On Thu, Dec 29, 2005 at 10:40:33AM -0800, H. Peter Anvin wrote: > It's not really in-between; generic RS RAID would be many times slower > than either; however, unlike raid10 it could survive *any* m failures > where m is the number of redundancy drives. > > The fundamental problem is that generic RS requires table lookups even > in the common case, whereas RAID-6 uses shortcuts to substantially > speed up the computation in the common case. RAID-6 is an important > corner of the problem space, since it deals with the unfortunately > fairly common problem of "disk failure discovered during recovery" > with RAID-5. > > That doesn't mean there couldn't be a problem space where it would > make sense (in fact, on the contrary), but it's still a substantial > engineering effort that would have to be justified. > > Heck, I might even be persuaded to look for generic RS shortcuts if > someone tempted me enough... Funny, I was just thinking about this last night ... The "obvious" case is distributed RAID (RAIS?), where one might want something to survive 50% loss or more. But as you know, the generic RS suffers from the same read-modify-write latency as all parity-based RAID levels except Daniel Phillips's RAID 3.5 [ http://sources.redhat.com/cluster/ddraid/ ]. For archival purposes though, slow writes might not be a big deal. At the margin, this use case begins to overlap various p2p technologies. Regards, Bill Rugolsky - 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
