On 06/30/09 03:00 AM, Andre van Eyssen wrote:
On Tue, 30 Jun 2009, Monish Shah wrote:
The evil tuning guide says "The ZIL is an essential part of ZFS and
should never be disabled." However, if you have a UPS, what can go
wrong that really requires ZIL?
Without addressing a single ZFS-specific issue:
* panics
* crashes
* hardware failures
- dead RAM
- dead CPU
- dead systemboard
- dead something else
* natural disasters
* UPS failure
* UPS failure (must be said twice)
* Human error (what does this button do?)
* Cabling problems (say, where did my disks go?)
* Malicious actions (Fired? Let me turn their power off!)
That's just a warm-up; I'm sure people can add both the ZFS-specific
reasons and also the fallacy that a UPS does anything more than
mitigate one particular single point of failure.
Actually, they do quite a bit more than that.
They create jobs, generate revenue for battery manufacturers, and tech's
that change
batteries and do PM maintenance on the large units. Let's not forget
that they add
significant revenue to the transportation industry, given their weight
for shipping.
In the last 28 years of doing this stuff, I've found a few times that
the UPS has actually
worked and lasted as long as the outage. Many other times, the unit is
failed (circuits),
or the batteries are beyond the service life. But really, something
approaching 40%
of the time they actually work out OK.
So they also create repair and recycling jobs. :-)
Don't forget to buy two UPSes and split your machine across both. And
don't forget to actually maintain the UPS. And check the batteries.
And schedule a load test.
The single best way to learn about the joys of UPS behaviour is to sit
down and have a drink with a facilities manager who has been doing the
job for at least ten years. At least you'll hear some funny stories
about the day a loose screw on one floor took out a house UPS and 100+
hosts and NEs with it.
Andre.
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