This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
RAID-7 is a marketting term created by Storage Computer, Inc. for what others here have described as RAID-4 with a write cache. John O'Brien (RAID7@world.std.com), (their marketting manager?) frequently posts here. His claims of ~10x improvement on I/O rates for VAXes have been shown to be poorly measured; the change in systems was not simply a RAID-for-modern-disk swap, but included increasing the CPU power by a factor of three and eliminating the HSC and old disk technology. He has also made difficult-to-substantiate claims about the growth and market success of his company relative to competitors. Thus, wise advice would be to take everything Mr. O'Brien says with a grain of salt (not bad advice for dealing with anyone, but especially true for dealing with vendors). The debate also appears here frequently as to whether or not you really WANT your RAID array doing write cacheing; Unix file systems may depend on specific ordering of writes and otherwise make assumptions that could leave you in trouble with power or disk failures. If write ordering is preserved, the danger is somewhat mitigated. That said, some posters here are pleased with their RAID7 arrays, and although comp.arch.storage opinion runs prevalently against Mr. O'Brien himself (and lately his pal Michael Willett who interestingly is quoted here from before he worked for Storage Computer), the possibility exists that the product is worthwhile.