The lvm_conf_file setting allows specification of an alternate lvm.conf file with the intent being to help isolate Cinder LVM operations from Host LVM operations.
This setting is currently only used when invoking a subset of the LVM commands that Cinder uses, namely pvs, vgs, lvs and lvdisplay. As a result the system /etc/lvm/lvm.conf is still utilised when running any other LVM commands.
One of the main reasons to use a private lvm.conf file, as I see it, is to allow the use of filter settings to limit the scanning of system devices for LVM PVs/VGs to only the relevant devices. However since only a subset of commands currently use the private lvm.conf in Cinder, it means that such filtering can only be applied in the Cinder private lvm.conf, e.g. /etc/cinder/lvm.conf, and not in the host system lvm.conf, /etc/lvm/lvm.conf.
Previous experience has shown that it can be very desirable to use filters in the Host lvm.conf to limit what devices are scanned in general, especially on server systems. For example if a customer volume is attached to a Cinder node for the purposes of backing it up, and it contains LVM VGs, automatic scanning may dynamically create Device Mapper entries for any LVs found. And there really isn't a need for the Host environment to know about the Cinder LVM environment.
Fergal.
I have prototyped changes to Cinder to allow for complete separate of Host and LVM environments if a Cinder private lvm.conf file is specified via the lvm_conf_file option.
I will put them up for initial review, but there are a couple of additional issues I identified while doing the work that I plan to split out and submit fixes for separately before finalising this work.
Fergal.