[ClusterLabs] fence_apc delay?

[Ken Gaillot]

On 09/05/2016 09:38 AM, Marek Grac wrote:
> Hi,
> 
> On Mon, Sep 5, 2016 at 3:46 PM, Dan Swartzendruber <dswartz at druber.com
> <mailto:dswartz at druber.com>> wrote:
> 
>     ...
>     Marek, thanks.  I have tested repeatedly (8 or so times with disk
>     writes in progress) with 5-7 seconds and have had no corruption.  My
>     only issue with using power_wait here (possibly I am
>     misunderstanding this) is that the default action is 'reboot' which
>     I *think* is 'power off, then power on'.  e.g. two operations to the
>     fencing device.  The only place I need a delay though, is after the
>     power off operation - doing so after power on is just wasted time
>     that the resource is offline before the other node takes it over. 
>     Am I misunderstanding this?  Thanks!
> 
> 
> You are right. Default sequence for reboot is:
> 
> get status, power off, delay(power-wait), get status [repeat until OFF],
> power on, delay(power-wait), get status [repeat until ON].
> 
> The power-wait was introduced because some devices respond with strange
> values when they are asked too soon after power change. It was not
> intended to be used in a way that you propose. Possible solutions:

I thought power-wait was intended for this situation, where the node's
power supply can survive a brief outage, so a delay is needed to ensure
it drains. In any case, I know people are using it for that.

Are there any drawbacks to using power-wait for this purpose, even if
that wasn't its original intent? Is it just that the "on" will get the
delay as well?

> *) Configure fence device to not use reboot but OFF, ON
> Very same to the situation when there are multiple power circuits; you
> have to switch them all OFF and afterwards turn them ON.

FYI, no special configuration is needed for this with recent pacemaker
versions. If multiple devices are listed in a topology level, pacemaker
will automatically convert reboot requests into all-off-then-all-on.

> *) Add a new option power-wait-off that will be used only in OFF case
> (and will override power-wait). It should be quite easy to do. Just,
> send us PR.
> 
> m,