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Re: RES: [Xen-users] SAN + XEN + FC + LVM question(s)

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Subject: Re: RES: [Xen-users] SAN + XEN + FC + LVM question(s)
From: Wendell Dingus <wendell@xxxxxxxxxxxxx>
Date: Mon, 15 Sep 2008 23:09:11 -0400 (EDT)
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Bruno, thanks to you and Javier both for your helpful replies.

Indeed I can see how the issue of an LVM *inside* an LVM could potentially be problematic. So far in my testing it has not been but then I've not introduced CLVM into the mix yet nor had it in service for any real period of time yet.

Doing some additional research I find numerous articles similar to this:

Getting at that *inner* LVM appears to not be without some effort. "kpartx" to add device mappings to /dev/mapper, pvscan, vgscan, lvchange, lvscan, etc... Based on that amount of effort I'm hopeful that no tools in the DOM0 would "accidentally" see that LVM data and do something to destroy it. I will continue to test things though.

Adding additional LV's and exposing them to the VM to grow the size of the VM possibly has a negative aspect, LV snapshots. If I am pushing 3 LV's into a VM from the DOM0 how can I make a single consistent snapshot for backup? Hmm...

Oh well a very interesting exercise with hopefully a very elegant solution.

Thanks again.

----- "Bruno Bertechini" <bruno.bertechini@xxxxxxxxxxx> wrote:
> Just complementing Javier's email, I have tested the "online" resizing
> inside domU's. It does not work without reboot or (re)-mount the drive /
> partition.
> Use LVM inside a domU seems to solve this problem, but you DO need to
> consider Javier's statements regarding "inner" and "outer" LVM's.
> IF we could find ANY documentation about it, and make sure this nested
> logical volume Will work witouht problem, I would say this is the *perfect*
> solution.
> We have *exactly* same scenario here : 03 hosts (dual Quad-Core cpus, 32Gb
> of RAM and EMC drive with several luns).
> I am on the SAME research of "Best practices" to use this combination.
> I think we find the 02 possibilities to GO.
> 1) LVM inside domU (downtime 0)
> 2) reboot the guest (domU) to "see" the changes (I would prefer this
> solution, since I personally don't like the Idea to "add" more LV's instead
> of resize them.
> Regards
> Bruno Bertechini
> -----Mensagem original-----
> De: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] Em nome de Javier Guerra
> Enviada em: segunda-feira, 15 de setembro de 2008 13:16
> Para: Wendell Dingus
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Assunto: Re: [Xen-users] SAN + XEN + FC + LVM question(s)
> On Mon, Sep 15, 2008 at 11:02 AM, Wendell Dingus <wendell@xxxxxxxxxxxxx>
> wrote:
> > Are there pitfalls or limitations I've not thought of here though? Is this
> > approach a "best practices" or is some other method considered "better"?
> AFAICS, you're right and ready to go.
> the only thing i don't like too much is using LVM inside DomU's.  in
> theory some scanning tools could confuse the LVM metadata inside those
> volumes with the 'outer' LVM metadata and wreck the whole thing.  i
> don't think it really happens in practice...
> if you want to avoid LVM inside DomU's, expanding a DomU drive become
> a little harder: you have to expand the LV from Dom0, and then somehow
> make that resizing known to the DomU kernel. usually that means (at
> least) unmounting the device, or even restarting.  after that, just
> resize the FS.
> if you can tolerate a few minutes of downtime to resize a DomU,
> consider avoiding the 'inner' LVM.  if not, or if you're confident no
> CLVM tool would confuse 'inner' and 'outer' LVM's, go with your plan.
> --
> Javier
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