[ClusterLabs] systemd's TasksMax and pacemaker

[Ken Gaillot]

Systemd version 227 introduced a new unit file option, TasksMax, to
limit the number of processes that a service can spawn at one time.
Depending on the version, the default is either 512 or 4,915.

It is conceivable in a large cluster that Pacemaker could exceed this
limit, so we are now recommending that users set TasksMax=infinity in
the Pacemaker unit file if building from scratch, or in a local
override if already deployed, to disable the limit.

We are not setting TasksMax=infinity in the shipped unit file in the
soon-to-be-released version 1.1.18 because older versions of systemd
will log a warning about an "Unknown lvalue". However, we will set it
in the 2.0.0 release, when we'll be making a number of behavioral
changes.

Particular OS distributions may have backported the TasksMax feature to
an older version of systemd, and/or changed its default value. For
example, in RHEL, TasksMax was backported as of RHEL 7.3, but the
default was changed to infinity.
-- 
Ken Gaillot <kgaillot at redhat.com>