On Wed, Oct 19, 2011 at 10:57:23AM +0200, Benny Lofgren wrote:
| On 2011-10-19 10.23, Paul de Weerd wrote:
| > | I think your methodology is fllawed. think of the situations when you
| > | have power loss, then shutdown is started and then power is back.
| > | or situations where you starting machine after blackout and then there
| > | is a blackout again...
| > | With good ups you at least have 'switch off after some time is gone"
| > | option.
| >
| > You can build a lot of logic in the way you do this, including 'switch
| > off after some time is gone'. Really the only thing you get with
| > 'good ups' is an indication of how long your battery is going to last,
| > which might even resemble something close to reality if you're lucky.
| >
| > Don't get me wrong: 'proper' upses have a lot of benefits, but that's
| > mostly related to the ease of doing this controlled power down in case
| > of blackouts.
|
| Well, Gregory is right in a way. The one flaw there is with my "poor man's
| UPS watchdog" is that there is no way to get the server going again if
| power is restored after the script decides to shut the server down but
| before the UPS runs out of battery juice and actually shuts off the
| power to the server.
|
| In that case, when power is restored the server will never have had its
| power cycled, so can never turn back on again even if you set its bios to
| boot when power is applied regardless of its state before power outage.
|
| What we can do in that case is to "almost" power it off, that is, shut
| down all services, get down to single user mode, unmount all volumes
| except for root which is remounted read-only, and then just wait.
|
| Either the server will eventually die, in which case it will boot back up
| as good as new when power is next applied, or the power will get back on
| in which case the script can detect that and simply do a reboot.
So .. what is the fundamental difference from a 'real' UPS that can
signal the machine itself that power is going down ? You get to do
the same steps "in case power is restored while we're going down".
Paul 'WEiRD' de Weerd
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