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FW: [Xen-devel] questions about production use

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Subject: FW: [Xen-devel] questions about production use
From: "Williamson, Mark A" <mark.a.williamson@xxxxxxxxx>
Date: Mon, 26 Jan 2004 15:58:29 -0000
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Whoops ;-) - here's the reply I sent to Nick's e-mail.  Accidentally
forgot to send it to the list as well...

>> [me talking about virtual disks]
> Sort of like LVM but run by Xen?  Tha sounds very interesting.

Yes, it is rather like LVM.  The user space virtual disk tools manage
allocation of disk extents and keep track of which extents belong to
which virtual disks.  Then when you create the Virtual Block Device that
domains will use to access that virtual disk, Xen does all the address
translation, so it just appears like another other Xen block device to
the guest OS.

>> [me talking about the free pool]
> That sounds like just the job.  We'd also want to be able to access
> all the virtual disks from Domain0 for administrative purposes (backup
> / transfer to a new host etc) but I guess that is possible.

Xen 1.2 and above have "automatic plumbing" of virtual block devices:
you create a virtual block device for a domain and it "just knows" that
it's there, a bit like hot plugging.  You can use this to add a virtual
disk to a device node in dom0, do stuff with it, then remove it from
dom0 again.  However, it is not safe to have two writers to one
filesystem (unless the filesystem explicitly supports it, which most
don't) and even with one reader and one writer, it's probably not a good
idea - you'll have to take virtual disks offline (by at least unmounting
them in the domain that's using them) before accessing them from dom0.
I expect you'd gathered this, though...

>> [me talking about re-exporting devices from dom0]
> Excellent!
> How much of this and the above is implemented now?  Should I be
> checking out Xen 1.2 and reading the docs?

Virtual Disks are in the 1.2 and unstable trees right now.  There was an
implementation of virtual disks in versions 1.0 and 1.1 but the rewrite
adds support for the new Python-based toolchain and some new features.
AFAIK nobody has used the new VD tools "in anger" yet but it's been
working pretty solidly in testing.  At some stage after we've got 1.2
released (which will be soon) I'll be adding some more whizzy features
to the unstable tree but these would probably be backwards compatible
with 1.2.

The virtual disk howto is now slightly out of date but I will be
updating it presently.  If you get stuck, there's always interactive
help on the mailing list ;-) - I can feed back any discussions we have
to make the docs better.

The ability to re-export devices from one domain that just appear like
an ordinary Xen block device to another domain is still at the design
stage, as yet.  However, this is fairly high on the priority list at the



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