[ClusterLabs] systemd's TasksMax and pacemaker

[Jan Pokorný]

On 15/11/17 11:16 +0100, Jan Pokorný wrote:
> On 14/11/17 15:07 -0600, Ken Gaillot wrote:
>> It is conceivable in a large cluster that Pacemaker could exceed
>> this limit
> 
> [of 512 or 4915 tasks allowed per service process tree, possibly
> overridden with systemd-system.conf(5) configuration],
> 
>> 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.
> 
> Thanks for broadcasting this ;)
> 
>> 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.
> 
> Note also Ansible playbooks and other means of automation and
> provisioning may have a final say on the system-wide default limit.
> These unattended changes shall also be put into decision whether you
> need a local customization of pristine pacemaker service file, at
> least for versions prior to 2.0.0, to avoid any unpleasant surprises
> with process proliferation limiting ("anti-fork-bomb") systemd.

Just noting that _not_ having TasksMax= actively used in the pristine
service file to defuse possibly enforced system-wide constraints with
tasks-per-service limit as extremely low as about 7 (or indeed,
service-local redefinition to that effect instead) may result
in a restart loop for the main pacemaker's process due to Restart=
directive.

Another reason to use fencing (if of course this is certain to be
spotted from the view of other nodes at all, I don't know).

-- 
Poki
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