Re: [Xen-devel] Xen & Automated Disk Management for Domains

[Michael Vrable <mvrable@xxxxxxxxxxx>]

On Thu, Aug 05, 2004 at 06:56:58PM +0100, Ian Pratt wrote:
> > One feature that would be really nice to have is the ability to easily
> > manage the disk images used by the Xen domains, so that I could, for
> > example, quickly load a particular Linux distribution on a set of LVM
> > logical volumes, then start a new Xen instance with those.
> 
> I'd certainly like to see more support for this.
> 
> Copy-on-write (CoW) sparse disks are pretty high on everyone's
> wish list. In principle, it should be possible to do this today,
> but it's a pain to setup: use LVM snapshot with the log device as
> a loop interface connected to a sparse file.

[Just to make sure we're on the same page since I didn't specify
earlier, I'm using LVM2 and not LVM1.]

I haven't gotten snapshots working yet...in my last attempts, I've
gotten a kernel NULL pointer dereference when trying to create a
snapshot.  I haven't put much effort into tracking the problem down, so
I'm not even sure if it's Xen-related yet, but I'd like to look into it
more soon.  Perhaps with XenoLinux 2.6 ready to run as Domain 0, I can
give that a shot as well--I'm currently using 2.4.

At the moment, I'm simply using LVM as a convenient way to create and
destroy partitions for new domains.  (Disk space is cheap, and I've
found this easier to manage than I think using sparse files and the
loopback driver would be.)

> [The LVM snapshot device is a bit 'odd' for our purposes as
> writes occur in-place, with the old block contents being copied
> to a log device. It would make more sense if we slightly modified
> the code to have the writes go to the log and leave the original
> device pristine. ]

Very true, but I don't know how much work this would be.  Since I
haven't been able to test, I don't know how multiple COW images of the
same underlying disk would work in the current implementation (if at
all).  They would certainly work better with the behavior you describe.

> It would certainly be very nice to have xend assist with setting
> all this up, and have a nice way of naming images etc (leveraging
> LVM wherever possible).
> 
> xend is intended to be extensible for exactly this sort of
> thing. Mike's writing a manual for it, which should be going into
> the tree RSN.

I've been reading over some of the code for xend the last few days, so I
at least have some ideas for how to proceed.  I'm hoping that I wouldn't
be stepping on anyone else's toes or duplicating work if I did work on
this.

Oh, and to everyone who has helped build Xen: Thanks!  It's been a lot
of fun to use.

--Michael Vrable


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