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Re: [Xen-users] Diskless Xen (PXE, NFS, ...)

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Subject: Re: [Xen-users] Diskless Xen (PXE, NFS, ...)
From: Thomas Treutner <thomas@xxxxxxxxxx>
Date: Wed, 28 Oct 2009 19:45:28 +0100
Cc: Javier Guerra <javier@xxxxxxxxxxx>
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On Wednesday 28 October 2009 17:07:39 Javier Guerra wrote:
> On Wed, Oct 28, 2009 at 10:53 AM, Thomas Treutner <thomas@xxxxxxxxxx> wrote:
> > Hello,
> >
> > I'd like to ask if someone has experience with setting up Xen (I use 3.2
> > from Lenny) in a diskless scenario - so not just the DomUs on a central
> > storage (therefore, I use open-iscsi), but also Xen itself and the Dom0?
> not first-hand experience; but a couple advices:

thanks for your input!

> - be sure to initiate the iSCSI devices on Dom0.  IOW, the DomU's
> won't know their storage isn't local.  

Yep, I did it that way, using a block-iscsi script floating around in the 
Internet somewhere and then iscsi: instead of phy: etc. 

> That means the same LUNs have 
> to be available on every Dom0, so they'll be ready when the DomU gets
> migrated.

Of course. I think I resolved my problems in the second try. Problem was I was 
changing my IP subnet after I discovered the nfsroot/bridging troubles to 
separate that clearly. Being new to open-iscsi, I wasn't aware that 
in /etc/iscsi/nodes and sendtargets old information was lurking around ;-)
So the iSCSI-initiator was confused about which path to take (my target host 
was still reachable at the old address too) and had hickups. After resolving 
that the initiator still tried at the old address first, which ran into a 
timeout, so live migration was very slow (~20s of waiting for vdb actions).
Resolved that too and now works like a charm!

> - you can get rid of NFS totally, either put your Dom0 in iSCSI too,

I've searched for that, it seems to be quite a hack without HBAs to me, even 
for vanilla kernels. 

> or just keep the initrd.  there's no requirement for initrd to yield
> to another root.  in either case, you'll find it much easier to
> manage.

Hm, that would be interesting. So the Dom0 would reside in a ramdisk? Wouldn't 
one need quite some (additional) amounts of RAM for that? I have a node with 
just 2G which isn't very much for Xen anyways, when subtracting 256-512M for 
Dom0. And I think I would need something like NFS anyways for the DomU.cfgs, 
if I don't want to regenerate the initrds every time. 

But thanks anyways for your ideas!


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