Re: [linux-lvm] Identifying useable block devices

[Marius Vollmer <marius.vollmer@redhat.com>]

[ I am not subscribed, so please keep me in CC.  I'll just reply to
  myself, sorry for breaking the threading.
]

Peter Rajnoha wrote:

> For now, these flags are only documented directly in libdevmapper.h
> (as they were only meant to direct udev rules and these situations
> were all audited directly by communicating with other teams).  I could
> probably add a few lines to the man page directly though as others
> could use this even when reading udev database...

That would be great!

> However, for your purpose, I'd better use
> DM_UDEV_DISABLE_OTHER_RULES_FLAG which just tells that everything else
> other than DM/LVM related should skip this device.

Hmm, DM_UDEV_DISABLE_OTHER_RULES_FLAG is (now) set for thin volumes, as
far as I can tell.  This is what lead me down this rabbit hole in the
first place: UDisks2 _does_ ignore events for nodes with
DM_UDEV_DISABLE_OTHER_RULES_FLAG set, and since Fedora 20, this causes
it to ignore thin volumes.

The use of DM_UDEV_DISABLE_OTHER_RULES_FLAG or any other such flag in
UDisks2 looked like a ugly hack to me, so I started looking for
alternatives.

The best option seemed to be to ignore any DISABLE flag in UDisks, and
to set UDISKS_IGNORE for LVM2 block devices that do not have the
/dev/VG/LV symlink.

Now you say that DM_UDEV_DISABLE_OTHER_RULES_FLAG is actually the Right
Way, but it seems to be buggy re thin volumes.  Correct?

(Of course, UDisks2 should not ignore _events_ but should ignore
_nodes_.  Otherwise, it will get confused when a node acquires a DISABLE
flag later on, which happens to thin pools.  What a mess! :-)