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LVM poll daemon overview
========================

(last updated: 2015-05-09)

LVM poll daemon (lvmpolld) is the alternative for lvm2 classical polling
mechanisms. The motivation behind new lvmpolld was to create persistent
system service that would be more durable and transparent. It's suited
particularly for any systemd enabled distribution.

Before lvmpolld any background polling process originating in a lvm2 command
initiated inside cgroup of a systemd service could get killed if the main
process (service) exited in such cgroup. That could lead to premature termination
of such lvm2 polling process.

Also without lvmpolld there were no means to detect a particular polling process
suited for monitoring of specific operation is already in-progress and therefore
it's not desirable to start next one with exactly same task. lvmpolld is able to
detect such duplicate requests and not spawn such redundant process.

lvmpolld is primarily targeted for systems with systemd as init process. For systems
without systemd there's no need to install lvmpolld because there is no issue
with observation described in second paragraph. You can still benefit from
avoiding duplicate polling process being spawned, but without systemd lvmpolld
can't easily be run on-demand (activated by a socket maintained by systemd).

lvmpolld implement shutdown on idle and can shutdown automatically when idle
for requested time. 60 second is recommended default here. This behaviour can be
turned off if found useless.

Data structures
---------------

a) Logical Volume (struct lvmpolld_lv)

Each operation is identified by LV. Internal identifier within lvmpolld
is full LV uuid (vg_uuid+lv_uuid) prefixed with LVM_SYSTEM_DIR if set by client.

such full identifier may look like:

  "/etc/lvm/lvm.confWFd2dU67S8Av29IcJCnYzqQirdfElnxzhCdzEh7EJrfCn9R1TIQjIj58weUZDre4"

or without LVM_SYSTEM_DIR being set explicitly:

  "WFd2dU67S8Av29IcJCnYzqQirdfElnxzhCdzEh7EJrfCn9R1TIQjIj58weUZDre4"


LV carries various metadata about polling operation. The most significant are:

VG name
LV name
polling interval (usually --interval passed to lvm2 command or default from lvm2 
		  configuration)
operation type (one of: pvmove, convert, merge, thin_merge)
LVM_SYSTEM_DIR (if set, this is also passed among environment variables of lvpoll
		command spawned by lvmpolld)

b) LV stores (struct lvmpolld_store)

lvmpolld uses two stores for Logical volumes (struct lvmpolld_lv). One store for polling
operations in-progress. These operations are as of now: PV move, mirror up-conversion,
classical snapshot merge, thin snapshot merge.

The second store is suited only for pvmove --abort operations in-progress. Both
stores are independent and identical LVs (pvmove /dev/sda3 and pvmove --abort /dev/sda3)
can be run concurently from lvmpolld point of view (on lvm2 side the consistency is
guaranteed by lvm2 locking mechanism).

Locking order
-------------

There are two types of locks in lvmpolld. Each store has own store lock and each LV has
own lv lock.

Locking order is:
1) store lock
2) LV lock

Each LV has to be inside a store. When daemon requires to take both locks it has
to take a store lock first and LV lock has to be taken afterwards (after the
appropriate store lock where the LV is being stored :))