. But I couldn't find
any clue about it hosting a xen disk (either dmesg or ps tree)

My current suspect is that may be the win 7 are using improper driver
during boot time?
(But both emulated / PV device should be accessible, hmm)

Suggestions on diagnose are welcome!
I don't have any specific suggestions regarding figuring out what's going on, but I do have a suggestion for a work-around. Based on what you've said, what you want is to use a domU running FreeNAS to provide storage for other domU's on the same system. The de-facto method of doing this sanely is to use a block-storage protocol instead of a network filesystem to export the storage devices, and import the block-device into dom0 (or a PV domU running as a storage driver domain) then pass them to the VM's (or, if you're not using them as boot devices, just import them in the VM directly). iSCSI is what usually gets used for this, although if FreeNAS supports it I'd suggest using ATAoE (which require near zero client-side setup when the client is Linux (literally just load the module)) or NBD (which requires significantly less client-side setup than iSCSI) as they're both much simpler to set-up on the client side and have much less protocol overhead than iSCSI. While I've never seen anyone try with a NAS running as a domU itself, I know all three options work fine with Xen when set up correctly.

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