>> But to understand how to best utilize an array with a fixed number of
>> drives, I add the following constraints:
>> - N+P should follow ZFS best-practice rule of N={2,4,8} and P={1,2}
>> - all sets in an array should be configured similarly
>> - the MTTDL for S sets is equal to (MTTDL for one set)/S
>
> Yes, these are reasonable and will reduce the problem space, somewhat.
Actually, I wish I could get more insight into why N can only be 2, 4,or
8. In contemplating a 16-bay array, I many times think that 3 (3+2) + 1
spare would be perfect, but I have no understanding what N=3 implicates...
>>
>>
>> While its true that RAIDZ2 is /much /safer that RAIDZ, it seems that
>> /any /RAIDZ configuration will outlive me and so I conclude that
>> RAIDZ2 is unnecessary in a practical sense... This conclusion
>> surprises me given the amount of attention people give to
>> double-parity solutions - what am I overlooking?
>
> You are overlooking statistics :-). As I discuss in
> http://blogs.sun.com/relling/entry/using_mtbf_and_time_dependent
> the MTBF (F == death) of children aged 5-14 in the US is 4,807 years, but
> clearly no child will live anywhere close to 4,807 years.
Thanks - I hadn't seen that blog entry yet...
> #define MTTR_HOURS_NO_SPARE 16
>
> I think this is optimistic :-)
Not really for me as the array is in my basement - so I assume that I'll
swap in a drive when I get home from work ;)
> There are many more facets of looking at these sorts of analysis,
> which is
> why I wrote RAIDoptimizer.
Is RAIDoptimizer the name of a spreadsheet you developed - is it
publically available?
Thanks,
Kent
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