Richard Elling wrote:
> Haudy Kazemi wrote:
>> How would one calculate system reliability estimates here? One is a
>> RAIDZ set of 6 disks, the other a set of 8. The reliability of each
>> RAIDZ set by itself isn't too hard to calculate, but put together,
>> especially since they're different sizes, I don't know.
>
> We just weigh them accordingly. MTTDL for a 6-disk set will be better
> than for an 8-disk set, though that seems to be counter-intuitive for
> some folks. Let me see if I can put some numbers together later this
> week...
OK, after some math we can get some idea...
Using the MTTDL[1] model for a default disk (500 GBytes, 800k hours MTBF,
24 hours logistical response, 60 GBytes/hr resync) we get:
config MTTDL[1] (yrs)
--------------------------------------------
6-disk raidz 75,319
8-disk raidz 40,349
2x 6-disk raidz 37,659
6-disk raidz + 8-disk raidz 26,274
2x 8-disk raidz 20,175
As you would expect, the MTTDL for the 6-disk scenario is better than for
the 8-disk scenario. So it follows that the MTTDL for a pair of 6-disk
raidz sets is better than for a pair of 8-disk raidz sets and the 6+8
scenario is in between.
The MTTDL[1] model is described at:
http://blogs.sun.com/relling/entry/a_story_of_two_mttdl
-- richard
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