Re: Triple parity and beyond

Stan Hoeppner wrote:

Late reply.  This one got lost in the flurry of activity...

<snip>

We must follow different definitions of "redundancy".  I view redundancy
as the number of drives that can fail without taking down the array.  In
the case of the above 20 drive RAID15 that maximum is clearly 11
drives-- one of every mirror and both of one mirror can fail.  The 12th
drive failure kills the array.

IMHO, 'redundancy' is not the important thing here, and may conflate two 
things - 'how much storage is spent on things other than my data (mirrors, 
parity)' [storage efficiency] and 'if the universe is trying to screw me 
over, how many disk losses can I survive?' [failure resilience]

Your 11 disks number is best-case, but quicksort has taught me to always 
look at average-case and worst-case as well. What you describe has very good 
best-case failure resilience, but its worst-case resilience is poorer. It 
has better best-case, average-case, *and* worst-case performance, but has 
considerably worse storage efficiency.

All of those need to be weighed in deciding which to use; raid 15 being 
'just better' isn't something that can be claimed. It depends on the 
workload.

<snip>

Knowing this is often critical from an architectural standpoint David.
It is quite common to create the mirrors of a RAID10 across two HBAs and
two JBOD chassis.  Some call this "duplexing".  With RAID10 you know you
can lose one HBA, one cable, one JBOD (PSU, expander, etc) and not skip
a beat.  "RAID15" would work the same in this scenario.

This architecture is impossible with RAID5/6.  Any of the mentioned
failures will kill the array.

And again, these address different issues. For one, there's multipath - with 
dual-ported drives, multipath, etc. you can tolerate HBA failure; that 
renders part of the issue something of a canard.

Adding a second JBOD is really not an apples-to-apples comparison, either - 
it's not likely to be a useful configuration in the same situations as lend 
themselves to parity >> 2. Certainly, no-one is advocating that support for 
RAID 10 be removed, and mdadm is certainly capable of handling a manually-
created RAID 15 today.