On 1/27/21 3:35 AM, Carlos E. R. wrote:
A question about "battery": is it still an argument if you system is backed up 
by an UPS? I thought the batteries on controller cards were paramount only on 
systems that are not protected by a convenient UPS. 
The idea is, I assume, that battery backed hardware raid detects that
the power died and commits everything to disk before finally powering
down the HDs.

The hardware RAID controllers that we use (3-ware/LSI/Broadcom) have
super-capacitors instead of lithium batteries.  The battery is used to power
cache RAM that holds unwritten data in the event of a system crash.  I don't
think there's time for the controller to complete writes to disk, it doesn't
power the disks after all.

An UPS will keep the entire computer running till it finally gives up,
or either the human or software powers it down before the battery runs down.

Not really the same thing, but both avoid disasters.

My experience with UPS may be limited, but back in the day I ran a 10-KVA
UPS to power a group of Sun Microsystem servers.  It was a nice UPS with a
ferroresonant transformer that conditioned power as well as running the
10-KVA load for 30-minutes.  In my experience, we experienced more unplanned
power outages caused by the UPS itself than if we directly connected to the
mains.  After we retired the Suns and switched to SuSE I once managed to
keep the main server up and running continuously for a bit more than 4-years,
without the UPS.  Without a reboot!  It was a busy server too, with a hardware
RAID controller.