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Re: extending LVM logical volumes for Xen root partitions is NOT so simple!

woods%planix.ca@localhost ("Greg A. Woods") writes:

>I'm assuming (without yet having read any code) that indeed it is the
>xbd(4) driver that's enforcing the size limits of the disklabel, just in
>the way any traditional disk device driver would do. My mistake was
>thinking of it more as a simple pass-thru device to access the back-end
>LV in the dom0 and I hadn't really paid attention to the fact that it
>supports partition sub-devices just as all disk drivers do.

Yes, xbd(4) is a traditional disk device driver.

- the raw partition is limited by the disk size.
- other partitions are limited to the partition size in disklabel.

So, using the raw partition 'd' (for x86) instead of 'a' would look

>Indeed that's the root of my subject line -- the documentation (ch17 of
>The NetBSD Guide in particular) makes no mention of putting disklabels
>on logical volumes, or of adjusting diskabels when resizing LVs (an
>explicit note in 17.8.3 mentions resizing the filesystem before/after
>resizing the LV, but with no mention of disklabels); and with my prior
>experience with LVM and similar of using one LV per filesystem I've
>mistakenly made the assumption that the burden of managing per-LV
>disklabels was entirely unnecessary.

It's something that only exists for Xen VMs, because the LV is
interpreted as a real disk image again.

>Perhaps though, and because of history and circumstance, many Xen admins
>will be giving a single LV for each domU, or at least for each "system
>volume" (perhaps with user data being additional LVs), and especially
>with install tools like systinst this volume ends up as a partitioned
>emulation of a whole disk.

You usually access the virtual disk only from the virtual machine, after
all you cannot do it concurrently on Dom0 and DomU without risking
data corruption.

>still considering the relative merits (in an all-NetBSD hosting
>environment) of doing the sysinst in the domU or in the dom0.

Treat the VM like a real machine. If you think the Dom0 could access it,
you always need to take special care.

                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."

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