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. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
