On Thu, August 21, 2014 3:54 pm, Matt wrote:
>> Hate to change the conversation here but that's why I hate hardware
>> RAID.
I love hardware RAID. 3ware more than others. In case of hardware RAID it
is tiny specialized system (firmware) that is doing RAID function. In the
specialized CPU (I should have called it differently) inside hardware RAID
controller. Independent on the rest of computer, and needing only power to
keep going. Tiny piece of code, very simple function. It is really hard to
introduce bugs into these. Therefore you unlikely will have problem on
device level. To find the status of the device and its components
(physical drives) you always can use utility that comes from hardware
vendor, you can have even web interface if it is 3ware.
>> If it was software RAID, Linux would always tell you what's going on.
It does. And so does hardware RAID device. And most of them (3ware in
particular) do not do offline (i.e. delaying boot) check/rebuild, but they
do it online (they are being operational in degraded state, and do
necessary rebuild with IO present on the device, they just export
themselves to Linux kernel with the warning of being degraded RAID during
boot).
Software RAID, however, has a disadvantage (more knowledgeable people will
correct me wherever necessary). Software RAID function is executed by main
CPU. Under very sophisticated system (linux kernel), as one of the
processes (even if it is real time process), on the system that is
switching between processes. Therefore, RAID task for software RAID lives
in much more dangerous environment. Now, if it never finishes (say, kernel
panics due to something else), you get inconsistent device (software RAID
one), and it is much much much harder task to bring that to to some extent
consistent state than, e.g., to bring back dirty filesystem that lives on
the sane device. This is why we still pay for hardware RAID devices. I do.
Just my 2c.
Valeri
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247
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