On Tue, 2010-04-06 at 19:25 +0200, Hans de Goede wrote:
> Hi,
>
> On 04/06/2010 06:29 PM, David Lehman wrote:
> > On Tue, 2010-04-06 at 10:58 -0400, Peter Jones wrote:
> >> On 04/06/2010 06:06 AM, Hans de Goede wrote:
> >>> Hi,
> >>>
> >>> On 04/06/2010 11:54 AM, Ales Kozumplik wrote:
> >>>> On 03/30/2010 02:30 PM, Hans de Goede wrote:
> >>>>> Hi All,
> >>>>>
> >>>>> While doing what should be testing a simple iscsi related patch,
> >>>>> I encountered the following issue:
> >>>>>
> >>>>> Take a system with a single disk, sda, which has a /boot on
> >>>>> sda1 and a PV on sda2. This PV is the PV for the 1 PV VG:
> >>>>> VolGroup, which contains LV's lv_swap, lv_root and lv_home.
> >>>>>
> >>>>> "Attach" an iscsi disk to this system, which becomes sdb,
> >>>>> which has a /boot on sdb1 and a PV on sdb2. This PV is the PV
> >>>>> for the 1 PV VG: VolGroup, which contains LV's lv_swap and
> >>>>> lv_root.
> >>>>>
> >>>>> Notice that:
> >>>>> 1) The 2 VG's have the same name
> >>>>> 2) Only sda has a lv_home LV.
> >>>>>
> >>>>> Now in the filter UI select only disk sdb to install to, then
> >>>>> the following may (depending on scanning order) happen:
> >>>>>
> >>>>> Assume sdb gets scanned first by devicetree.py:
> >>>>> - when scanning sdb2, handleUdevLVMPVFormat() will
> >>>>> call "lvm lvchange -ay" for all LV's in this VG
> >>>>> (as seen by udev, more on that later).
> >>>>> - at this point, sda has not been scanned yet, so
> >>>>> isIgnored has not been called for sda2 yet, and thus
> >>>>> lvm_cc_addFilterRejectRegexp("sda2") has not been called
> >>>>> yet.
> >>>>> - thus lvm lvchange sees both sda2 and sdb2, it complains
> >>>>> that there are 2 identically named VG's and picks the one
> >>>>> using the sda2 PV.
> >>>>
> >>>> Maybe we should stop the installation at this point and tell the user
> >>>> that he named two VGs the same and needs to address this before
> >>>> proceeding with the installation? Because otherwise we will need to do
> >>>> too many changes for a corner case that only occurs infrequently. And we
> >>>> still won't be completely happy with them.
> >>>>
> >>>
> >>> That won't work, as there actually are no duplicate VG's when looking only
> >>> at the devices the user selected in the filter UI, the problem is
> >>> that lvm at this point does not honor what we've selected in the filter UI
> >>> and what not. Which is caused by the way we build the ignore these devices
> >>> cmdline argument for lvm.
> >>
> >> Perhaps we should be generating an lvm.conf with a proper filter section for
> >> this instead? It's not really an ideal solution :/
> >
> > It might be worth passing lvm a full list of the devices it is allowed
> > to look at instead of telling it which devices to ignore.
>
> I've been thinking in that direction too, and I like it, but ...
>
> > We will know
> > the full list of PVs at activation time.
>
> Do we? Currently we activate LV's from handleUdevLVMPVFormat()
> when we find the first PV.
Right, but lv.setup calls vg.setup, which raises an exception if
len(self.parents) < self.pvCount, so that's taken care of.
>
> But if you've ideas to change this, I'm all ears. We could delay
> bringing up the LV's until the VG actually has pv_count parents,
> but this still won't save us from duplicate VG name issues.
>
> Hmm, but if we were to use the VG's UUID as name in the device
> tree then this could work. This would probably require quite a bit
> of reworking of the code though, as I think there are assumptions
> that devicetree name == VG name in quite a few places.
I'm not even sure what we do right now if we encounter two VGs with the
same name. I do see that we try to look up the VG by name in
handleUdevLVMPVFormat. I think it would only help to do all lookups on
existing VGs by UUID instead of name, at least while populating the
tree.
>
> Note that this problem is less obscure then it seems, it can
> be triggered by this PITA called software RAID. If we have a PV on
> a software RAID mirror, then lvm will see the PV 3 times, and semi randomly
> (or so it seems) pick one (*). So we really must make sure
> we've scanned all possible lower level devices before activating
> LV's. I've been thinking about this, and a patch for this should not
> be all that invasive.
Have you seen this happen, or are you speculating? It seems to me we
would have hit this long ago if it were so simple to set up. I suppose
it's possible that we have and just don't know it.
>
> *) Although we could simply not care in this case, as at the end of
> populating the tree we tear down everything, and the next activation
> the ignore list will be ok and the right PV will get used.
>
> Regards,
>
> Hans
>
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