Re: [gentoo-user] {OT} Need a new server

[Michael Orlitzky]

On 09/13/2013 05:47 PM, Grant wrote:
>
> I had no idea.  How awesome.  So the entire array shows up as /dev/sda
> when using a real hardware controller?  Just enable an extra kernel
> config option or two and it works?
>

Yep.


>> Yes. RAID10 both stripes and mirrors. So you can lose one, and it's
>> definitely mirrored on another drive. Now you have three drives. If you
>> lose another one, is it mirrored? Well, maybe, if you're lucky. There's
>> a 2/3 chance that the second drive you lose will be one of the remaining
>> mirror pair.
>>
>> Recommendation: add a hot spare to the system.
>
> Would the hot spare be in case I lose 2 drives at once?

It's just to minimize the amount of time that you're running with a
busted drive. The RAID controller will switch to the hot spare
automatically without any human intervention, so you only have to keep
your fingers crossed for e.g. 3 hours while the array rebuilds. This is
as opposed to 3 hours + (however long it took the admin to notice that a
drive has failed).


>   Isn't that extraordinarily unlikely?

If the failures were random, yes, but they aren't -- they just seem that
way. The drives that you use in a hardware RAID array should ideally be
exactly the same size and have the same firmware. It's therefore not
uncommon to wind up with a set of drives that all came off the same
manufacturing line at around the same time.

If there's a minor defect in a component, like say a solder joint that
melts at too low of a temperature, then they're all much more likely to
fail at around the same time as the first one.


> Are modern SSDs reliable enough to negate the need for mirroring or do
> they still crap out?

I don't have any experience with SSDs, but a general principle: ignore
what anyone says, mirror them anyway, and make lots of backups.