[lvm-devel] [PATCH] Never scan internal LVM devices.

[Petr Rockai]

Hi,

the attached patch disables scanning for any LVM-created internal
devices, such as mirror images, mirror logs, snapshot COW areas etc.

There are two reasons for this: First, scanning these devices can
disrupt more complex operations (primarily lvconvert) involving
deactivation of these internal devices. See BZs 596453, 606931 and
607316.

Second that even if such a device contains a PV label (or an MDA), it
should never be used by LVM as a PV. Since these logical volumes are
never visible to the user (disregarding any bugs in LVM) and they cannot
be created by the user, they always exist as a part of a composite
logical volume.

Even if a PV was stacked in an LV, and this LV was composite (so it is
using some internal LVs as its components), it is desirable that this
*and only* this composite LV is seen by LVM as a PV. If the PV label or
MDA is propagated into the component LVs (such as mimages) and these are
found during a scan, this would lead to the PV being found multiple
times which is certainly undesirable. Cf. the situation with MD
component devices, which is analogous.

For all I can tell, this patch would fix all the above-mentioned BZs,
which are all high-priority test blockers.

Yours,
   Petr.

PS: There are other options to address the disruption issue that is
causing all the above bugzillas. Nevertheless, all other options seem to
be too complex to implement and would run a high risk of regressions in
unrelated areas of functionality -- we have discussed this last week
with Alasdair. I believe the proposed patch is relatively simple and
does not introduce any bad behaviours (nor eliminates any desirable
behaviours).

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