2012-01-08 5:37, Richard Elling пишет:
The big question is whether they are worth the effort. Spares solve a 
problem and only impact availability in an indirect manner. For single-parity
solutions, spares can make a big difference in MTTDL, but have almost no impact
on MTTDL for double-parity solutions (eg. raidz2).

Well, regarding this part: in the presentation linked in my OP,
the IBM presenter suggests that for a 6-disk raid10 (3 mirrors)
with one spare drive, overall a 7-disk set, there are such
options for "critical" hits to data redundancy when one of
drives dies:

1) Traditional RAID - one full disk is a mirror of another
   full disk; 100% of a disk's size is "critical" and has to
   be prelicated into a spare drive ASAP;

2) Declustered RAID - all 7 disks are used for 2 unique data
   blocks from "original" setup and one spare block (I am not
   sure I described it well in words, his diagram shows it
   better); if a single disk dies, only 1/7 worth of disk
   size is critical (not redundant) and can be fixed faster.

   For their typical 47-disk sets of RAID-7-like redundancy,
   under 1% of data becomes critical when 3 disks die at once,
   which is (deemed) unlikely as is.

Apparently, in the GPFS layout, MTTDL is much higher than
in raid10+spare with all other stats being similar.

I am not sure I'm ready (or qualified) to sit down and present
the math right now - I just heard some ideas that I considered
worth sharing and discussing ;)

Thanks for the input,

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